Dr. Jeff Masters' WunderBlog

Ernesto near hurricane strength

By: JeffMasters, 08:28 PM GMT on august 31, 2006

Tropical Storm Ernesto is near hurricane strength, headed for a landfall near the North Carolina/South Carolina border this evening. The latest center report from the Hurricane Hunters at 3:37pm EDT found a central pressure of 991 mb, just 1 mb higher than Ernesto's pressure when he was a hurricane south of Haiti. Radar animations from the Wilmington radar don't show an eyewall forming yet, put the intensity and number of spiral rain bands is increasing. Again, it is a good thing Ernesto has only a few hours over water to intensify, or this would have been a Category 2 hurricane in another day. The latest surface wind measurements from the SFMR instrument carried on NOAA's P-3 aircraft found highest winds of 67 mph, on the southeast side of the storm (Figure 1). Wind observations from offshore buoys have been as high as 42 mph sustained with gusts to 50 mph this afternoon. Rainfall amounts up to 8 inches have been estimated from Wilmington radar in some small pockets, and amounts of 4 inches are common across North and South Carolina. An additional 4-8 inches will fall over much of North Carolina, making fresh water flooding the main hazard of the storm. A storm surge of up to 5 feet near the coast will also cause some problems.


Figure 1. Wind analysis of the NOAA P-3 Hurricane Hunter data from 3:30pm EDT 8/31/06.

Ernesto is under wind shear of 10-20 knots, thanks to southwesterly upper-level winds from the trough of low pressure pulling the storm to the north. This shear is keeping the storm from organizing as quickly as it would otherwise. Water temperatures under the storm are about 30 C, which is very favorable for intensification. The eastern portion of the storm is over the axis of the very warm Gulf Stream Current.

Hurricane John
The Air Force Hurricane Hunters reached Hurricane John this afternoon, and ofund that it had weakened to a Category 2 hurricane. This probably occurred because of interaction with the mountainous terrain of Mexico. However, John is now pulling away from the coast of Mexico, and may be able to re-intensify. The forecast track of the storm takes it very near to the tip of the Baja Peninsula, and John could be the strongest hurricane to affect Baja since Hurricane Liza of 1976 brushed the peninsula as a Category 4 storm. Wind shear is light and forecast to remain low, and sea surface temperatures (Figure 2) are a very warm 30 C under the hurricane--about 1-2 degrees C above normal for this time of year. John's strength is likely to be controlled by difficult to predict eyewall replacement cycles over the next two days.


Figure 2. Current sea surface temperatures along the Pacific coast. Temperatures in the Gulf of California may not be accurate, due to difficulties retrieving the temperature via satellite measurements in such a narrow body of water. The red line separating blue colors from yellow marks the 26 C isotherm--the critical temperature needed to sustain a tropical cyclone. Note the very cool waters extending from the California border southwards along the coast. This long stretch of cool water will make it difficult for John to hold together if it tries to approach California. Image credit: NOAA/AOML.

Is John a threat to the U.S.?
In yesterday's blog, I discussed in detail the historical record of the five Eastern Pacific tropical cyclones that have affected the U.S. with tropical storm force winds. The latest model guidance and official forecast now suggest that the U.S. is not at risk from John.

Super Typhoon Ioke
Super Typhoon Ioke hit tiny Wake Island with the left side of its eyewall this morning. A storm surge of 8 feet with 50 foot waves probably battered the island. Observations from Wake showed winds of 78 mph gusting to 100 mph and a pressure of 934 mb before the instrument failed at 2:18am EDT this morning. A drifting buoy (52609) about 100 miles east of Wake apparently took a direct hit, and measured a pressure of 921.5 mb in the eye:

Measurements from drifting buoy 52609:

Date/Time Pressure
-------------------
8/30/06 16 995.9
8/31/06 00 970.5
8/31/06 02 939.6
8/31/06 03 921.5
8/31/06 05 936.7
8/31/06 06 951.7

Unfortunately, the buoy has no wind measurement equipment. Ioke continues to be a borderline Category 4/5 super typhoon, and is not expected to decline to category 3 strength for several more days. This would probably make it the longest-lived Category 4 or higher storm on record anywhere.

African tropical waves
The tropical wave near 18N 50W is surrounded by a large cloud of dry air and African dust and has lost almost all of its thunderstorm activity near the center. Development is not likely until Sunday at the earliest, when it may find a moister environment near the Lesser Antilles Islands.

A tropical wave near 12N 36W, 700 miles southwest of the Cape Verde Islands, has shown a small increase in thunderstorm activity today. Some slow development of this system is possible over the next few days.

A spinning area of clouds few hundred miles north of Puerto Rico is associated with an upper level low pressure system. Development is not expected of this system.

All of the global models are calling for development of a tropical wave that wil come off the coast of Africa this weekend. The models are not very good at forecasting development of tropical systems coming off the coast of Africa; it will be interesting to see if this consensus forecast is correct.

Next update
Tonight, I'll be talking live at 8:45pm EDT on the Barometer Bob show:

http://www.barometerbobshow.com/

Jeff Masters

Updated: 08:59 PM GMT on august 31, 2006

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Ernesto strengthens as it approaches the Carolinas

By: JeffMasters, 01:54 PM GMT on august 31, 2006

Tropical Storm Ernesto is gathering strength, headed for a landfall near the North Carolina/South Carolina border late this afternoon. Radar animations from the Charleston radar shows a steadily organizing system, with a strong band of heavy rain wrapping around the west side of the center. Ernesto may have a partial eyewall and be a strong tropical storm with 65-70 mph winds at landfall. Wind observations from offshore buoys have been as high as 30 mph sustatined with gusts to 42 mph this morning. The Hurricane Hunters left the storm at 6am EDT this morning, and reported a pressure of 996 mb before they left. This is not far from Ernesto's 990 mb pressure it had when it was a hurricane with 75 mph winds south of Haiti. A NOAA Hurricane Hunter is due in the storm by noon, and an Air Force airplane at 2pm EDT.


Figure 1. Current long-range radar out of Charleston.

The north side of Ernesto is under some significant wind shear of 20 knots, thanks to southwesterly upper-level winds from the trough of low pressure pulling the storm to the north. As Ernesto moves further north, the shear will increase, likely putting a limit on the amount of intensification the storm can do. Water temperatures under the storm are about 30 C, which is very favorable for intensification. The eastern portion of the storm is over the axis of the very warm Gulf Stream Current.

Hurricane John
Hurricane John is a dangerous Category 3 hurricane this morning, just 100 miles off the coast of Mexico. The storm is moving parallel to the coast, but any deviation to the right would bring the intense core to the coast, making it one of the strongest Pacific hurricanes ever to strike Mexico. The forecast track of the storm takes it very near to the tip of the Baja Peninsula, so John has a chance to make a double hit on Mexico. John could be the strongest hurricane to affect Baja since Hurricane Liza of 1976 brushed the peninsula as a Category 4 storm. Only seven Category 4 Eastern Pacific hurricane have hit Mexico in recorded history. Wind shear is light and forecast to remain low, and sea surface temperatures (Figure 2) are a very warm 30 C under the hurricane--about 1-2 degrees C above normal for this time of year. John's strength is likely to be controlled by difficult to predict eyewall replacement cycles over the next three days, and could be a Category 4 hurricane when it encounters Baja. An Air Force Hurricane Hunter aircraft is scheduled to investigate John at 11am PDT today.


Figure 2. Current sea surface temperatures along the Pacific coast. Temperatures in the Gulf of California may not be accurate, due to difficulties retrieving the temperature via satellite measurements in such a narrow body of water. The red line separating blue colors from yellow marks the 26 C isotherm--the critical temperature needed to sustain a tropical cyclone. Note the very cool waters extending from the California border southwards along the coast. This long stretch of cool water will make it difficult for John to hold together if it tries to approach California. Image credit: NOAA/AOML.

Is John a threat to the U.S.?
In yesterday's blog, I discussed in detail the historical record of the five Eastern Pacific tropical cyclones that have affected the U.S. with tropical storm force winds. The latest model guidance still suggests the possibility John could come far enough north to affect the U.S., but it is looking increasingly unlikely. A ridge of high pressure is expected to build in and force John to the west about the time the storm reaches Baja. The GFDL model, however, takes the storm about halfway up the Gulf of California before turning it westward across Baja and away from North America.

Super Typhoon Ioke
The incredible Category 4 Super Typhoon Ioke made almost a direct hit on tiny Wake Island in the Pacific, passing just to the northeast of the island. The island experienced the eyewall of the left front quadrant of the super typhoon, but probably missed the calm of the eye. Observations from Wake showed winds of 78 mph gusting to 100 mph and a pressure of 934 mb before the instrument failed at 2:18am EDT this morning. Ioke continues to be a borderline Category 4/5 super typhoon, and is not expected to decline to category 3 strength for several more days. This would probably make it the longest-lived Category 4 or higher storm on record anywhere.

African tropical waves
The large spiral of low clouds near 18N 50W surrounded by a large cloud of dry air and African dust continues to spin, and now has some thunderstorm activity near the center. This thunderstorm activity developed last night and stayed on through the morning, an indication that the dry air surrounding this system is starting to dilute. As the environment continues to moisten, this system will have the potential for development. Development would likely not happen until Sunday at the earliest, when it should be near the northern Lesser Antilles Islands.

A tropical wave near 12N 37W, a few hundred miles southwest of the Cape Verde Islands, has changed little in organization the past day. Some slow development of this system is possible over the next few days, but doesn't appear likely.

A spinning area of clouds few hundred miles north of Puerto Rico is associated with an upper level low pressure system. Development is not expected of this system.

I'll be back this afternoon with an update. Tonight, I'll be talking live at 8:45pm EDT on the Barometer Bob show, live from northeast Florida:

http://www.barometerbobshow.com/

Jeff Masters

Updated: 02:52 PM GMT on august 31, 2006

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Ernesto, John, Ioke

By: JeffMasters, 09:02 PM GMT on august 30, 2006

Tropical Depression Ernesto steamed north across Florida today, crossing over Lake Okeechobee, headed for an exit from the Florida coast north of Cape Canaveral. The storm has maintained its integrity, as seen in satellite animations and radar imagery. Although Ernesto's winds have dropped below tropical storm force, its central pressure has stayed about 1002 mb, only a 1 mb rise from when it made landfall. The considerable blow-up of heavy thunderstorm this afternoon over Ernesto's center is due to the normal daytime increase in thunderstorm activity due to solar heating of the Florida landmass. Rainfall amounts over Florida have generally been below 4 inches, and Ernesto has not been much of a problem for the state.

Once Ernesto re-emerges into the Atlantic early Thursday morning, it will re-intensify over water. None of the forecast models or the official NHC forecast are calling for this to become a hurricane, though. The passage over Florida has weakened it to the point where it would take more time over water than Ernesto will have. It is possible that Ernesto will intensify very little, as happened when it popped off the coast of Cuba. The most likely intensity at its second landfall in South Carolina is 40-55 mph.

Hurricane John
Hurricane John strengthened into a very dangerous Category 4 hurricane this afternoon, and is already bringing tropical storm force winds to the Mexican coast. John is expected to move parallel to the coast over the next two days, but close enough to bring hurricane force winds to the coast at times. Any slight deviation towards the coast will bring the hurricane's dangerous core ashore, and would make John one of the strongest hurricanes ever to hit the Pacific coast of Mexico. The area of the coast that pokes out farthest into the ocean, just south of Manzanillo, is at highest risk of a strike, and the latest 12Z (8am EDT) runs of both the GFDL and NOGAPS models are calling for a strike here. Wind shear is light and forecast to remain low, and sea surface temperatures (Figure 1) are a very warm 30 C under the hurricane--about 1-2 degrees C above normal for this time of year.


Figure 1. Current sea surface temperatures along the Pacific coast. Temperatures in the Gulf of California may not be accurate, due to difficulties retrieving the temperature via satellite measurements in such a narrow body of water. The red line separating blue colors from yellow marks the 26 C isotherm--the critical temperature needed to sustain a tropical cyclone. Note the very cool waters extending from the California border southwards along the coast. This long stretch of cool water will make it difficult for John to hold together if it tries to approach California. Image credit: NOAA/AOML.

Is John a threat to the U.S.?
Eastern Pacific hurricanes are most likely to impact the U.S. in El Nino years, where the ocean along the Mexican coast heats up to much above normal temperatures and can fuel very intense hurricanes. This was the case in 1997, when Category 5 Hurricane Linda, the most intense hurricane ever observed in the Eastern Pacific, moved parallel to the coast and threatened California. The National Hurricane Center issued several advisories for Linda alerting San Diego to the possibility of receiving tropical storm force winds from Linda. The storm turned out to sea before reaching California, however.

This is not an El Nino year, but recent warming of the waters in the Eastern Pacific has led some El Nino experts wondering if a late-arriving El Nino might be on the way. Water temperatures along the Pacific coast of Mexico are 1-2 degrees C above normal all the way to the California coast, giving 2006 the possibility of allowing a tropical storm to reach California. It is very rare for an Eastern Pacific storm to move far enough north to affect the Arizona or California. Since 1900, only four tropical cyclones have brought tropical storm force winds to the Southwestern United States: an unnamed tropical storm that made landfall near Long Beach, CA, in 1939 (52 mph winds south of L.A.); the remnants of Hurricane Joanne in 1972; the remnants of Hurricane Kathleen in 1976 (76 mph wind gust at Yuma, AZ); and the remnants of Hurricane Nora in 1997. In addition, a hurricane just missed making landfall in October 1858 and brought hurricane force winds to San Diego and tropical storm force winds all the way to Los Angeles.

In order to affect California, a tropical cyclone would have to be moving quickly, so the the cold waters off the coast would not weaken it too fast. The alternative would be for the storm to barrel up the narrow Gulf of California, where water temperatures remain warm all the way to the end. To my knowledge, no such storm has ever been able to shoot more than half way up the narrow Gulf of California before dashing itself to pieces on the rugged terrain on either side. I'd be surprised if John manages to bring tropical storm force winds to the U.S.

Super Typhoon Ioke
The incredible Category 4 Supertyphoon Ioke continues to rumble towards tiny Wake Island in the Pacific. The entire population of the island has been evacuated to Hawaii. NOAA's National Ocean Service has a station on Wake Island, and the current conditions at 3pm EDT were sustained winds of 34 mph gusting to 44 mph, and the pressure at 999 mb and falling rapidly. It is expected that Ioke's storm surge may completely submerge the island. Ioke has a significant wave height of 50 feet, meaning that 1/3 of the waves in the storm are higher than that. Wow!

African tropical waves
A tropical wave near 12N 36W, a few hundred miles southwest of the Cape Verde Islands, has changed little in organization today. Some slow development of this system is possible over the next few days.

The large spiral of low clouds near 18N 46W surrounded by a large cloud of dry air and African dust continues to spin, but the thunderstorm activity near the center remains near zero this afternoon. The wave has some potential for development if it can find a moister environment. This is not likely until Sunday at the earliest, when it may be near Puerto Rico or the Bahamas.

I'll be back Friday morning with a new update.

Updated: 09:06 PM GMT on august 30, 2006

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Ernesto spares Florida, heads for the Carolinas

By: JeffMasters, 01:57 PM GMT on august 30, 2006

Tropical Storm Ernesto waited until the final hours before landfall to finally put its act together, much to the benefit of South Florida. The pressure dropped from 1005 to 1001 mb as the storm came ashore about midnight, but the winds did not have time to adjust to the lower pressure, and Ernesto still had just 45 mph winds at landfall. A tropical storm in the developing phase is a fussy thing, and a number of ingredients have to come together just right for rapid intensification. I believe that the presence of Cuba to the south and the Florida Peninsula to the north, along with the particular pattern of upper air flow that existed, combined to create a turbulent air pattern with multiple vortices that made consolidation of the storm around just one central vortex difficult. One could see these multiple vortices in long radar loops last night, and it was not until just before landfall that Ernesto managed to consolidate around a single center and start to intensify. Had the storm had another 24 hours over the warm waters, it would have been a hurricane.


Figure 1.Total precipitation from the Miami radar.

So today, residents of Florida should be feeling good. There will be some heavy rains moving through periodically, but flooding should be minor. So far, Ernesto has dumped rain amounts less than four inches. Winds are too low to do any damage, but the windsurfers in South Florida get an unexpected boon. Virginia Key, one mile from downtown Miami, had sustained winds of 33 mph, gusting to 41 mph this morning at 8am. The Rickenbacker Causeway connecting Virginia Key to downtown Miami is a popular windsurfing spot I've spent many days windsurfing at, and I'm sure the windsurfers out out in force today to take advantage of Ernestos's unexpectedly modest winds. Sustained winds at other offshore buoys were generally in the 20-25 mph, with a peak wind gust at 50 mph seen at Vaca Key in the past hour.

Ernesto's forecast track
There is no change to the forecast. Ernesto is maintaining a well-organized appearance on radar animations, and should only slowly weaken during its 1-day long plus passage up the length of Florida. The storm will re-emerge into the Atlantic early Thursday morning and re-intensify over water. If the storm stays close to shore and makes landfall in South Carolina, it will probably come ashore Thursday night as a tropical storm. If the storm moves more offshore and makes landfall near the Outer Banks of North Carolina, it has extra time over warmer water, and will have a chance to be a Category 1 hurricane. The chances of Ernesto becoming a hurricane are a little less than it appeared yesterday, since it made landfall in Florida as a weaker than expected storm.

Hurricane John
The most serious situation in the tropics today is off the west coast of Mexico, where Category 3 Hurricane John is. John has just completed an eyewall replacement cycle, and is expected to intensify into a Category 4 hurricane today. The Air Force Hurricane Hunters are in the storm now, since it presents a serious threat to the coast of Mexico from Acapulco to Manzanillo.

Super Typhoon Ioke
The incredible Category 5 Spertyphoon Ioke continues to trek over the Western Pacific, and is expected to submerge tiny Wake Island later today. The entire population of the island has been eveacuated to Hawaii.

African tropical waves
An tropical wave near 12N 34W, a few hundred miles southwest of the Cape Verde Islands, has a pronounced surface circulation one can see in QuikSCAT satellite data from 3:52am EDT this morning. The amount of heavy thunderstorm activity has decreased since yesterday, and any development should be slow to occur. The thunderstorm activity associated with this wave was being enhanced yesterday by a process known as upper-level divergence. When the the winds at high levels diverge (blow outward from a common center), then air from the surface must rise to fill the vacuum created. As this surface air rises, the moisture in it condenses, fueling thunderstorms. Thunderstorms created by this mechanism make a tropical disturbance look more impressive than it really is. The wave has moved away from this area of upper divergence, reducing the amount of thunderstorm activity.

The large spiral of low clouds near 18N 45W surrounded by a large cloud of dry air and African dust continues to spin, but the thunderstorm activity near the center has dropped to nearly nil this morning. The wave is under a very favorable upper-level environment for development, with low shear and an upper-level anticyclone on top. However, water vapor satellite imagery shows that the air on all sides is very dry. The GFS model is showing that this wave will not be able to find a moister environment until Sunday at the earliest, when it may be near Puerto Rico or the Bahamas.

I'll be back late this afternoon with an update on Ernesto, John, Ioke, and the rest of the tropics.

Jeff Masters

Updated: 02:18 PM GMT on august 30, 2006

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Time's almost up for Ernesto

By: JeffMasters, 08:34 PM GMT on august 29, 2006

The latest Hurricane Hunter report from 3:17 pm continues to indicate that Ernesto is not strengthening. The surface pressure remains steady at 1005 mb, and the maximum sustained surface winds steady at 45 mph. Ernesto's appearance on satellite imagery is still improving, with upper-level outflow being established to the north, and the size and depth of heavy thunderstorm activity increasing.
Key West radar animations show an increase in organization and intensity of the spiral rain bands. Ernesto still has until midnight to intensify, and should come ashore in the Everglades tonight with peak winds of 60 mph or lower.

Marathon in the Keys has had several heavy squalls go through, and reported a peak wind gust of 32 mph so far this afternoon. Top winds at some of the offshore buoys have been 35 mph, gusting to 40 mph.

Forecast track
There is no change to the forecast for Florida, landfall will be tonight near midnight in the Everglades, followed by a 1-day long plus passage up the length of Florida, followed by a re-emergence into the Atlantic on Wednesday night or Thursday morning. Ernesto will re-intensify over water. If the storm stays close to shore and makes landfall in South Carolina, it will probably come ashore Thursday night as a tropical storm. This is the solution of the GFDL model. If the storm moves more offshore and makes landfall near the Outer Banks of North Carolina, it has extra time over warmer water, and will probably be a Category 1 hurricane. This is the solution of the UKMET model and Canadian model. The GFS and NOGAPS models have a solution in between. The models differ markedly in what direction Ernesto will go after landfall, with several models taking Ernesto up the coast into New England, and several northwest into Ohio.

Spectacular imagery
Since yesterday, the GOES-East satellite has been in rapid scan mode where it delivers one satellite image per minute. For those of you on high-bandwidth connections, the animation of these images available at http://hadar.cira.colostate.edu/ramsdis/online/RSOgeflt.html (Colorado State University) is most impressive.


Figure 1. Visible satellite image from 2:30pm EDT 8/29/06 over the Atlantic. Image credit: Navy Research Lab.

Ignore the wave I said to pay attention to, and pay a little attention to the wave I said to ignore--and the wave I didn't even mention
The concentrated area of thunderstorms I mentioned this morning, halfway between Africa and the Lesser Antilles Islands, near 9N 36W, has fallen apart. I no longer expect development of this system. However, another tropical wave to its east, at 12N 28W, has developed a very pronounced rotation this afternoon, and has some heavy thunderstorm activity on its south side. This wave has potential for some slow development over the next few days as it moves west at 20 mph. It's under 15 knots of wind shear. The thunderstorm activity associated with this wave is being enhanced by a process known as upper-level divergence. When the the winds at high levels diverge (blow outward from a common center), then air from the surface must rise to fill the vacuum created. As this surface air rises, the moisture in it condenses, fueling thunderstorms. Thunderstorms created by this mechanism make a tropical disturbance look more impressive than it really is. The upper divergence will go away over the next 24 hours, reducing the amount of thunderstorm activity the wave has.

The large spiral of low clouds near 18N 43W that I said to ignore this morning may be worth watching yet, although I don't think it will ever develop. A small burst of heavy thunderstorms developed on the east side of the center of circulation this afternoon, and the wave is under low enough wind shear for development, 10 knots. However, water vapor satellite imagery shows this wave embedded in a large area of dry air and African dust that should preclude development for at least the next two days.

I may have a short update tonight in Ernesto puts on a burst of intensification.

Jeff Masters

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Ernesto moves into the Florida Keys

By: JeffMasters, 04:08 PM GMT on august 29, 2006

The Hurricane Hunter reports from late this morning indicate that Ernesto's pressure remains steady at 1005 mb, and the maximum sustained surface winds steady at 45 mph. Ernesto's appearance on satellite imagery is much improved, with upper-level outflow being established to the north, and the size and depth of heavy thunderstorm activity increasing. Key West radar animations show an increase in organization and intensity of the spiral rain bands, and it won't be long before the Hurricane Hunters observe a falling pressure and rising winds. We are very lucky this storm does not have another day over water, or it would be near Category 2 hurricane status. Ernesto has only about 12 more hours over water, and should come ashore in the Everglades tonight with peak winds of 60 mph.

Marathon in the Keys had a heavy squall go through at 11:09am EDT, and reported a wind gust of 27 mph. Winds at the offshore buoy on Sombrero Key near Marathon were sustained at 35 mph, gusting to 40 mph at 11am this morning. Winds will continue to rise in the Keys this afternoon, and a 1-3 foot storm surge can be expected.

The latest forecast models are still in excellent agreement, calling for a landfall in the Everglades tonight, a long 2-day passage up the length of Florida, followed by a re-emergence into the Atlantic and possible re-intensification to a Category 1 hurricane before a second landfall in the Carolinas. Ernesto should then slowly push inland over the mid-Atlantic states, and may create inland flooding problems due to its slow movement northwards.


Figure 1. A tropical wave to watch, and one to ignore--visible satellite image from 9:45am EDT over the Atlantic.

New tropical wave to watch
A concentrated area of thunderstorms has developed about halfway between Africa and the Lesser Antilles Islands, near 9N 36W. This tropical wave is showing some rotation, and had winds up to 60 mph in some of the heavier thunderstorms, according to a 4:15 am EDT pass from the QuikSCAT satellite. The GFS model does develop this system into a tropical storm by Friday, and predicts it will be a threat to the Lesser Antilles Islands early next week. This wave is very similar in position and appearance to the wave that spawned Ernesto. Wind shear is about 10-15 knots over the wave, which is low enough to allow some slow development. Sea Surface Temperatures are 84-86F (29-30 C), which is very favorable for development. However, the intensity of the heavy thunderstorm activity has declined markedly this morning, and the wave is not a threat to develop until Thursday at the earliest. A large spiral of low clouds to the wave's north-northwest is associated with another tropical wave. This wave we can ignore, since it is embedded in a large area of African dust that should inhibit development. The computer models are very bullish in developing waves coming off the coast of Africa in the next two weeks, and I expect we'll have at least two new named storms by the time the peak of hurricane season arrives, September 10.

I'll be back with an update by 4:30 pm EDT today, and include some thoughts on the anniversary of Katrina.

Jeff Masters

Updated: 04:18 PM GMT on august 29, 2006

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Ernesto slowly strengthing, headed towards the Keys

By: JeffMasters, 12:03 PM GMT on august 29, 2006

A quick update on Ernesto--the latest Hurricane Hunter mission at 6:30am EDT found only 45 mph winds at the surface, and a 1005 mb pressure. The Hurricane Hunters earned their money today, as they reported moderate turbulence during many of their legs. This often happens in intensifying storms right next to land, as the air flowing over the land to water is very turbulent. Ernesto is slowly strengthening, but probably only has enough time to make it to a 60 mph tropical storm before landfall on the mainland Florida Peninsula in Everglades National Park. Satellite imagery shows a slowly expanding area of intense thunderstorms near the center, and some decent upper-level outflow on the east side. There is dry air on the storm's west side, and this is being pumped into Ernesto by a small upper-level low to is west. This influence should wane today as the upper low weakens and moves off.

You can see some ill-defined spiral bands on the Key West radar, but the storm's low-level organization is poor.

The latest forecast models are all in excellent agreement, calling for a landfall in the Everglades tonight, a long passage up the spine of Florida, followed by a re-emergence into the Atlantic and possible re-intensification to a Category 1 hurricane before a second landfall in the Carolinas.


Figure 1. Tropical Storm Ernesto at 1:45 pm EDT Monday August 28, as presented by the NOAA Visualization Program.

Next update
I'll have an update by 12 pm EDT, and will discuss what might come after Ernesto. There is a new tropical wave in the mid-Atlantic that has some potential for development.

Jeff Masters

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Ernesto headed for Florida

By: JeffMasters, 09:16 PM GMT on august 28, 2006

Ernesto continues its trek across Cuba, and has dumped over 8 inches so far at Guantanamo. No reports of injuries or damage have come out of Cuba thus far, and Ernesto's top winds have only been 40 mph for the duration of its trek. The storm has maintained a large envelope of spinning clouds and moisture around its center, and is already starting to fire up some heavy thunderstorms north of Cuba as its center prepares to emerge into the Florida Straits. However, the system is very disorganized at present and will not be able to strengthen quickly once it moves over water.

The 12Z (8am EDT) model runs are all in, and they don't show any significant changes to the track or expected intensity of Ernesto for its Florida landfall. Ernesto is expected to pop off the Cuban coast tonight, cross the Florida Straits, and make landfall somewhere between Miami and Naples. The SHIPS intensity model is calling for Ernesto to be a tropical storm with 55 mph winds at landfall, while the GFDL model thinks Ernesto will be a Category 1 hurricane with 75-80 mph winds. The GFDL is probably too strong, since its 6 hour forecast verifying at 2pm today predicted 60 mph winds, and the real winds were only 40 mph. With only 18-36 hours over water, Ernesto's winds will most likely be in the 50-75 mph range at landfall. My best guess is for a very wet tropical storm with maximum winds of 60 mph hitting the Everglades.

The models' expectations of what might happen to the Carolinas later in the week are very different, since small changes in the forecast track put the storm over land where it weakens, or over the warm Gulf Stream, where it can intensify dramatically. The worst case model is the GFDL, which brings a strong Category 2 hurricane to the South Carolina coast Thursday night. The best case scenario is a total miss, with the storm going out to sea, as depicted by the Canadian model (although it's unclear if the storm might circle back and strike the coast next week).

Finally, my summary of the computer models and where to find them
I've put together a detailed desciption of the computer models we provide on the wunderground.com computer model plot for each tropical storm. A brief description of the important models is given, along with a web site to get the data from. I've linked this description page under the "My links" section at the right of my blog, and it is also available on the tropical page. Here's a condensed version:

Types of hurricane forecasting models
The best hurricane forecasting models we have are "global" models, that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called "dynamical" models. The four best hurricane forecast models--GFDL, GFS, UKMET, and NOGAPS--are all global dynamical models. These models take several hours to run on the world's most advanced supercomputers.

Non-global models
The BAMM model (Beta and advection model, medium layer) is included on wunderground.com's computer model page. The BAMM is a simple trajectory model that is very fast to run, and did the best of any individual model at 3-5 day track forecasts in 2005. Since this model is always available, we have included it along with the "big four". In general, one should not trust the BAMM model for the 1-2 day time period when output from "the big four" are available. "The big four" are generally not available for tropical disturbances, and for these situations we post plots of a number of other non-global models such as the LBAR, A98E, etc. All of these models are described in detail on NHC's web site.

Model performance in 2005
The National Hurricane Center issues annual verification reports comparing model performance to the official NHC forecast. The 2005 report found that the official NHC forecast was usually the best forecast, but was closely matched by taking an average of the "big four" models to come up with a consensus forecast. There are several techniques used to come up with these consensus model forecasts. The three best techniques are called the GUNA, CONU, and Florida State University Superensemble (FSSE). The FSSE model was developed by FSU with funding from the private company Weather Predict, and is not available to the public. The performance of the "big four", official forecast, and consensus models are plotted below.

Among individual track models, the GFDL did the best at 1-2 day forecasts, and the UKMET and BAMM (not shown in the plot) did very well at 4-5 day forecasts. For intensity, the SHIPS model (which we post in the lower right corner of the wunderground.com computer model forecast image) was the best performer. The SHIPS model is run using input from the GFS model.


Figure 1. Forecast performance in 2005, compared to a simple "Climatological and Persistence' (CLIPER) model. OFCL=Official NHC forecast. The "big four" global dynamical models are the GFDL, GFS, NOGAPS, and UKM (UKMET). Three methods of averaging the "big four" and coming up with a consensus model forecast are CONU, GUNA, and FSSE (Florida State Super Ensemble). The Official forecast and the three consensus forecasts did the best at all time periods. Among individual models, the BAMM model(not shown) did the best at the 3-5 day range, followed closely by the UKMET. The GFDL did the best in the 1-2 day range. Image credit: National Hurricane Center.

Next update
I'll be live on Internet Partnership Radio (www.ipr365.com), formerly Radio NHCWX, tonight at 8:30 pm EDT. I'll be talking with host Mike Watkins about Ernesto. My next blog will be Tuesday morning.

Jeff Masters

Updated: 09:30 PM GMT on august 28, 2006

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Ernesto ashore in Cuba; Florida and the Carolinas next

By: JeffMasters, 01:22 PM GMT on august 28, 2006

The hurricane season of 2006 has claimed its first victim, a woman washed away by Ernesto's storm surge in Haiti yesterday. However, it appears that we have not had a repeat of the massive flooding disaster of 2004, when Hurricane Jeanne brushed Haiti, killing 3000. Ernesto is still pounding Haiti and the Dominican Republic with relentless heavy rains as it moves ashore into Cuba this morning. Rainfall amounts of 8 inches have been reported in Barahona, Dominican Republic, and amounts as high as 20 inches may have fallen in Haiti. Cuba is receiving its own torrential rains, but is being spared a significant storm surge or high winds. Ernesto is barely a tropical storm, and I expect it will be a tropical depression Tuesday morning when it pops off the coast of Cuba into the Florida Straits. Given the great success of Hispaniola and Cuba at weakening the storm, the threat to Florida is now much reduced. Let's analyze the possibilities.


Figure 1. Surface wind analysis at 9:30pm EDT last night shows Ernesto had maximum winds of only 40 knots (45 mph) in a small circle (lightest blue color) between Cuba and Haiti.

Ernesto and Florida
The NOAA jet flew last night, and last night's 00Z (8pm EDT) set of model runs have come into much better agreement on a track into South Florida Tuesday night or Wednesday morning. Two model runs are available from the latest 6Z (2am EDT) model cycle, and continue the trend of pushing the forecast tracks to the east. The GFS and GFDL models have virtually identical tracks for their 72-hour forecasts, with Ernesto as a Category 1 hurricane with 75 mph winds and a pressure of 983 mb when it hits Key Largo Tuesday night or early Wednesday morning. This more easterly track is believable given Ernesto's recent northwesterly track.

Wind shear remains low and is forecast to remain low for the duration of Ernesto's life, so the key factor controlling his intensity will be interaction with land. To get an idea of how quickly Ernesto might reintensify once emerging into the Florida Straits, it is helpful to search the historical record. I found two storms similar to Ernesto that crossed the tip of Haiti, hit Cuba as hurricanes, then emerged into the Florida Straits. Hurricane Inez of 1966 hit Cuba as a Category 3 hurricane, then weakened to a tropical storm when she emerged into the Florida Straits. It took three days before she was able to re-intensify into a Category 2 hurricane in the Gulf. The August 1894 Category 2 hurricane that hit Cuba took only one day to re-intensify from a tropical storm into a Category 2 hurricane once it emerged into the Florida Straits. This storm hit Ft. Myers as a Category 2 storm. Dissipation of Erensto is also possible, as happened to a 1916 tropical storm that hit Cuba on a similar track and died as it emerged into the Florida Straits. A 1928 tropical storm following a similar path never regained hurricane strength, despite spending two full days over the Gulf after encountering Cuba. So, history is against Ernesto becoming anything stronger than a Category 1 hurricane upon landfall in South Florida, and I believe landfall as a tropical storm is more likely. If Ernesto does take a more westerly track up the west coast of Florida towards Sarasota, landfall as a Category 1 hurricane could occur.

Ernesto is a more significant threat to the Carolinas
While much of the focus of attention has deservedly been on Ernesto's impact on Florida, I believe the best chance of Ernesto hitting the U.S. as a Category 2 or 3 hurricane will come in the Carolinas. The GFDL model has Ernesto as a borderline Category 1 or 2 hurricane with a pressure of 975 mb Thursday night upon landfall in South Carolina. The GFS, UKMET, and Canadian models predict that Ernesto will stall off the Carolina coast, as the trough of low pressure drawing it northeastwards accelerates away. High pressure will then build in, forcing Ernesto back to the west towards the Carolina coast. If this happens, Ernesto will have plenty of time over the warm Gulf Stream, and could easily reach Category 2 or 3 strength before making landfall in the Carolinas. The NOGAPS model depicts a similar scenario, but predicts Ernest will stall further north, then move west, threatening the Mid-Atlantic states.

Next update
I'll have an update late this afternoon when the 12Z (8am EDT) model runs are in. I will also post an explanation of which models to trust, and where to get plots of the output from the various models. The Hurricane Hunters have gone home; they will not be flying the storm while it is over Cuba, so there will be nothing new to report from them.

Jeff Masters

Updated: 01:46 PM GMT on august 28, 2006

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Haiti weakens Ernesto

By: JeffMasters, 08:13 PM GMT on august 27, 2006

Ernesto is has been clobbering Hispanolia all morning with tropical storm force winds and torrential rains. But now, the island has bitten back. Ernesto is struggling to hold his eye together, thanks to the mountainous terrain on the southwestern peninsula of Haiti. The 1:10pm and 3pm EDT Hurricane Hunter eye reports found a surface pressure estimated at 1007 mb, a big increase from this morning's 995 mb. Moreover, the eye was substantially tilted, with the calm at the surface about 25 miles south of the calm at the 10,000 foot flight level. The plane could find maximum winds of only 35 mph during that 2-hour period, so Ernesto will probably be downgraded to a tropical storm with the 5pm advisory. The center of the storm is just south of some very mountainous terrain, and this is significantly disrupting Ernesto. The upper level outflow still looks strong, wind shear is weak, and an upper-level anticyclone (clockwise-rotating region of winds) is still in place, so Ernesto will no doubt reorganize tonight once he moves away from Haiti. However, given Ernesto's small size and the difficultly he is having with Hispanolia, there is hope that the expected 1-2 day traverse of Cuba will significantly weaken him. It may take Ernesto a day or two to regain hurricane strength once he emerges into the Florida Straits. This bodes well for the Florida Keys, which may dodge another hurricane. I think that only if Ernesto makes landfall north of Tampa will he have time to organize into a major hurricane.

The Keys evacuate
Monroe County emergency management is taking no chances, and has ordered a mandatory evacuation of all visitors and non-residents in the Florida Keys beginning at 1 pm this afternoon. A local state of emergency has been declared by Monroe County at noon today. All County and state Parks are encouraged to close this afternoon. At 10 am Monday, an evacuation for all mobile home residents will go in effect.


Figure 1. A view of the southwestern peninsula of Haiti, looking towards Cuba. Note the mountainous terrain and totally denuded of trees. Deforestation in Haiti has claimed over 99% of all the forests. The lack of forest cover to absorb Ernesto's torrential rains will cause deadly floods throughout southern Haiti today and Monday. Image credit: Google Earth.

The intensity forecast
There is no major change to the intensity forecast. As long as Ernesto is not interacting with Cuba or Hispanolia, he should strengthen. A low-shear environment with an upper-level anticyclone (clockwise rotating region of winds) has developed on top of the storm. These conditions are highly favorable for intensification. Ernesto is over waters of 30-31 C (86-88 F), and these waters warm up to nearly 90F in the narrow channel between Jamaica and Cuba. These warm waters extend to great depth, and the total heat content available to fuel rapid intensification is high. However, given the sharp decrease in intensity of Ernesto, thanks to its interaction with Hispanolia, I no longer expect Ernesto will have enough time to reorganize and attain Category 2 status for its Cuba landfall. Expect a Category 1 hurricane.

The track forecast
The 12Z (8am EDT) models are in, and continue to show Florida will get a double hit--first the Keys, then the main Peninsula. The GFDL model is the furthest west, favoring a hit in the Panhandle as a Category 3 hurricane. The GFS is the farthest east, calling for a hit on Key Largo as a tropical storm. The other models are in between. At this point, there is no reason to disqualify any of these model solutions as unreasonable. It still appears that New Orleans can breathe easy, as Ernesto should miss that city by a wide margin. However, residents of the U.S. southeast coast need to be prepared for tropical storm conditions if Ernesto crosses Florida and then re-intensifies off the Southeast coast. Several of the models indicate a track curving along the Southeast Coast just offshore, which may bring tropical storm conditions to Georgia, South Carolina, and North Carolina. Ernesto is not a threat to New England.

The storm surge forecast
Ernesto should not generate as high of storm surge in the Keys as Hurricane Wilma did last year. Storm surge heights should be four feet or less. Cuba will block the formation of the mound of storm surge water that Ernesto is piling up today and Monday, and the storm will have to generate a new mound of storm surge water once it crosses into the Florida Straits. North of the Keys, any hit along the west coast of Florida as a Category 2 or higher storm will generate a 10 foot or higher storm surge, due to the shallow waters extending out for a great distance from the coast.

Next update
The Hurricane Hunters will be in Ernesto until about 5pm EDT this afternoon. The next mission is at 8pm EDT tonight. The NOAA jet is scheduled to fly tonight as well, so we'll have our first set of higher-reliability model runs Monday morning. The NOAA P-3 gets its first action Monday morning, and will fly their SFMR instrument that measures surface winds over the entire area. I may have a short update tonight if there's a significant change to report, otherwise I'll see you in the morning.

Jeff Masters

Updated: 08:15 PM GMT on august 27, 2006

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Hurricane Ernesto smashes Haiti; Cuba and Florida next

By: JeffMasters, 12:49 PM GMT on august 27, 2006

Ernesto is now a hurricane, the first hurricane of the 2006 season. This first hurricane of season appears likely to do something the Hurricane Season of 2005 can't boast of--get its name retired. Ernesto is delivering a deadly blow to Haiti, and Cuba and Florida are next in line. The 6am Hurricane Hunter eye report found surface winds of 75 mph, good enough for upgrading the storm to a hurricane. The pressure fall has only been an unspectacular 2 mb in the past 12 hours, but the improvement in satellite appearance has been spectacular. An eye is now clearly visible on infrared satellite imagery, and upper-level outflow channels have opened to the north and south. Wind shear has fallen to 5-10 knots, and an upper-level anticyclone (clockwise-rotating region of winds) has developed over Ernesto, a highly favorable situation for strengthening.

The eye of Ernesto will pass just south of or over the southwestern tip of Haiti today, pounding that impoverished nation with hurricane force winds and rains of up to 20 inches. I expect the death toll will be in the hundreds.


Figure 1. Tropical Cyclone Heat Potential for the Caribbean. Value of over 90 kJ/m^2 are commonly associated with rapid deepening phases of hurricanes.

The intensity forecast
The upper level winds have calmed down significantly in the past 12 hours, and the low-shear environment with an upper-level anticyclone is expected to remain for the duration of Ernesto's life. Furthermore, Ernesto is over waters of 30-31 C (86-88 F), and these waters warm up to nearly 90F in the narrow channel between Jamaica and Cuba. These warm waters extend to great depth, and the total heat content available to fuel rapid intensification is high. In fact, the highest heat content waters anywhere in the Atlantic, 120 kJ/cm^2, are found here. Anything over 90 kJ/cm^2 are considered high enough to fuel rapid intensification, and I expect Ernesto will be a Category 2 hurricane when it hits Cuba.

Once Ernesto encounters Cuba, it should go down at least one Category in strength. The eastern end of Cuba is very rugged and will interfere with the storm's circulation. Exactly how long Ernesto spends over Cuba will be critical in determining what kind of blow the Florida Keys will receive. Ernesto will most likely emerge from Cuba into the Florida Straights as a tropical storm. The extremely warm waters with high heat content in the Florida Straights should fuel rapid intensification once more, after a 12-hour reorganization period. I expect the order for visitors to evacuate the Lower Keys will be given this afternoon, the 7th evacuation order for the Keys in the past 3 years. I think it is unlikely Ernesto could affect the Keys as anything stronger than a Category 2 hurricane with 105 mph winds. A hit as a tropical storm or Category 1 hurricane is more likely. If Ernesto spends another day or two traversing the warm waters along the west cost of Florida, then it could grow to a major Category 3 or 4 hurricane. Probably the best scenario would be to have Ernesto pop off Cuba and proceed straight north to the Everglades, spending very little time over water. This might allow Ernesto to affect Florida only as a tropical storm.

The track forecast
The models have come into better alignment now. They unanimously predict a stronger trough of low pressure than originally forecast will act to pull Ernesto northwards across Florida, and then northeastwards out to sea. The exact highest risk Florida landfall is difficult to pin down so far in advance, and everywhere from Miami to Pensacola is at risk. It appears now that New Orleans can breathe easy, as Ernesto should miss that city by a wide margin. Residents of North Carolina should be alert, as Ernesto may brush the Outer Banks after traversing Florida.

The storm surge forecast
The waters along the west coast of Florida are very shallow, and extend out far into the Gulf of Mexico. This creates an ideal environment for a large storm surge to build, and storm surge heights over 10 feet are likely if Ernesto comes ashore as a Category 2 or stronger hurricane along the west coast of Florida. If Ernesto takes a track parallel to the coast and just offshore, a large storm surge could affect a very long portion of the Florida coast, causing immense damage.

Next update
The hurricane hunters have left Ernesto, and the next Hurricane Hunter mission into Ernesto is at 2pm EDT this afternoon. The NOAA jet is scheduled to fly tonight, so we'll have our first set of higher-reliability model runs Monday morning. The NOAA P-3 gets its first action Monday morning, and will fly their SFMR instrument that measures surface winds over the entire area. I'll have an update this afternoon when new model runs and Hurricane Hunter information becomes available.

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Jeff Masters

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Ernesto jumps towards Hispanolia

By: JeffMasters, 02:19 AM GMT on august 27, 2006

The Hurricane Hunters left Ernesto at 7:30pm EDT this evening, and found that the center had jumped about 50 miles east-northeast of the previous center. This sort of center reformation is common in strengthening tropical storms subject to significant wind shear, and we should not be surprised if another center shift occurs before the shear finally starts to decrease Sunday afternoon.

The new center position is bad news for Haiti and the Dominican Republic, which will now be subject to torrential flooding rains. Considerable loss of life may occur in Haiti, where deforestation will allow flood waters to rampage unchecked down the barren hillsides of the nation's rugged terrain. The new center position is also bad news for Cuba, whose entire length will now get a pounding from Ernesto.


Current IR satellite image of Ernesto.

The new center position is good news for the U.S., since Ernesto's closer approach to Cuba and Hispanolia will keep the storm weaker than it otherwise would have been. In the past few hours, the cloud pattern on infrared satellite imagery has gotten more fragmented, which may be due to storm starting to struggle due to interaction with the island of Hispanolia. It could also be a temporary adjustment due to reorganization of the storm around the new center, or an increase in wind shear, which is still a substantial 15-20 knots.

The forecast
The new set of model runs show more agreement that Ernesto will make it to the Gulf Coast by Friday as a hurricane, possibly a major hurricane. All four major global models are forecasting a landfall between Louisiana and Florida by the end of the week, so the threat to Texas appears lower now. It is still possible that Ernesto could stall shortly before landfall and assume an unpredictable path, however. The GFS model's solution of bringing Ernesto over Key West and inland farther north near Tampa appears unrealistic, due to the failure of the model to put the surface center in the same place as the center at mid levels of the atmosphere.

Next update
The next Hurricane Hunter mission into Ernesto is at 2am EDT Sunday morning. I'll have an update in the morning. I apologize for my late update tonight; a thunderstorm took out my Internet service all afternoon and evening.

Jeff Masters

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Updated: 02:23 AM GMT on august 27, 2006

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Ernesto slowly strengthening

By: JeffMasters, 03:20 PM GMT on august 26, 2006

The Hurricane Hunters found a strengthening Tropical Storm Ernesto this morning. Their final report at 5:30am EDT had a pressure of 997mb, good enough for 50 mph winds at the surface. Water vapor satellite loops show a large upper level low forming over the western Caribbean, and this low is moving westward at nearly the same speed as Ernesto. Counterclockwise flow of air around the upper low is causing 15-20 knots of shear over Ernesto, and this can be seen in the water vapor loop and visible satellite loop causing a flattening of the clouds on the west side of the storm. However, the overall appearance of Ernesto on satellite imagery is slowly improving, with upper-level outflow developing on the east side, and more low-level spiral bands forming. Long range radar animations from Puerto Rico show these spiral bands. Rain amount of 1-3 inches have fallen across the island so far today, with isolated totals as high as 8 inches. As long as Ernesto does not grow any closer to the upper level low to his west, slow intensification should occur the remainder of today.

As Ernesto passes south of Haiti today, the mountainous southwestern peninsula could experience life-threatening flash floods. This is common in this part of Haiti, even for modest tropical storms. Deforestation of the hillsides has left them little protection from floodwaters.


Figure 1. Current storm total rain in Puerto Rico.

The intensity forecast
The latest 06 GMT (2am EDT) model runs are in, and they again portray a conflicting picture of what may happen. Two of the four major global models--UKMET and GFS--dissipate or severely weaken the storm by Monday. However, the other two major models, the NOGAPS and GFDL, foresee a major hurricane in the Gulf that hits the Florida Panhandle on Friday. The Canadian model takes a major hurricane to the Louisiana coast on Friday. It appears that the two models poo-pooing Ernesto--the UKMET and GFS--are not doing a good job initializing the storm, and this is causing an overly optimistic weakening. Ernesto is holding his own again the shear, and is much stronger today than these models were predicting in their runs yesterday. The NHC official forecast of a continued slow intensification into a Category 1 hurricane by Monday seems like a reasonable one. After that, an explosive deepening phase into a major Category 3 or 4 hurricane is certainly plausible, given the very warm waters and low wind shear forecast to develop. However, keep in mind that our computer models are very poor at forecasting the behavior of upper-level lows in the tropics, and the upper low to the west of Ernesto could easily be in a different location and bring significant shear to the system. In addition, intensity forecasts more than 48 hours out are very unreliable. Witness some of the intensity forecasts for Debby made two and three days ago, which called for her to be a Category 1 or 2 hurricane today. Today, Debby is a tropical depression on the verge of dissipation. With all that said, I believe that Ernesto will be a major hurricane in the Gulf of Mexico at some point.

Jamaica, the Cayman Islands, and Cuba are going to get a pounding from Ernesto as he approaches hurricane strength Sunday and Monday. Washed out roads, downed power lines and trees, and minor roof damage will be common in those islands. Ernesto is under too much shear and does not have enough time to intensify into more than a strong tropical storm or weak Category 1 hurricane before affecting those islands, and the full force of Ernesto is likely to be reserved for the U.S. Gulf Coast. Those of you with plans to be in Cancun or Cozumel can expect a day or so of airport closures due to high winds between Monday night and Tuesday night.

The track forecast
As Ernesto crosses into the Gulf of Mexico on Tuesday, a trough of low pressure will be swinging across the eastern U.S. and should pull the storm on a more northerly track. Most of the models are showing that this trough will be strong enough to bring the storm all the way to the coast between Louisiana and the Florida Panhandle. However, the trough may not be strong enough to do this, and Ernesto could get stuck in the Gulf for a week, potentially heading westwards towards Texas as a new ridge of high pressure builds in. A subsequent trough could then turn the storm northwards into the coast at some later time. The UKMET model and GFS model prefer this solution. At this point, there is not enough information to say which solution is most likely, and residents from Texas to the Florida Keys need to be prepared for this storm to affect them.

The next system?
A large tropical wave with plenty of spin came off the coast of Africa yesterday is is currently over the Cape Verde Islands. Some of the computer models forecast that this wave will develop, but this is very unlikely for at least the next three days. The Saharan Air Layer analysis is showing a large amount of dry air spiraling into the center of this wave. There are currently no other threat areas to be concerned with. In the long range, the GFS is forecasting a very busy first week of September, with two or more tropical storms developing between Africa and the Lesser Antilles.

Hurricane Ioke
Hurricane Ioke in the Central Pacific underwent an eyewall replacement cycle and is now a Category 4 hurricane with 150 mph winds today. However, now that this cycle is complete, the storm should attain Category 5 status again today. It's not a threat to any land areas, but if it remains a Category 4 hurricane or higher the next four days, as forecast models are calling for it to do, it might set a world record for the longest-lived Category 4 or higher storm. The 6Z GFDL model run has Ioke at 875mb in 4 days, the lowest forecast pressure ever made by the GFDL model. However, bear in mind that the GFDL model in the Central Pacific is not being run with its full coupled ocean model, so these extreme forecasts of Ioke's intensification are probably overdone. The GFDL is not the only model having trouble with Ioke--the 11pm discussion last night from the Honolulu Hurricane Center noted:
Big issue with the track guidance for the 06z model suite is that the UKMET track bounced off the date line...resulting in an erroneous mirror image track...eastbound back into the central Pacific. This threw all of the consensus model tracks off...bending them back sharply eastward.

Our tracking map software had a similar problem, and we're using some older software that doesn't have that problem right now for Ioke.


Figure 2. Visible satellite image of Category 5 Hurricane Ioke, 2030 GMT 8/25/06. Image credit: Navy research Lab.

Next update
The next Hurricane Hunter mission into Ernesto is at 2pm EDT this afternoon. I'll have an update sometime between 3-7pm today.

Jeff Masters

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Ernesto gets its name

By: JeffMasters, 09:18 PM GMT on august 25, 2006

The Hurricane Hunters visited Tropical Depression Five this afternoon, and at 1:30pm EDT found a surface circulation center and maximum sustained surface winds of 40 mph, making this Tropical Storm Ernesto. Visible satellite images from this afternoon show a a sheared system, with the low level circulation center completely exposed. Upper level winds from the northwest are creating 10-20 knots of shear, which is keeping all the heavy thunderstorm activity confined to the southeast (downwind) side. However, this heavy thunderstorm activity is building towards the center, and the storm has some solid spiral bands forming. The storm is intensifying in the face of the shear. Radar from Aruba shows some of the outer spiral bands.


Figure 1. Current satellite image of Ernesto.

The models
The latest 12Z (8am EDT) model runs are in, and they portray a conflicting picture of what may happen. Three of the four major global models--UKMET, GFS, and NOGAPS--dissipate or severely weaken the storm by Monday. The Canadian model, which has been the most aggressive in making Ernesto a hurricane in the Gulf of Mexico, now keeps the system a weak tropical storm all the way to landfall in the Florida Panhandle on Tuesday. The European Center model has a similar forecast. Oddly, the large upper-level trough of low pressure over the central Caribbean is forecast by these models to move west away from Ernesto, and an upper level high pressure system to build on top of the storm. This situation should act to lower the shear and aid in intensification, and that it what the official NHC forecast is calling for. It is unusual for the models to forecast a favorable shear environment, yet dissipate a storm, and serves to show the limitations of these models in making hurricane intensity forecasts. I have very little confidence in any of the Ernesto intensity forecasts--including the official NHC forecast of a hurricane in the Gulf of Mexico on Tuesday. High wind shear could still destroy this storm on Sunday. Ernesto should slowly intensify through Saturday, but beyond that, I have no idea. It's best to prepare for the worst, and hope for the best. All portions the U.S. coast from the Florida Keys to Brownsville, Texas are at risk from this storm.

Given the high degree of uncertainty in Ernesto's intensity, those of you traveling to or from Jamaica, Cuba, the Cayman Islands, and Cancun should play "wait and see" as long as you can. At this point, it is likely that Jamaica, eastern Cuba, and southwest Haiti will get tropical storm conditions on Sunday. Jamaica's airports will probably close about 2am Sunday, and remain closed into late Sunday night. Ernesto probably does not have enough time to intensify to a hurricane before reaching Jamaica. Airports in the Cayman Islands will probably close by late morning on Sunday, and reopen Monday afternoon. Ernesto could be a Category 1 hurricane for the Caymans.

Hurricane Ioke
Hurricane Ioke in the Central Pacific has reached Category 5 status with 160 mph winds today. Ioke is the first hurricane in the Central Pacific since 2002. It's not a threat to any land areas, but is most impressive on satellite imagery. The 12Z GFDL has Ioke at 880mb in 5 days, the lowest forecast pressure I've seen from the GFDL model.


Figure 2. Visible satellite image of Category 5 Hurricane Ioke, 2030 GMT, 8/25/06. Image credit: Navy Research Lab.

Max Mayfield to step down
Max Mayfield announced today that he is stepping down as head of the National Hurricane Center at the end of the year, according to press reports from the Miami Herald and Florida Sun Sentinel. Mayfield, 57, admitted that the last two hurricane seasons wore him out. Ed Rappaport, the center's deputy director and a veteran forecaster may be next in line for the job, after Max steps down on January 3. Max will be missed--his expert guidance of NHC has no doubt saved many lives, and I will miss his calm and intelligent presence at the helm of NHC.

Next update
The next Hurricane Hunter mission into Ernesto is at 2am EDT Saturday morning. My next update will be Saturday morning between 9-11am, or earlier if there's something major to comment on. I don't have my model summary piece completed yet, but will do so as soon as I get it done.

New JeffMasters blog for dial-up users
There is new blog site for those of you suffering on slow dial-up connections, or for those of you who don't want to see the comments:

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Jeff Masters

Updated: 09:30 PM GMT on august 25, 2006

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Max Mayfield to retire as head of NHC

By: JeffMasters, 06:22 PM GMT on august 25, 2006

Max Mayfield announced today that he is stepping down as head of the National Hurricane Center at the end of the year, acccording to press reports from the Miami Herald and Florida Sun Sentinel. Mayfield, 57, admitted that the last two hurricane seasons wore him out. Ed Rappaport, the center's deputy director and a veteran forecaster may be next in line for the job, after Max steps down on January 3. Max's expert guidance of NHC has no doubt saved many lives, and I will miss his calm and intelligent presence at the helm of NHC.


Max Mayfield at the 2006 American Meteorological Society meeting, extolling what he wants engraved on his tombstone: "Don't look at the center track forecast line! Look at the cone of possible center locations."

I'll have an update on TD 5 between 4pm and 5pm EDT.

Jeff Masters

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Ernesto is on the way

By: JeffMasters, 01:42 PM GMT on august 25, 2006

Tropical Depression Five is looking much more organized this morning, and will likely be named Ernesto with the NHC 11am EDT advisory. Satellite intensity estimates from NOAA using a the standard "Dvorak technique" are the same for Debby and TD 5 this morning. Since Debby has a name, Ernesto should getting his name, as well. The visible satellite images from this morning show a large and expanding area of intense thunderstorms, some solid spiral bands forming, and the beginnings of some decent upper-level outflow. A QuikSCAT satellite pass at 7:40am EDT showed winds of up to 50 mph near the center, and I imagine this will be the maximum sustained wind speeds for the 11am advisory.


Figure 1. Current satellite image of TD 5.

Wind shear
As usual, the major question is, "what is wind shear doing, and what is the wind shear forecast?" My top hurricane bookmark is the real-time wind shear analysis done by the University of Wisconsin's CIMSS group. This image, updated every 3 hours from data taken by the GOES-East satellite, shows yellow contour lines of what the shear is, in knots. Pink arrows show what direction the shear is coming from. The shear is the difference between the winds at high altitude (200 mb) and low altitude (850 mb). Hurricanes like wind shear close to zero, but can tolerate shear up to 20 knots and still slowly intensify. As we can see from the winds shear plot from this morning at 9Z (5am EDT) (Figure 2), Tropical Storm Five was under about 10 knots of shear, from the west. This is low enough to allow some intensification, and that is what we are seeing this morning. However, Tropical Storm Debby was under about 20 knots of shear, and has not been able to intensify this morning.


Figure 2. Wind shear estimate at 9Z (5am EDT) Fri Aug 25 2006. Image credit: University of Wisconsin's CIMSS group.

To the west of TD 5 we see a large area of very high shear of up to 50 knots, south of Jamaica. This area of high shear is associated with a strong upper level trough of low pressure that has been a common feature over the Caribbean this season. The trough is moving westward, away from TD 5, but TD 5 is racing fast enough westward that it may catch up to this high shear area and get torn apart on Sunday. The 8pm UKMET and 2am GFS models both have TD 5 dissipating by Sunday, and this is reasonable forecast. However, the trough is forecast to split apart on Monday and leave a region of low shear very favorable for intensification of TD 5 in the western Caribbean. If the storm does manage to survive into Monday, we could end up with a dangerous hurricane in the Gulf of Mexico by the middle of next week. The latest runs of the GFDL and Canadian models prefer this solution, bringing the storm across Cuba and into the eastern Gulf of Mexico as a hurricane. This is also a reasonable forecast. If you have travel plans to Jamaica, Cuba, or Cancun in the coming days, I would wait as long as possible to change them until we see if TD 5 will survive.

Debby
Tropical Storm Debby remains a minimal tropical storm today, with 20 knots of vertical wind shear putting a damper on her intensification. She could slowly intensify in response to some warmer waters along her path the next two days. By the time Debby turns north this weekend, she could attain hurricane status, but this is looking unlikely now. Early next week, Debby is expected to get caught up in the jet stream and die in the North Atlantic.

New African wave
Several of the computer models are predicting that the tropical wave coming off the coast of Africa today will develop into a tropical storm early next week.

Next update
The next Hurricane Hunter mission into TD 5 is at 2pm EDT this afternoon, and I'll have an update when the reports from their mission come in. Also, I'm getting a lot of questions about which models are best, where to find model output, etc, and I will try to post a quick summary of this info this afternoon.

Jeff Masters

Updated: 01:45 PM GMT on august 25, 2006

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Tropical Depression Five forms

By: JeffMasters, 07:55 PM GMT on august 24, 2006

The Hurricane Hunters found a closed circulation and surface winds of 35 mph in the tropical wave moving through the Lesser Antilles, so this system is now Tropical Depression Five. Here's the 3:30pm EDT statement from NHC:

Reports from an Air Force reconnaissance plane indicate that the tropical wave moving westward through the Windward Islands has developed a closed wind circulation...and advisories on either a tropical depression or tropical storm will be initiated at 5 PM AST. Maximum winds at this time appear to be just below tropical storm strength...but it is possible that stronger winds may be observed prior to advisory time.

The new center that developed 100 miles north of the South American coast this morning is beginning to consolidate, with a new line of intense thunderstorms developing to the south of the center. This new spiral band joins the older spiral band to the east, which has brought wind gusts as high as 51 mph to the Islands today. It is clear that this will be Tropical Storm Ernesto in short order.

Current conditions in the islands
Barbados reported sustained winds of 32 mph at 5am this morning, and wind gusts as high as 51 mph. Togabo had wind gusts to 36 mph, and sustained winds of 33 mph with wind gusts to 43 mph were observed on St. Lucia. A QuikSCAT satellite pass from 6:30am EDT shows a large area of 35 mph winds to the north of the center. Radar from Martinique shows the heavy bands of rain spreading over the islands. Winds on Martinique have gusted to 35 mph so far today.

Wind shear
Upper level winds out of the west are creating about 5-10 knots of wind shear over the center, which is not significantly inhibiting development. The shear is forecast to remain low through the next five days. There is a zone of very high shear to the system's north, but it is forecast to retreat to the west ahead of the developing storm. The evolution of the shear pattern over the past 24 hours has matched what the GFS has said would happen, so this gives me confidence that the shear forecast is correct, at least for the next day. This system does have the potential to become a hurricane by early next week.

The computer models
The latest 8am EDT (12Z) computer model runs were all initialized with the old center position, and thus are unreliable. We'll have to wait until the 18Z (2pm EDT) model runs are available late tonight before we can put much stock in any of the computer model solutions. With this in mind, here is my what the latest 12Z computer models say:

The Canadian model continues to be very consistent, and develops Ernesto into a hurricane south of Jamaica, that then tracks into the Gulf of Mexico. The NOGAPS model is also consistent, assuming a more southerly track will occur with little development due to close proximity to the South American coast. This forecast is already incorrect, and can be discounted. The GFS takes a weak tropical storm across the Dominican Republic on Saturday, then into the Bahamas. The GFDL takes a strong tropical storm into Haiti on Sunday, then on a long path over eastern Cuba. On Tuesday, the GFDL has the system emerging from the coast of Cuba as a weak tropical storm and passing through the Florida Keys. The run-to-run consistency of the GFDL has been poor, and both the GFDL and GFS have not done a good job forecasting the initial track of the storm so far.


Figure 1. Current satellite image of TD 5.


Figure 2. Preliminary model tracks for TD 5.

Debby
Tropical Storm Debby remains a minimal tropical storm today, but is expected to slowly intensify in response to some warmer waters along her path the next two days. By the time Debby turns north this weekend, she could attain hurricane status. Early next week, Debby is expected to get caught up in the jet stream and die in the North Atlantic.

My next update will probably be Friday morning.

Jeff Masters

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Caribbean wave develops a new circulation

By: JeffMasters, 04:23 PM GMT on august 24, 2006

A powerful tropical wave that has the potential to become a serious hurricane is sweeping through the southernmost Lesser Antilles Islands today, bringing tropical storm force wind gusts and heavy rains. An important development has occurred in the past two hours--a new circulation center developed near 12.5N 63W, about 100 miles north-northwest of the original center near the South American coast. This new center lies between St. Vincent and Grenada, and southwest winds observed last hour in Grenada confirms that a closed circulation now exists at the surface. The old center near the South American coast now looks likely to dissipate. Inflow of warm, moist air into its center was too restricted by the presence of the South American land mass, and thus a new center farther north along the axis of this tropical wave was able to form and take over.

Current conditions in the islands
Barbados reported sustained winds of 32 mph at 5am this morning, and wind gusts as high as 51 mph. Togabo had wind gusts to 36 mph, and sustained winds of 33 mph with wind gusts to 43 mph were observed on St. Lucia. A QuikSCAT satellite pass from 6:30am EDT shows a large area of 35 mph winds to the north of the center. Radar from Martinique shows the heavy bands of rain spreading over the islands. Winds on Martinique have gusted to 35 mph so far today.

Wind shear
What happens in the next few hours in crucial in determining if we have a serious hurricane to worry about in a few days, or just a another harmless tropical blob. The storm is very vulnerable to wind shear right now as it reorganizes. The center of circulation is almost completely exposed, with just one spiral band of heavy thunderstorms connected to the northeast side of the center. Upper level winds out of the west are creating about 10 knots of wind shear over the center, keeping the band of thunderstorms pushed to the downwind side of the center. If the shear can increase a little this afternoon, it may disrupt the storm enough to keep it from developing today. The new center location also puts the storm closer to the large area of dry air and Saharan dust that covers much of the eastern Caribbean. This may also help disrupt the storm.

However, I think 97L will overcome these obstacles. Wind shear is probably low enough to allow the storm to reform, and I wouldn't be surprised to see the center jump again more to the east, to be underneath the strongest thunderstorms. The shear is forecast to remain low through the next five days. There is a zone of very high shear to the system's north, but it is forecast to retreat to the west ahead of the developing storm.

The computer models
The latest 8pm and 2am EDT computer model runs were all initialized with the old center position, and thus are unreliable. The new set of model runs using data from 12 GMT (8am EDT) are not going to be any better, since the new center formed about 2 hours later. We'll have to wait until the 18 GMT (2pm EDT) model runs are available late tonight before we can put much stock in any of the computer model solutions. With this in mind, here is my summary from my previous blog about what the latest computer models say:

The Canadian model continues to be very consistent and very gung-ho, developing 97L into a strong tropical storm on Saturday, south of Jamaica, then taking the storm into the Gulf of Mexico as a hurricane. The NOGAPS model is also consistent, assuming a more southerly track will occur with no development due to close proximity to the South American coast. The GFS takes a weak tropical storm across the Dominican Republic on Saturday, then into the Bahamas. The GFDL has the same idea, but has a much stronger system that becomes a Category 1 hurricane in the Bahamas on Monday. The run-to-run consistency of the GFDL has been poor, and both the GFDL and GFS have not done a good job forecasting the initial track of the storm so far.

What the new center means for this storm
The separation of its center from the coast removes the primary impediment to intensification for 97L. It looks more likely that this storm will develop into at least a strong tropical storm, and probably a hurricane. The track such a hurricane might take is highly uncertain, but the more northerly center increases the risk for Haiti, the Dominican Republic, the Bahamas, and the U.S. East Coast.


Figure 1. Current satellite image of Invest 97L.


Figure 2. Preliminary model tracks for disturbance 97L approaching the Lesser Antilles Islands.

Debby
Tropical Storm Debby remains a minimal tropical storm today, but is expected to slowly intensify in response to some warmer waters along her path the next two days. By the time Debby turns north this weekend, she could attain hurricane status. Early next week, Debby is expected to get caught up in the jet stream and die in the North Atlantic.

Why the 2am run of the GFDL failed
The 2am EDT run of the GFDL model failed on 97L this morning, so I was quoting the results from the 8pm EDT run last night. I got an email from Morris Bender of the GFDL project this morning on why the 2am GFDL run failed:

The vortex initialized from our initialization was very weak as the initialization process spins up the storm to match the observed initial winds. Since the easterlies in the lower level were very strong in the GFS analysis, it initialized a disturbance with very weak vorticity.

As you can see at hour 0 there was almost no circulation initially. As a result, our grid movement shut down at 3 hours and the inner nests could no longer follow the storm for the rest of the forecast.

The previous runs also had a very weak initial disturbance but there was enough of a pressure gradient that the inner nests continued to follow the vortex and eventually with the high resolution it developed into a significant tropical cyclone.

In the 6z run, the weak disturbance moved out of the high resolution inner grid, into the coarse resolution, and so all we had left was a very weak disturbance that could not be resolved in the coarse outer mesh.


As you can see, getting the computer models to work on weak disturbances is a difficult business! We should not put too much faith in the computer models for any weak system; it is too difficult for the models to get the starting conditions of the storm correct.

I'll have an update this afternoon after the Hurricane Hunters check out 97L.

Jeff Masters

Updated: 05:55 PM GMT on august 24, 2006

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Strong tropical wave blows into the Lesser Antilles

By: JeffMasters, 01:29 PM GMT on august 24, 2006

A powerful tropical wave that has the potential to become a serious hurricane is sweeping through the southernmost Lesser Antilles Islands today, bringing tropical storm force wind gusts and heavy rains. Barbados reported sustained winds of 32 mph at 5am this morning. Togabo had wind gusts to 36 mph, and wind gusts to 43 mph were observed on St. Lucia. A QuikSCAT satellite pass from 6:30am EDT shows a large area of 35 mph winds to the north of the center, which appears to be near 11N 59W, near the islands of Trinidad and Tobago and just off the South American coast. The storm's organization has steadily increased since yesterday, and the Hurricane Hunters are tasked to investigate this afternoon to see if a tropical depression has formed. The storm has two main areas of intense thunderstorms visible on satellite imagery this morning that are almost disconnected. I expect the northern mass will dissipate today, since it is much farther from the center of circulation.

Wind shear is favorable in a small area over the storm--5-10 knots--and is forecast to remain low through the next five days. However, there is a zone of very high shear to the system's north, so the forecast of low shear could easily change. A large area of dry air and Saharan dust covers much of the eastern Caribbean, and may be a modest impediment to intensification. The primary difficulty for the storm lies in its close proximity to South America. The storm center may hug the coast through Saturday, limiting its development. The storm should bring heavy rains and winds near tropical storm force over the ABC Islands of Aruba, Bonaire, and Curacao on Friday. After that, there is a lot of uncertainty.

The computer models
The latest 8pm and 2am EDT computer model runs have a variety of solutions. The Canadian model continues to be very consistent and very gung-ho, developing 97L into a strong tropical storm on Saturday, south of Jamaica, then taking the storm into the Gulf of Mexico as a hurricane. The NOGAPS model is also consistent, assuming a more southerly track will occur with no development due to close proximity to the South American coast. The GFS takes a weak tropical storm across the Dominican Republic on Saturday, then into the Bahamas. The GFDL has the same idea, but has a much stronger system that becomes a Category 1 hurricane in the Bahamas on Monday.

The run-to-run consistency of the GFDL has been poor, and both the GFDL and GFS have not done a good job forecasting the initial track of the storm so far. A more southerly track betwen Jamaica and Honduras like the Canadian and NOGAPS models are suggesting is probably more reasonable. If 97L can survive the next two days and separate from the South American coast--which it has at least a 50/50 chance of doing--I believe it will probably develop into a serious hurricane, as the GFDL and Canadian models have been suggesting. The track such a hurricane might take is highly uncertain, but it appears that Jamaica, Cuba, and the Yucatan would be at highest risk in the Caribbean. No part of the U.S. coast can be ruled out as a target in the longer term.

Got travel plans to the Caribbean this week? Don't change them yet. This appears to be an all-or-nothing kind of situation, and we could get nothing. It may not be until Saturday that we have a reasonable idea if this storm will be a major threat.


Figure 1. Current satellite image of Invest 97L.


Figure 2. Preliminary model tracks for disturbance 97L approaching the Lesser Antilles Islands.

Debby
Tropical Storm Debby remains a minimal tropical storm today, but is expected to slowly intensify in response to some warmer waters along her path the next two days. By the time Debby turns north this weekend, she could attain hurricane status. Early next week, Debby is expected to get caught up in the jet stream and die in the North Atlantic.

I'll have an update this afternoon after the Hurricane Hunters check out 97L.

Why the 2am run of the GFDL failed
The 2am EDT run of the GFDL model failed on 97L this morning, so I was quoting the results from the 8pm EDT run last night. I got an email from Morris Bender of the GFDL project this morning on why the 2am GFDL run failed:

The vortex initialized from our initialization was very weak as the initializaiton process spins up the storm to match the observed initial winds. Since the easterlies in the lower level were very strong in the GFS analysis, it initialized a disturbance with very weak vorticity.

As you can see at hour 0 there was almost no circulation intially. As a result, our grid movement shut down at 3 hours and the inner nests could no longer follow the storm for the rest of the forecast.

The previous runs also had a very weak initial disturbance but there was enough of a pressure gradient that the inner nests continued to follow the vortex and eventually with the high resolution it developed into a signficant tropical cyclone.

In the 6z run, the weak distrubance moved out of the high resolution inner grid, into the coarse resolution, and so all we had left was a very weak disturbance that could not be resolved in the corase outer mesh.


As you can see, getting the computer models to work on weak disturbances is a difficult business! We should not put too much faith in the computer models for any weak system; it is too difficult for the models to get the starting conditions of the storm correct.

Jeff Masters

Updated: 01:53 PM GMT on august 24, 2006

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Distubance approaching the Caribbean grows stronger

By: JeffMasters, 08:21 PM GMT on august 23, 2006

A tropical wave near 11N 55W, about 400 miles east of the southernmost Lesser Antilles Islands, is moving westward towards the Caribbean at about 20 mph. Thunderstorm activity has increased markedly today in association with this wave (labeled 97L by NHC), and it has the potential to develop into a tropical depression by Thursday. The Hurricane Hunters are tasked to investigate it on Thursday afternoon. The thunderstorm activity is still pretty disorganized, and mostly lies to the west of the center of circulation, so noon Thursday is the earliest it is likely to reach tropical depression status.

Wind shear is favorable, a low 5-10 knots, but is a very high 30-40 knots just to the north, and any movement of this high shear zone to the south--or 97L to the north--might disrupt it. A large area of dry air and Saharan dust lies to the north, and this has slowed development today. The storm will spread heavy rains and wind gusts to 40 mph across Trinidad, Tobago, Grenada, and the South American coast of Venezuela on Thursday, and the ABC Islands of Aruba, Bonaire, and Curacao on Friday. Development into a tropical depression--or a tropical storm--is highly dependent on how close 97L comes to the coast of Venezuela. The southeast Caribbean off the Venezuelan coast has been climatologically unfriendly to developing tropical storms. The presence of the South American land mass so close cuts off a key source of moisture for a developing storm, and many vigorous looking disturbances and tropical storms have died here.

The computer models' take on things
The latest 8am EDT computer model runs are in. The Canadian model continues to be gung-ho, developing 97L into a strong tropical storm on Saturday, south of Jamaica. The NOGAPS model assumes a more southerly track will occur, and develops 97L Saturday once it moves off the coast of Columbia. NOGAPS then takes a weak storm along the north coast of Honduras, then across the Yucatan and into the Gulf of Mexico. The GFS assumes the storm will move further north than is reasonable, and does not develop it. I think the GFS solution can be discounted. The UKMET takes the storm along the north coast of South America, and does not develop it.

My take on things
I believe there's about a 60% chance 97L will become a tropical storm. It should come very close to developing into a tropical depression Thursday afternoon or evening. However, its close proximity to South America will probably keep it somewhere between a near-tropical depression and a 45-mph tropical storm until Saturday. If it survives into Saturday, intensification into a strong tropical storm or hurricane in the western Caribbean is possible. There may be additional obstacles to overcome by then--such as too much wind shear, or the Yucatan Peninsula or Honduras getting in its way.


Figure 1. Current satellite image of Invest 97L.


Figure 2. Preliminary model tracks for disturbance 97L approaching the Lesser Antilles Islands.

Debby
Tropical Storm Debby has deteriorated a bit this afternoon, thanks to dry air and cooler waters. Debby is expected to turn more northwestward over the weekend, and get pulled northwards and recurved into the prevailing westerly winds at high latitudes over cold waters early next week. Debby is not a threat to any land areas. The storm will be in a more favorable environment for intensification on Friday, and could eventually make it to Category 2 hurricane status, as predicted by the GFDL model.

I'll have an update Thursday morning.

Jeff Masters

Updated: 08:26 PM GMT on august 23, 2006

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Tropical wave approaching the Caribbean a threat; Debby gets her name

By: JeffMasters, 01:52 PM GMT on august 23, 2006

Forget about newly-named Tropical Storm Debby, now churning west-northwestward into oblivion in the open Atlantic. The area we need to focus on today is a tropical wave near 10N 53W, about 500 miles east of the southernmost Lesser Antilles Islands. This new wave is a threat to develop into a tropical depression later this week, once it crosses into the Caribbean. NHC has assigned this disturbance the name "Invest 97L", and has tentatively tasked the Hurricane Hunters to investigate it on Thursday afternoon.

While the wave does have the potential to eventually become a serious hurricane, it also has a number of hurdles to overcome, and it is more likely that it will never become a hurricane. Firstly, while satellite imagery does show some rotation at middle levels of the atmosphere, a pass by the QuikSCAT satellite at 5:11am EDT today showed no rotation at the surface, and just a slight wind shift. The system will have to develop a lot more spin, which will take time. There have been some intermittent bursts of heavy thunderstorm activity, which generated winds of up to 30 mph seen on the recent QuikSCAT pass, but the cloud pattern is very disorganized at present. Wind shear is favorable, a low 5-10 knots, but is a very high 30-40 knots just to the north, and any movement of this high shear zone to the south--or 97L to the north--might disrupt it. A large area of dry air and Saharan dust lies to the north, and this will interfere with development. The system should move through the Lesser Antilles Islands just north of the South American coast on Thursday and Friday, and this region of the Caribbean has been climatologically unfriendly to developing tropical storms. The presence of the South American land mass so close cuts off a key source of moisture for a developing storm, and many vigorous looking disturbances and tropical storms have died in the southeastern Caribbean.

The computer models forecast that wind shear over the south half of the Caribbean will remain low the rest of the week, so the further south 97L can stay, the more likely it is to develop. The 8pm EDT run of the GFDL last night did develop it into a tropical storm by tomorrow, but this model did a poor job with the track and took it too far north. The GFDL, UKMET, and NOGAPS models do not develop it. The Canadian model develops it early next week once it enters the central Caribbean south of Jamaica, and forecasts it will move into the Gulf of Mexico. If this system is going to develop, I suspect that the Canadian model has the right idea, and development will not occur until 97L reaches the central or western Caribbean.


Figure 1. Current satellite image of Invest 97L.


Figure 2. Preliminary model tracks for disturbance 97L approaching the Lesser Antilles Islands.

Debby
Tropical Storm Debby got a name last night, and is a modest 45-mph tropical storm headed west-northwest towards the north central Atlantic. Debby is expected to turn more northwestward over the weekend, and get pulled northwards and recurved into the prevailing westerly winds at high latitudes over cold waters early next week. Debby is not a threat to any land areas. The storm is in a moderately favorable environment for intensification, and could eventually make it to Category 1 hurricane status.

I'll have an update this afternoon.

Jeff Masters

Updated: 02:00 PM GMT on august 23, 2006

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Tropical Depression Four dusts the Cape Verdes; new threat approaching the Caribbean

By: JeffMasters, 08:57 PM GMT on august 22, 2006

Tropical Depression Four has increased its organization and is close to tropical storm status. It has made what is likely to be its closest pass to any land area--a brush of the Cape Verde Islands off the coast of Africa. The storm is moving towards the west-northwest away from these islands today, towards the open Atlantic. Mindelo reported sustained winds as high as 27 mph today, but no rain--just widespread dust. Other than pulling a lot of dust over these islands, the effect of the storm was minimal. The storm will provide a good case study for the NASA African Monsoon Multidisciplinary Analyses Project, which has several weather research aircraft based in the Cape Verdes Islands right now. The project aims to examine the formation and evolution of tropical hurricanes in the eastern and central Atlantic and their impact on the U.S. east coast, and the composition and structure of the Saharan Air Layer, and whether aerosols affect cloud precipitation and influence cyclone development.

TD Four is in a moderately favorable environment for intensification, and should be a tropical storm by morning. Wind shear is a low 5-10 knots. I expect this will be a Category 1 hurricane by Sunday, but it should stay far away from land. All of the models now have the storm recurving northwards well east of Bermuda early next week, as a trough of low pressure picks up the storm.

Figure 1. Visible satellite image from 2:30pm EDT Tue Aug 22 2006. TD Four is on the right of the image, just off the coast of Africa, and tropical disturbance 97L is in the left side of the image. A long line of cumulus clouds and thunderstorms nearly connects the two systems. Image credit: Navy Research Lab.

New threat approaching the Lesser Antilles
A tropical wave near 10N 47W has developed some rotation at mid levels, and is a threat to develop into a tropical depression later this week. NHC has assigned this disturbance the name "Invest 97L", and has tentatively tasked the Hurricane Hunters to investigate it on Thursday afternoon. There have been some intermittent bursts of heavy thunderstorm activity this afternoon near the center of circulation, but the cloud pattern is very disorganized at present. Wind shear is about 10-15 knots, the waters underneath are warm, and the wind shear is forecast to remain low for the next week along the system's path. The main impediment to development in the next two days will be the large area of dry air and Saharan dust to the wave's north. The system should move through the Lesser Antilles Islands on Thursday. The GFS and NOGAPS models hint at development in the Central Caribbean by Saturday, and a possible threat to Jamaica, Cuba, and Hispanolia by Sunday. We'll have to watch this system carefully, it has the potential to be trouble.


Figure 2. Preliminary model tracks for disturbance 97L approaching the Lesser Antilles Islands.

An area of thunderstorms in the western Caribbean near Cuba is drifting northwestwards into the Gulf of Mexico. Wind shear is a high 20 knots over this area, and upper level winds are not favorable for development.

I'll have an update in the morning.

Jeff Masters

Updated: 08:58 PM GMT on august 22, 2006

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Tropical Depression Four whips the Cape Verde Islands

By: JeffMasters, 12:49 PM GMT on august 22, 2006

Tropical Depression Four is making what is likely to be its closest pass to any land areas during its life--a brush of the Cape Verde Islands off the coast of Africa. The storm should get no closer than about 100 miles from the islands today, and its outer rain bands could bring 2-4 inches of rain and winds gusts to 50 mph to these islands today. The only station that reports hourly observations in these islands is Sal, which has seen the winds increase to a steady 17 mph so far this morning. Winds measured from the QuikSCAT satellite in a 4am EDT pass this morning were about 35 mph in the islands.

TD Four is a favorable environment for intensification, and should be Tropical Storm Debby later today. Wind shear is a low 5-10 knots. There is a large area of dry air and Saharan dust to the wave's north, but this is far enough away that it shouldn't inhibit intensification over the next day or two. The GFDL intensifies the storm up to a strong Category 1 hurricane by Saturday, and the official NHC forecast is close behind.

Unless you live in the Cape Verde Islands, this storm is unlikely to affect you. As we can see from the plot of historical storm tracks of August tropical depressions that formed near the location of the current storm (Figure 1), none of these storms that have started out as far north as this storm have affected any land areas other than the Cape Verde Islands. The GFS and GFDL models have the storm recurving northwards well east of Bermuda early next week, as a trough of low pressure picks up the storm. However, it is too early to have high confidence in this forecast. The UKMET, NOGAPS, and Canadian models all predict that this trough of low pressure will miss TD 4, which will continue on a more westerly path and possibly threaten Bermuda. The models have not done a great job with the track of this storm, which was 70 miles further south than the models predicted it would be last night.


Figure 1. Historical tracks of August tropical depressions that tracked near the current position of TD 4.

Elsewhere in the tropical Atlantic
An area of thunderstorms in the western Caribbean west of Jamaica is drifting northwestwards towards the Yucatan Channel and Gulf of Mexico. Wind shear is a high 20 knots over this area, and upper level winds are not favorable for development. The shear is expected to drop over the Gulf by Thursday, so we'll have to keep an eye on this system later this week. None of the computer models develop anything in the Gulf or Caribbean this week, though. The long-range GFS model is no longer predicting any new tropical storms forming anywhere in the Atlantic in the next 10 days.

I'll have an update on this system when it becomes Debby.
Jeff Masters

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Tropical Depression Four arrives

By: JeffMasters, 09:10 PM GMT on august 21, 2006

The strong tropical wave that moved off the coast of Africa Sunday has become large and well-organized enough to be classified as Tropical Depression Four. The depression is located southeast of the Cape Verde Islands off the coast of Africa, and is expected to track northwestwards just to the south of these islands over the next two days. The waters under the wave are 27-28 C, which is warm enough to allow some modest intensification. Wind shear is a low 5-10 knots. There is a large area of dry air and Saharan dust to the wave's north, but this is far enough away that it shouldn't inhibit intensification over the next day or two, and this depression should be Tropical Debby by Tuesday afternoon. The GFDL intensifies the storm up to a strong Category 1 hurricane by Sunday, and the official NHC forecast has it close to hurricane strength by Saturday.

Unless you live in the Cape Verde Islands, this storm is unlikely to affect you. As we can see from the plot of historical storm tracks of August tropical depressions that formed near the location of the current storm (Figure 1), none of these storms have affected any land areas other than the Cape Verde Islands. The GFS model has the storm recurving northwards well east of Bermuda early next week. The Cape Verde Islands, however, are under a Tropical Storm Warning, and will get some heavy rain and high winds from this system starting Tuesday morning.


Figure 1. Historical tracks of August tropical depressions that tracked near the current position of TD 4.

Sea surface temperatures cool to below the 26.5 C threshold tropical systems prefer north of the Cape Verdes Islands, so once the system moves north of about 15 N latitude, it may weaken a bit before it crosses back over warmer waters later in the week. The dry air and Saharan dust to the north of the Cape Verde Islands may cause some trouble for the storm later this week.


Figure 2. Visible satellite image of TD 4 at 1:45pm EDT Mon Aug 21 2006. Image credit: Navy Research Lab.

Elsewhere in the tropical Atlantic
An area of thunderstorms over the Bahamas is associated with an upper level low pressure system, and development is not expected. An area of thunderstorms west of Jamaica is drifting northwards towards Cuba and South Florida, and is associated with a tropical wave. Upper level winds are not favorable for development of this area, either. The long-range GFS model is predicting a series of 2 or 3 more tropical waves will emerge off the coast of Africa over the next two weeks and intensify into tropical storms. Each of these storms eventually recurves out to sea without affecting any land areas. It is possible to have a very active hurricane season and have all (or nearly all) of the storms miss land!

Jeff Masters

Updated: 11:43 PM GMT on august 21, 2006

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Debby in the making

By: JeffMasters, 01:42 PM GMT on august 21, 2006

A very strong tropical wave moved off the coast of Africa Sunday. The NHC and Navy have a system where they officially designate a disturbance as an "invest"--something worth investigating. This storm is called 96L, and if there were hurricane hunters within range, they would fly the storm, as the system already appears to be a tropical depression. However, NHC typically hesitates to label these systems fresh off the coast of Africa depressions until they hold together for at least a day. Many such systems fall apart within their first day over water. The waters under the wave are 27-28 C, which is .5-1.5 degrees C above the 26.5 C threshold for tropical cyclone formation--not great, but good enough. Wind shear is a low 5-10 knots. There is a large area of dry air and Saharan dust to the wave's north, and this will likely be a major inhibiting factor for this wave once it moves north of 15 N latitude.


Figure 1. Visible satellite image at 8am EDT Mon Aug 21 from the European satellite. Image credit: Navy Research Lab.

The wave is impressive on satellite imagery this morning--low level spiral banding has formed, and there is a clear low-level rotation of the clouds. There is also some upper-level outflow developing on the east side. We don't have a recent QuikSCAT pass to judge the surface winds. The GFS, NOGAPS, and GFDL models develop the wave into a tropical storm; the UKMET does not. I expect that the model consensus is correct--this system will be Tropical Storm Debby by Wednesday. All the models predict that the storm will pass well north of the Lesser Antilles Islands. The storm may bring tropical storm conditions to the Cape Verde Islands Tuesday and Wednesday, as it moves west or west-northwest at 15 mph just south of the islands. Although it is too early to be certain, the Cape Verde Islands will probably be the only land areas the storm will affect. The preliminary model runs point to a track that will eventually recurve the storm out to sea before affecting any other land.

Sea surface temperatures cool to below the 26.5 C threshold tropical systems prefer north of the Cape Verdes Islands, so once the system moves north of about 15 N latutide, it may weaken. There is also a lot of dry air and Saharan dust to the north that will cause trouble for it. The GFDL model does intensify the storm to a Category 1 hurricane, then weakens it as the dry air and cooler SSTs take their toll.


Figure 2. Preliminary model tracks for the Atlantic disturbance 96L.

Elsewhere in the tropical Atlantic, there are no threat areas to discuss, and the computer models are not forecasting any development through Wednesday.

Jeff Masters

Updated: 02:09 PM GMT on august 21, 2006

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New African wave

By: JeffMasters, 01:18 AM GMT on august 21, 2006

Well, our quiet days in the Atlantic didn't last very long, as it's time to talk about a new threat area. A very vigorous tropical wave has moved off the coast of Africa today, and has the potential to develop into a tropical depression later this week. The waters under the wave are 27-28 C, which is .5-1.5 degrees C above the 26.5 C threshold for tropical cyclone formation--not great, but good enough. Wind shear is a modest 10 knots. There is a large area of dry air and Saharan dust to the wave's north, and this will likely be the major inhibiting factor for this wave.

Both the GFS and NOGAPS models develop the wave into a tropical storm; the UKMET does not. The GFS predicts the storm will pass well north of the Lesser Antilles Islands.


Figure 1. Preliminary model tracks for the new African tropical wave.

I'll have much more on this system Monday, plus a look at the rest of the tropics.

Jeff Masters

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Another quiet weekend in the tropics

By: JeffMasters, 03:11 PM GMT on august 20, 2006

It's another quiet day in the tropical Atlantic. The persistent area of low pressure off the U.S. East Coast moved ashore this morning in Georgia. An area of heavy thunderstorms south of Jamaica is associated with a westward-moving tropical wave. Wind shear is high here, 20-30 knots, and development is not expected. The Intertropical Convergence Zone (ITCZ), the area of strong thunderstorms between Africa and the Lesser Antilles Island, is moderately active today, but no real threat areas stand out within it. The GFS and NOGAPS models continue to predict that a tropical storm will develop within the ITCZ by late in the week. However, any development here will have to contend with a large area of dry air and African dust that covers the entire eastern tropical Atlantic, north of the ITCZ.

Wind shear across most of the tropical Atlantic is forecast to drop to very low levels beginning Wednesday, and I expect we'll have at least one tropical storm developing by the end of the week.

Until then, enjoy these quiet days!
Jeff Masters

Updated: 03:14 PM GMT on august 20, 2006

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Jacksonville low keeps on spinning

By: JeffMasters, 01:46 PM GMT on august 19, 2006

The persistent low pressure system about 50 miles east of Jacksonville Florida continues to kick up heavy thunderstorms and high winds over the waters off the Florida coast. Wind shear from a protuberance of the jet stream is still a very hefty 40-50 knots, and is expected to remain over 40 knots through Sunday, so development of this system into a tropical depression is not expected. However, radar animations out of Jacksonville, FL show a healthy circulation and some strong thunderstorms on the east and south sides. Wind speeds at a buoy 45 miles east-northeast of St. Augustine have been about 25 mph gusting to 30 mph this morning. A QuikSCAT satellite pass at 7:06am EDT this morning showed some wind gusts as high as 45 mph in some of the thunderstorms. The low could affect Georgia and northern Florida today and Sunday much as a tropical depression would, bringing heavy thunderstorms and gusty winds.


Figure 1. Current radar out of Jacksonville, FL.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Elsewhere in the tropics
The strong tropical wave south of the Cape Verde Islands that the GFS model had been developing into a hurricane is now pretty ordinary looking. The GFS no longer develops this wave. A good general rule for model predictions of tropical storm formation:

1) If two or more of the reliable models (GFS, NOGAPS, GFDL, UKMET) are forecasting develoment, watch out.

2) If none of the reliable models are forecasting development, watch out. The models miss most tropical storm development.

3) If just one of the reliable models is forecasting development, you can probably discount it.

That being said, we have a case where two reliable models--the GFS and NOGAPS--are forecasting that the large tropical wave that will move off Africa Sunday will develop into a tropical storm by the middle of next week. However, the wave will have to contend with a large cloud of Saharan dust which has just emerged from the coast of Africa.

Wind shear remains high over the Caribbean today, but the GFS is forecasting that this will drop significantly by Wednesday, and remain very low for the ten days following. I expect at least one tropical storm will form in the Atlantic during the next seven days. One candidate might be a weak tropical wave currently in the mid-Atlantic near 11N 40W.

Jeff Masters

Updated: 01:50 PM GMT on august 19, 2006

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"Not dead yet", says East Coast disturbance

By: JeffMasters, 01:57 PM GMT on august 18, 2006

The persistent low pressure system 100 miles east-northeast of Jacksonville Florida refuses to die, despite my attempts to declare it dead last night. Wind shear from a protuberance of the jet stream is now a very hefty 40-50 knots, but the system has retained its circulation and still has some heavy thunderstorm activity firing up on its south side. Wind speeds as noted in a QuikSCAT satellite pass at 7:32am EDT this morning show that the strongest winds remain near 25 mph. This system is definitely an oddity--it refused to develop Wednesday when conditions appeared quite favorable with low wind shear, and now refuses to die under extremely hostile wind shear. The high shear is forecast to remain or increase today, and this system has little hope of gaining tropical depression status before it moves ashore into northern Florida and/or Georgia tonight. These areas can expect some spotty heavy rains from the storm.


Figure 1. Current long range radar out of Jacksonville, FL.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Tropical wave off of Africa
A strong tropical wave moved off the coast of Africa Thursday, near 7N 16W. The QuikSCAT pass from 3:17pm EDT yesterday revealed winds of up to 60 mph to the west of the center, but the thunderstorm activity has since quieted down. We don't have a recent QuikSCAT pass to evaluate the current winds. Visible satellite animations from this morning show good rotation, and this disturbance does have some potential for develoment a few days from now. SSTs are a bit cool at 27C (81F), but wind shear has declined from 20 knots last night to about 10 knots today. The past three days worth of GFS model runs have been developing this system into a hurricane that threatens the Lesser Antilles about seven days from now. None of the other models develop the system.

Jeff Masters

Updated: 02:38 PM GMT on august 18, 2006

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Wind shear destroys Carolina disturbance

By: JeffMasters, 01:25 AM GMT on august 18, 2006

As expected, high wind shear from a protuberance of the jet stream has destroyed the circulation of the disturbance off the coast of South Carolina. The remnants of this system will move ashore into northern Florida and/or Georgia on Friday, bringing some spotty heavy rains.


Figure 1. Current long range radar out of Jacksonville, FL.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Tropical wave off of Africa
A very strong tropical wave has moved off the coast of Africa today, and is centered near 7N 16W. The QuikSCAT pass from 3:17pm EDT reaveals winds of up to 60 mph to the west of the center. The wave has a lot of rotation, but not a closed circulation. The past two days worth of GFS model runs have been developing this system into a hurricane that threatens the Lesser Antilles in about a week. None of the other models develop the system. SSTs are a bit cool at 27C (81F), and wind shear is a fairly high 20 knots, so let's see how this cool water and moderate shear affects the system overnight before talking about whether it represents a future threat.

Jeff Masters

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Carolina disturbance still near tropical depression strength

By: JeffMasters, 01:29 PM GMT on august 17, 2006

The low pressure system centered about 100 miles east-southeast of Charleston, South Carolina remains very close to tropical depression strength. Winds this morning at the buoy 47 miles southeast of Charleston, SC have been about 25 mph, gusting to 30 mph. Sustained winds of 30 mph and a closed circulation at the surface are required before NHC will classify a system as a tropical depression. The storm does have a well-defined closed circulation, but only one area of intense thunderstorms, on the south side of the circulation. Strong upper level winds from the north are creating about 20 knots of winds shear over it, and pushing all the thunderstorm activity to the south side.

Long range radar animation out of Charleston, SC shows a big blob of precipitation, but not much organization to this system--little spiral banding is evident. The appearance on satellite imagery has improved some this morning, perhaps because the system is drifting southwestward over slightly warmer waters. There is still about a six hour window for this storm to become a tropical depression. However, wind shear is expected to increase rapidly to over 100 knots by tonight, which will surely destroy the storm. The remains may brings heavy rain to Georgia, northern Florida, and South Carolina Friday, but significant damage from this system is very unlikely. The Hurricane Hunters are on the schedule to go investigate the storm this afternoon, but I expect that this mission will be cancelled.


Figure 1. Current long range radar out of Charleston, SC.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Elsewhere in the tropics
There are no other threat areas to discuss. If you missed it, my discussion of the outlook for the remainder of August was posted yesterday.

Jeff Masters

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Hurricane Hunters don't find a depression

By: JeffMasters, 09:27 PM GMT on august 16, 2006

An Air Force Hurricane Hunter aircraft investigated the low pressure system centered about 150 miles southeast of the North Carolina/South Carolina border this afternoon, but did not find winds strong enough to support calling it a tropical depression. Peak winds were only 25 mph at the surface, and winds of 30 mph are required before NHC will classify a system as a tropical depression. The storm does have a well-defined closed circulation, but satellite imagery and long range radar out of Wilmington, NC haven't shown much change in the system's organization today. With warm 84F (29C) waters underneath and wind shear of only 10 knots, I wouldn't be surprised to see this storm become a tropical depression or even a weak tropical storm by Thursday afternoon. However, the system has to act fast, since it will have a very hostile environment by Thursday night. A trough swinging off the East Coast today is expected to drag a filament of very strong jet stream winds southwards to Florida by Thursday night. These jet stream winds are forecast by both the GFS and NAM models to bring 100-150 knots of wind shear over the disturbance, which will easily tear it apart. Considering that wind shear over 20 knots is unfavorable for tropical storms, the 5pm NHC tropical weather outlook, "upper-level winds become increasingly unfavorable for development on Thursday", is a little understated!

High pressure building in behind the trough of low pressure should force the system towards the west or southwest through Thursday night. South Carolina could see some heavy rains and gusty winds Thursday night into Friday from the storm, but a storm strong enough to cause significant damage would be a major surprise.


Figure 1. Current long range radar out of Wilmington, NC.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Elsewhere in the tropics
There are no other threat areas to discuss. If you missed it, my discussion of the outlook for the remainder of August was posted in my previous blog.

Jeff Masters

Updated: 09:36 PM GMT on august 16, 2006

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August tropical outlook

By: JeffMasters, 02:28 PM GMT on august 16, 2006

A well defined surface circulation has developed about 200 miles southeast of the North Carolina/South Carolina border this morning. Heavy thunderstorm activity is limited to the south side of this system, due to 10 knots of wind shear from northerly upper level winds. However, long range radar out of Wilmington, NC shows some impressive echoes and low level rain bands forming, and I imagine NHC will send out a Hurricane Hunter airplane this afternoon to see if a tropical depression has formed.

The computer models are forecasting that this system will not recurve out to sea, as the trough of low pressure that had been pulling this system to the north is now exiting the East Coast. High pressure is building in, which should force the system towards the coast over the next few days. Steering currents are weak, and the track of this system is very uncertain. The storm may go ashore over South Carolina (as favored by the GFDL model), northern Florida (as favored by the NOGAPS model), or perhaps North Carolina or Georgia. Wind shear is expected to increase significantly on Thursday from the north to the south, so this system lilkely does not have long to live.


Figure 1. Current long range radar out of Wilmington, NC.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Eleswhere in the tropics
The big burst of thunderstorms that developed in the Gulf of Mexico last night is gone, and no development is likely there today. A rotating area of clouds a few hundred miles northeast of the northernmost Lesser Antilles is associated with an upper level low pressure system. This is not expected to develop.

August hurricane outlook
What a difference a year makes! By this date in 2005, we were already up to Hurricane Irene, the 9th named storm of the season. Of those nine, four were hurricanes, and two (Dennis and Emily) were record-breaking Category 4 and 5 hurricanes. However, before we congratulate ourselves too much on a safe start to hurricane season, it is instructive to look at the plot of typical hurricane activity for the Atlantic (Figure 3). Peak hurricane season starts about August 18 and runs through October 18. The worst part of hurricane season is in front of us, and I do anticipate that conditions will get active. Witness 1998, when only one named storm occurred prior to August 19, and 10 named storms and 7 hurricanes formed by the end of September. A similar pattern of activity occurred in 2000, with only two named storm by this date, and a season total of 15 named storms. So, those of you who doubt NOAA and Dr. Gray's predictions of 15 named storms this season need to put your skepticism on hold.


Figure 3. Climatological Atlantic hurricane and tropical storm activity.

A major shift in the atmospheric pattern over the Atlantic began at the end of July, and portends an active hurricane season. June and July were characterized by a much stronger than normal Bermuda High, with surface pressures up to 7 mb higher than normal over the Atlantic. Taking a look at the surface pressures the past ten days (Figure 4), we see that surface pressures are now up to 7 mb lower than normal over much of the Atlantic, a complete reversal of the situation in June and July. Lower surface pressures are more conducive for hurricane formation, and drive weaker trade winds. Weaker trade winds mean less evaporative cooling of the ocean, allowing the ocean to heat up more than usual.


Figure 4. Sea level pressure (top) and departure from normal (bottom) for the 10 days ending August 12, 2006. Image credit: NOAA Climate Prediction Center.

We can see that the tropical Atlantic has warmed considerably, relative to normal, between July 28 (Figure 5) and August 12 (Figure 6). The Caribbean has warmed about 1/2 a degree C, relative to normal, and the blue pool of cooler than normal waters near the Bahamas has shrunk. The 2-week forecast from the GFS model predicts a continuation of these conditions, and I expect that the ocean will continue to warm to much above normal levels through September (although not as warm as 2005).


Figure 5. Sea surface temperature departure from normal for July 28, 2006.


Figure 6. Sea surface temperature departure from normal for August 12, 2006.

Wind shear
Wind shear was higher than normal in June and July, and has decreased to near normal levels since August 1. The exception is the region from the Bahamas north along the U.S. East Coast, which has still seen higher than average wind shear, due to the presence of strong upper-level low pressure systems. The 2-week GFS forecast continues to call for strong upper-level low pressure systems to roam the waters of the Atlantic, bringing hostile wind shear to any budding tropical systems that venture near. However, the wind shear averaged over the entire tropical Atlantic is expected to be near normal, and I expect that some systems will begin finding "holes" in the shear and manage to develop during the remainder of August.

Dry air
Outbreaks of Saharan dust and associated dry air have been common this year. The dust acts to reduce the amount of sunlight reaching the ocean, keeping SSTs cool. The dust and dry air also interfere with the formation process of tropical storms. The Saharan dust season peaks in June and July, and should begin a slow decline the remainder of hurricane season. Plots of relative humidity from the latest 2-week GFS forecast support this idea.

Vertical instability
Another important ingredient for tropical storm formation which I haven't talked much about is vertical instability. Simply put, if the air near the surface is very warm and the air at high altitudes is very cold, this is an unstable atmosphere. When air is unstable, thunderstorm and tropical storm activity is enhanced. This occurs because in an instable atmosphere, air from the surface can rise further and faster than air in a stable atmosphere. Rising air pulls up the moist air from the surface to colder regions aloft, where the moisture can condense and fall as rain. Since warm air is less dense than cold air, rising air in an unstable atmosphere finds itself less dense than its surroundings, since it started out very warm to begin with. Thus, the air will continue to rise, until it reaches a region of the atmosphere where the stability is high. In tropical cyclones, this often happens at about 50,000 feet--the beginning of the Stratosphere, a very stable layer of air where temperature increases with height.

Instability over the tropics during the 2006 hurricane season (Figure 7) has been below average. The ocean temperatures have been close to normal, which keeps the atmosphere more stable. Also, the general atmospheric ciculation has brought more stable air into the tropical Atlantic than we saw in 2005. However, with the weak trade winds we've been seeing this August allowing the oceans to heat up to much above normal, I expect that instability will increase to near normal levels by the beginning of September, enhancing hurricane formation.


Figure 7. Vertical instability in 2006 (blue lines) compared to normal (black lines) for the Eastern Caribbean (left) and tropical Atlantic (right). Image credit: CIRA (Cooperative Institute for Research in the Atmosphere).

Steering pattern
The large scale jet stream pattern and associated positioning of the Bermuda High has remained unchanged since early June, and is forecast to remain the same into early September. This pattern puts a trough of low pressure over the U.S. East Coast, which will act to recurve storms approaching the U.S. I expect that many of the strongest hurricanes we will see this year will recurve harmlessly out to sea, perhaps threatening only Bermuda. However, some systems may not recurve in time, which puts the U.S. East Coast at higher risk than average for a hurricane strike. North and South Carolina have the highest risk of any region of the U.S., since they stick out farthest into the ocean. This steering pattern also favors higher than normal hurricane activity in the northernmost Lesser Antilles Islands, such as the Virgin Islands, Anguilla, and St. Maarten. The U.S. Gulf Coast has a lower than average chance of a hurricane strike with this steering pattern. Given that this steering pattern has held for so long, the odds are that it will remain in place through at least mid-September.

Summary
The relatively quiet hurricane season we've been enjoying is not going to last. A very active period will start, as soon as the atmosphere destabilizes a bit more. If one believes the long-range 2-week outlook from the GFS model, the current quiet period should last another 4-12 days. Around August 21, I expect it will appear that a switch has been thrown, and the Atlantic will be very active indeed. Expect our first hurricane in the Atlantic by August 26, and a very active September. However, I do expect we will get many recurving storms that will miss land, and that this hurricane season will be similar to the ones we experienced in 1995-2003.

Jeff Masters

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New disturbance in the Gulf of Mexico

By: JeffMasters, 09:43 PM GMT on august 15, 2006

The old frontal boundary that spawned the distubance off the East Coast we're watching has spawned a new disturbance over the Gulf of Mexico this afternoon. A large blow-up of intense thunderstorms south of New Orleans marks the center of this disturbance. With wind shear only 5-10 knots, this region is probably the bigger threat to develop than its sister east of Florida, which is under higher wind shear. Developments from old fronts are usually slow, and I expect Thursday is the earliest we would have to worry about a tropical depression forming. Steering currents are weak, but a slow motion to the south or southwest away from Louisiana should begin tonight. The NHC has not run their package of preliminary model runs on this system yet, but I will post them as soon as they do.

East coast of Florida disturbance
Heavy thunderstorms continue over the waters east of Florida this evening along a broad area of low pressure that has developed from the remains of an old cold front. Wind shear is 10-15 knots in a narrow band along this old front, which is low enough to allow some development to occur. This disturbance remains poorly organized, with only a moderate amount of heavy thunderstorm activity. The Hurricane Hunter mission scheduled for today was canceled and rescheduled for Wednesday.

The computer models are forecasting that any system that forms in this area will begin moving northwestwards towards the Carolinas over the next two days, in response to a trough of low pressure swinging across the eastern U.S. When the trough moves out to sea on Thursday, high pressure is forecast to build back in, forcing the system back towards the west, or leaving it nearly stationary off the East Coast. None of the computer models forecast that the storm will grow to anything stronger than a 40-mph tropical storm.


Figure 1. Current satellite of the Gulf of Mexico.


Figure 2. Preliminary models tracks for the East Coast disturbance.

I'll be back Wednesday with my Atlantic tropical outlook for the rest of August.

Jeff Masters

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Tropical update; 2nd warmest July on record

By: JeffMasters, 02:06 PM GMT on august 15, 2006

Heavy thunderstorms continue over the waters east of Florida this morning along a broad area of low pressure that has developed from the remains of an old cold front. Wind shear is 10-15 knots in a narrow band along this old front, which is low enough to allow some development to occur. Visible satellite imagery from this morning shows that this disturbance is poorly organized, with no signs of a surface circulation and only a moderate amount of heavy thunderstorm activity. I expect the Hurricane Hunter mission scheduled for today into this system will be canceled.

Development along old fronts is usually slow, so the earliest we should expect a tropical depression to form in this area is Wednesday. There is a significant amount of wind shear on either side, plus plenty of dry air to the north, so anything that develops will likely struggle to intensify.

The computer models are forecasting that any low that forms along the old front will begin moving northwestwards towards the Carolinas over the next three days, in response to a trough of low pressure swinging across the eastern U.S. When the trough moves out to sea on Thursday, high pressure is forecast to build back in, forcing the system back towards the west, or leaving it nearly stationary off the East Coast. None of the computer models forecast that the storm will grow to anything stronger than a 40-mph tropical storm.


Figure 1. Current satellite of the Florida region.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Wave off of Africa
The tropical wave that moved off the coast of Africa yesterday is near the Cape Verde Islands today. The wave is looking much less organized this morning, with lower thunderstorm activity. It is crossing into cooler waters of only 26C, which may be partly responsible for this loss of thunderstorm activity. Wind shear is about 10-15 knots, which could allow some slow development, but I don't expect that this wave will develop. Water temperatures in front of it are even cooler, and wind shear is forecast to be marginal. None of the computer models develop this system.

Second hottest July on record in the U.S.
July 2006 was hotter than the scorching summer of 1988 across the U.S., and was the second warmest July on record, according to data released by the National Climatic Data Center yesterday. The average temperature of 77.2F was second only to the Dust Bowl year of 1936, which had an average temperature of 77.5F. January to July has been the warmest such year-to-date period on record. The nationally averaged year-to-date temperature was 55.3F, or 3.2F above the 1901-2000 average. The previous record of 54.8F was set in 1934. The July heat wave broke more than 2,300 daily temperature records for the month and eclipsed more than 50 records for the highest temperatures in any July. The heat was most extreme in South Dakota, where temperatures averaged 10F above normal in some areas. The all-time state temperature record of 120F set in 1936 was matched on July 15. July of 2006 ranked as the 26th driest July in the 112-year record, and severe drought conditions from Texas northwards to the Dakotas have been responsible for billions of dollars in agricultural losses.


Figure 4. Rank by county of temperatures for July 2006.

Is it Global Warming?
Is this year's heat wave due to global warming? Well, you can't blame one hot summer in one country on global warming. When one plots up the average U.S. July temperature from 1895-2006 (Figure 5), it is apparent that there has been about a 1F warming in July temperatures since the late 1800's. However, there is a lot of natural variability. While it has been very hot in July during the past 20 years, there was a period in the 1930s was was equally hot. Nevertheless, the globe has warmed about 1.4F in the past since 1970, and this trend has continued this year. Globally, July 2006 was the third warmest July on record since records began in 1880 (1.01F/0.56C above the 20th century mean) and the sixth warmest year-to-date (January-July) (0.92F/0.51C).

An overwhelming majority of climate scientists believe that a significant portion of the warming since 1970 is due to anthropogenic (human) causes, and is likely to accelerate over the next few decades. Assuming that the forecasts of these scientists are correct, we should expect summers like the Summer of 2006 to be commonplace 10-15 years from now. I know I'm planning on doing a lot more swimming in Lake Superior in coming summers, something I've never been able to do until this year.


Figure 5. Average July temperature for the U.S. from 1895-2006. Image credit: National Climatic Data Center.

Sea ice continues to shrink
Santa's shop on the North Pole Ice Cap continues to grow more endangered. Northern Hemisphere sea ice extent, as measured from passive microwave instruments onboard NOAA satellites, was the lowest for any July on record in 2006 at 8.7 million square kilometers (Figure 6). The previous July low extent record was set in 2005 with 9.1 million square kilometers.


Figure 6.Departure from normal of sea ice extent over the North Pole for July, 1979 - 2006. Image credit: National Climatic Data Center.

I'll be back Wednesday with my Atlantic tropical outlook for the rest of August.

Jeff Masters

Updated: 02:10 PM GMT on august 15, 2006

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Watching three tropical waves

By: JeffMasters, 02:13 PM GMT on august 14, 2006

Heavy thunderstorms continue over the waters east of Florida this morning along an old cold front. Wind shear is 10-15 knots in a narrow band along this old front, which is low enough to allow some development to occur. A non-tropical area of low pressure developed yesterday near 28N 74W, a few hundred miles east of Cape Canaveral, Florida. Preliminary forecast tracks for this low are plotted in Figure 1. However, visible satellite imagery from this morning reveals that a new area of low pressure is developing much closer to the Florida coast near 27.5N 78W, about 150 miles east of Vero Beach. Long range radar out of Melbourne is also showing an increase in thunderstorm activity off the coast in association with this developing low. This low is drifting slowly southwards, and we don't have any premilinary model tracks from NHC for it. My guess is that this new low will dominate the circulation and the old low NHC has been tracking will dissipate later today. The East Coast of Florida near Vero Beach and West Palm Beach could get heavy rain today as this new low continues to develop.

Development along old fronts is usually slow, so the earliest we should expect a tropical depression to form from either of these two lows is Tuesday. There is a significant amount of wind shear on either side, plus plenty of dry air to the north, so anything that develops will likely struggle to intensify. The Hurricane Hunters are scheduled to investigate this system on Tuesday afternoon.

The computer models are forecasting that any low that forms along the old front will begin moving northwestwards towards the Carolinas over the next three days, in response to a trough of low pressure swinging across the eastern U.S. When the trough moves out to sea on Thursday, high pressure is forecast to build back in, forcing the system back towards the west, or leaving it nearly stationary off the East Coast. None of the computer models forecast that the storm will grow to anything stronger than a 45-mph tropical storm.


Figure 1. Current satellite of the Florida region.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Tropical wave in the eastern Caribbean
A tropical wave a few hundred miles west of the Lesser Antilles Islands is moving west-northwestward at 10-15 mph. While the wave is an area of low wind shear (5-10 knots), it is embedded in a large area of dry air, which is keeping thunderstorm activity minimal. A QuikSCAT pass from 5:48am EDT today showed no surface circulation and top winds below 20 mph. The dry air should keep any development slow today. Wind shear is expected to remain below 10 knots near the wave's location through Tuesday, but the long-term prospect of this system becoming a hurricane is very low. The wave is headed for the Western Caribbean, where high wind shear associated with an upper-level low pressure system will dominate all week. The Hurricane Hunters mission scheduled for today was canceled, due to the wave's lack of organization. NHC did not run their package of preliminary model tracks on it this morning, and this may be the last mention I give of this system.

New wave off of Africa
A large and impressive tropical wave has moved off the coast of Africa today, a few hundred miles southeast of the Cape Verdes Islands. The wave has a well-defined circulation at mid-levels, and is under about 10 knots of wind shear. It could develop into a tropical depression in the next few days as it moves west-northwest just south of the Cape Verdes Islands.

Figure 3. Preliminary models tracks for the Cape Verdes Islands disturbance.

I'll be back with an update Tuesday, unless some significant development occurs. On either Tuesday or Wednesday, I plan to post my outlook for the remainder of August. Is today's activity a sign the tropics are heating up?

Jeff Masters

Updated: 03:28 PM GMT on august 14, 2006

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Watching the waters near Florida

By: JeffMasters, 02:03 PM GMT on august 13, 2006

Heavy thunderstorms are on the increase in the waters east of Florida this morning along an old cold front that pushed off the coast. Wind shear is 10 knots along this old front, and is forecast to remain 10 knots or lower through Tuesday. This is low enough to allow some development to occur, and we'll have to watch this area closely. Most of the computer models are forecasting that something will develop along the front, as early as Monday. The models are very uncertain about where such a development might go. If something develops relatively close to Florida, the preferred track appears to be across Florida and into the Gulf of Mexico. A more northerly track towards the Carolinas is also one of the model solutions. If something develops further from Florida, the preferred track is northeastward towards Bermuda. We'll just have to wait and see where the focus of development may be along the old front.


Figure 1. Current satellite of the Florida region.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Tropical wave near Barbados
A tropical wave near Barbados is producing heavy rain in the southern Lesser Antilles Islands today as it moves westward at 10-15 mph. Barbados has reported some heavy rain with this system, but winds have been below 20 mph on the island, and the QuikSCAT satellite did not see any winds over 20 mph in a 6:14am EDT pass today. While the wave is an area of low wind shear (5-10 knots), it is embedded in a large area of dry air, which should keep development slow today. Wind shear is expected to remain below 10 knots in the Eastern Caribbean through Tuesday, so the wave has some potential for development once the dry air surrounding it becomes more dilute.


Figure 3. Preliminary models tracks for the Barbados tropical wave.

I'll have an update this afternoon if changing conditions warrant; otherwise, see you Monday morning!

Jeff Masters

Updated: 07:30 PM GMT on august 13, 2006

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Another quiet weekend in the tropics

By: JeffMasters, 01:41 PM GMT on august 12, 2006

High wind shear and dry air continue to dominate the tropical Atlantic. There is a tropical wave about 500 miles east of the Lesser Antilles Island that has a surface circulation, but this wave is embedded in a very large area of dry air, and thunderstorm activity is presently very limited. The computer models are indicating the possiblity of some development off the coast of the Carolinas by Tuesday, but any storm here is likely to be a threat only to Bermuda. The long range 2-week GFS forecast calls for a continuation of the current pattern, with high wind shear remaining as a major deterrent to tropical cyclone formation until the end of August.

Since there's not much of interest to report in my blog, let me call attention to another blog on the site. Margie Kieper is putting together a very ambitious series of blogs documenting the effect of Katrina's storm surge. Today, she documents the damage at Grand Island, LA. Each day for the next 20 days she'll march down the coast to Mobile, AL, showing what happened to each 10-mile long section.


Figure 1. Damage to Grand Island, LA in the wake of Katrina. Image credit: NOAA.

I'll be back with an update on Sunday.
Jeff Masters

Updated: 01:45 PM GMT on august 12, 2006

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Quiet Atlantic; more on Supertyphoon Saomai

By: JeffMasters, 02:11 PM GMT on august 11, 2006

High wind shear continues to dominate the tropical Atlantic, and there's little to be concerned with today. The remains of the tropical wave that the Hurricane Hunters investigated earlier this week as it moved through the Lesser Antilles Islands are just south of Haiti. A hint of a circulation at mid levels of the atmosphere developed this afternoon, but the associated heavy thunderstorm activity is limited. The wave is under about 10-20 knots of wind shear, and any development of this system will be slow.

Supertyphoon Saomai:
In China, the death toll has risen to over 100 in the wake of Supertyphoon Saomai, which slammed into the coast south of Shanghai Thursday as a Category 4 storm with 135 mph winds. The death toll will no doubt rise higher today as the remains of Saomai spread heavy rains through the same region of China hit by Tropical Storm Bilis, which killed more than 600 people last month.


Figure 1. Supertyphoon Saomai as it passed north of Taiwan, August 10, 2006 at 1:22 GMT. At maximum strength, Saomai was a Category 5 storm with 160 mph winds. The image was taken by the Department of Defense F-15 satellite. Image credit: NOAA Environmental Visualization Project.

The media is calling Saomai the worst typhoon to hit China in 50 years, but there is some dispute about just how strong the storm was at landfall. Here is comparison of intensities from three different agencies at Saomai's landfall at 12 GMT August 10:

U.S. Joint Typhoon Warning Center: 1-min sustained winds of 135 mph, Cat 4.
Japan Meteorological Agency: 1-min sustained winds of 100 mph, Cat 2.
Hong Kong Observatory: 1-min sustained winds of 115 mph, Cat 3.

So, these three agencies all using the same satellite data couldn't agree on the strength of this typhoon within two Saffir-Simpson categories! This underscores the difficulty of trying to determine if global warming is causing an increase in Category 4 and 5 hurricanes--even today with much better tools and training, experts still can't agree on storm intensities with the accuracy needed for such a study.

This was discussed in more detail in a paper published this year by Kamahori, Yamazaki, Mannoji, and Takahashi of the the Japan Meteorological Agency (JMA) in the on-line journal Scientific Online Letters on the Atmosphere - a new journal produced by the Meteorological Society of Japan. The study compares typhoon intensities in the Northwest Pacific since 1977 as compiled by the Joint Typhoon Warning Center (JTWC) and the JMA. The JTWC data was used in the famous Webster et. al study from 2005 that found a worldwide 80% increase in Category 4 and 5 tropical cyclones since 1970. A key element of their conclusions was the data from the Northwest Pacific, which make up about 50% of global Category 4 and 5 storms. The JMA group found that using JTWC's dataset, the number of days when a Category 4 or 5 typhoon was present increased from about 10 per year in 1977-90, to 17 per year during 1991-2004--a 70% increase. However, the JMA data for the same time period showed a 40% decrease in Category 4 and 5 typhoon days. The authors concluded, "We do not have sufficient evidence to judge which dataset is reasonable." I would have to agree--until we get a coordinated major re-analysis effort of all the tropical cyclone data for the globe, it is dangerous to make conclusions about whether global warming is causing an increase in tropical cyclone intensities. I think it is likely there has been some increase, but it is nowhere nearly as large as the 80% increase reported by Webster et. al.

Jeff Masters

Updated: 09:50 PM GMT on august 11, 2006

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Quiet in the Atlantic; Supertyphoon Saomai hits China

By: JeffMasters, 02:03 PM GMT on august 10, 2006

The tropical wave that crossed through the Lesser Antilles Islands last night is now in the eastern Caribbean, south of Puerto Rico. While there is some substantial thunderstorm activity associated with the wave visible on satellite imagery and long range radar out of San Juan, there is no trace of a surface circulation anymore, and the wave is much less organized than it was yesterday. A good pass by the QuikSCAT satellite at 5:52am EDT this morning barely shows a wind shift associated with the tropical wave. The Hurricane Hunter mission scheduled for this afternoon has been cancelled, and no more missions are scheduled. The wave is not a threat to develop. Wind shear is 20 knots over the wave and is not forecast to lessen significantly over the next three days, so this wave missed its best chance at developing.

Elsewhere in the tropics
There are no other threat areas to discuss in the Atlantic. The computer models are indicating that the frontal zone off the East Coast of the U.S. may spawn a tropical storm sometime in the period 2-5 days from now. Any development here would most likely be a threat only to Bermuda.

Supertyphoon Saomai hits China
Supertyphoon Saomai slammed into the coast south of Shanghai, China this morning. Saomai was a Category 4 storm with sustained winds of 150 mph and a central pressure estimated at 910 mb at landfall. It's been an unusually tough typhoon season for China, which has endured five strikes by tropical cyclones, and much death and destruction. Saomai (the Vietnamese word for the planet Venus) is pounding the same region of China hit by Tropical Storm Bilis, which killed more than 600 people last month.


Figure 1. Supertyphoon Saomai at landfall in China. Image from the Japanese MTSAT satellite, 5:30am EDT Thu Aug 10 2006. Image credit: Navy Research Lab

Who wants a supertyphoon?
Who wants a supertyphoon? Apparently, Korea does. An excerpt from a Korean news story:

http://english.chosun.com/w21data/html/news/200608/200608090024.html

Nation Prays for a Typhoon in the Midst of Unbearable Heat

It appears as though Koreans will have no respite from the sweltering heat, with scorching weather set to continue for at least 10 more days and temperatures hitting a new high on Malbok--traditionally regarded as one of the three hottest days of the lunar year--on Wednesday.

Typhoon Saomai -- this year's eighth - held a promise of cooling off the Korean Peninsula, but has disappointingly veered off toward southern China. The Korea Meteorological Administration predicted that only a typhoon will be able to bring down the insufferable temperatures.

I'd say it's nuts to refer to a supertyphoon that "disappointingly veered off!" But then again--this summer's heat has been hot enough to scramble eggs on a sidewalk, not to mention one's brains. Almost all of the Northern Hemisphere has suffered through exceptional heat.

I'll be back Friday morning with the latest update.
Jeff Masters

Updated: 08:33 PM GMT on august 10, 2006

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Hurricane Hunters do not find a tropical depression

By: JeffMasters, 07:20 PM GMT on august 09, 2006

The Hurricane Hunters just finished investigating the tropical wave moving into the Lesser Antilles Islands, and found no closed surface circulation. The top winds were 34 mph at flight level of 1000 feet, and 23 mph at the surface. Wind shear of 10-20 knots due to upper-level winds from the southeast are interfering with this storm's organization, and have blown the main area of thunderstorms away from the center to the northwest of the storm. Strong upper-level westerly winds from an upper-level low pressure system to the north are expected to bring significant amounts of shear over the system for the next two days. The shear may weaken enough to allow a tropical depression to form in the next day or two, but it will be a struggle for this system to get organized. There are the beginnings of some upper level outflow apparent on satellite imagery, but no real low-level spiral banding occurring yet. Pressures are falling and it has been raining at Barbados, Martinique, and St. Lucia, but winds have been under 20 mph, except at Martinique, where the wind gusted to 52 mph at 3:23pm EDT. Martinique/Guadaloupe radar presents a nice picture of the storm's rainbands.

This morning's GFDL model continues to predict that the wave will develop by Thursday into a weak tropical storm, which will move through the Caribbean Sea to a point south of Haiti on Saturday, where high wind shear will dissipate it. None of the other computer models develop the storm at all.

The wave will move through the Lesser Antilles Islands tonight and Thursday morning, bringing winds of 20-30 mph and heavy rain to Barbados, St. Lucia, Martinique, and surrounding islands. Puerto Rico and the Dominican Republic may also receive these effects, on Friday and Saturday, respectively. The Hurricane Hunter mission for tonight was cancelled, and has been rescheduled for 2pm EDT Thursday.


Figure 1. Preliminary model tracks for the Lesser Antilles tropical wave.

Elsewhere in the tropics
A broad non-tropical low pressure system located about 800 miles southwest of the Azores is drifting southward, and is not expected to develop over the next two days. Some of the computer models are forecasting that development is possible by Sunday or Monday, though.

Jeff Masters

Updated: 09:40 PM GMT on august 09, 2006

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Lesser Antilles wave a threat to become a tropical depression

By: JeffMasters, 02:23 PM GMT on august 09, 2006

A tropical wave moving westward at 15-20 mph near 13N 57W, about 300 miles east of the Lesser Antilles Islands, has made a significant comeback this morning after losing its spin and most of its heavy thunderstorm activity last night. The system has re-gained its spin, and a new blow-up of intense thunderstorms has developed over the circulation center. It remains to be seen if the storm can hang onto this thunderstorm activity; 10-20 knots of wind shear are still interfering, and this system has had a history of alternately looking organized, then disorganized, as the shear waxes and wanes. Strong upper-level westerly winds from an upper-level low pressure system to the north are responsible for this shear, and this low is expected to continue to bring significant amounts of shear over the system for the next two days. It is possible that the shear may weaken enough to allow a tropical depression to form in the next day or two, but it will be a struggle for this system to get organized. Pressures are falling at the two buoys about 100 miles to the north of the storm's center, though, and peak winds at 9am EDT at one of these buoys was a sustained 28 mph. We don't have a recent QuikSCAT satellite pass to judge the winds that way, unfortunately. There are the beginnings of some upper level outflow apparent on satellite imagery, but no real low-level spiral banding occurring yet.

The wind shear is greatest to the system's north, so the further south it stays, the more likely it is to develop. The Saharan Air Layer (SAL) of very dry air is 100-200 miles to the system's north, which is probably far enough away that it will not impact it over the next day or two.

Last night's GFDL model predicted that the wave will develop by Thursday into a weak tropical storm, which will move through the Caribbean Sea to a point south of Haiti on Saturday, where high wind shear will dissipate it. None of the other computer models develop the storm at all.

The wave should move through the Lesser Antilles Islands tonight and Thursday morning, bringing winds of 30-40 mph and heavy rain to Barbados, St. Lucia, Martinique, and surrounding islands. Puerto Rico may get socked on Friday with these conditions, and the Dominican Republic on Saturday. The Hurricane Hunters are on call to investigate the wave today at 2pm EDT today, and I'll have an update this afternoon on what they find.


Figure 1. Preliminary model tracks for the mid-Atlantic tropical wave.

Elsewhere in the tropics
Clouds and showers stretching from the Bahamas through South Florida are associated with an upper level low pressure system. High wind shear, dry air, and cool air temperatures are expected to keep this area from developing. A broad non-tropical low pressure system located about 800 miles southwest of the Azores is drifting southward, and is not expected to develop over the next two days. Some of the computer models are forecasting that development is possible by Sunday or Monday, though.

Jeff Masters

Updated: 02:34 PM GMT on august 09, 2006

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NOAA forecasts fewer hurricanes in 2006

By: JeffMasters, 08:26 PM GMT on august 08, 2006

NOAA's August 8 hurricane forecast was issued today, and calls for a less active season than their May 22 outlook did. However, they still predict a 75% chance of a more active than usual hurricane season, with 12-15 named storms, 7-9 hurricanes, and 3-4 intense hurricanes. An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. NOAA's May 22 forecast called for 13-16 named storms, 8-10 hurricanes, and 4-6 major hurricanes. The key statistic that should cheer us up is the forecast reduction in the number of intense hurricanes, by 1-2.



NOAA follows the lead of the other major forecasting groups, which have all reduced their forecast number of named storms and hurricanes by 1 or 2 since May. Here's a comparison of what the four groups currently are forecasting:

NOAA Aug 8 forecast: 12-15 named storms, 7-9 hurricanes, 3-4 intense hurricanes.
Dr. Bill Gray Aug 3 forecast: 15 named storms, 7 hurricanes, 3 intense hurricanes.
Cuba Institute of Meteorology Aug 1 forecast: 15 named storms, 9 hurricanes. Last named storm ends in mid-November.
Tropical Storm Risk, Inc. Aug 4 forecast: 16 named storms, 8 hurricanes, 3-4 intense hurricanes.

Why the reduction in storms?
NOAA cites three main reasons for reducing their forecast numbers:

1) Sea surface temperatures anomalies (departures from normal) cooled during June and July. This happened due to stronger trade winds over the Atlantic. In addition, surface pressures have been higher than average. Hurricane formation is enhanced when lower surface pressures are present.

2) La Nina died quicker than expected. This has resulted in higher wind shear over the Atlantic.

3) The persistent upper-level ridge (and associated westward extension of the Bermuda High) over the eastern U.S., which contributed to the extremely active 2003-2005 hurricane seasons, is not present this year.

NOAA does not make seasonal forecasts of where hurricanes might make landfall, but notes that similar above-normal seasons have historically averaged 2-3 landfalling hurricanes in the continental United States and 2-3 hurricanes in the region around the Caribbean Sea.

Tropical wave approaching the Lesser Antilles
A tropical wave moving westward at 15-20 mph near 13N 49W, about 700 miles east of the Lesser Antilles Islands, is looking more impressive this afternoon. A blow-up of intense thunderstorms has developed over the circulation center, which is a key sign that a tropical depression may be trying to form. It remains to be seen if the storm can hang onto this thunderstorm activity; 10-20 knots of wind shear are still interfering. This shear has been oscillating in strength during the last two days, periodically blowing away all the heavy thunderstorms the storm has managed to build. I wouldn't be surprised to see a sudden increase in shear rip away most of this current burst of deep convection. However, I think it more likely that the storm will hang onto this burst and become a tropical depression by Wednesday night.

The wind shear is greatest to the system's north, so the further south it can stay, the more likely it is to develop. If the storm does make it into the Caribbean, its chances are much better than if it turns more west-northwest and takes aim at Puerto Rico. The wind shear to the north is expected to retreat a bit to the north over the next two days, and should stay in the 10-20 knot range over the wave, which is low enough to allow a tropical depression to form. Dry air will continue to be a problem for the wave, and will likely keep development slow.

Last night's GFDL model predicted that the wave would develop by Thursday into a weak tropical storm, and move through the central Lesser Antilles Islands and into the Caribbean Sea. This morning's run of the GFDL model is no longer showing any development, and none of the other computer models develop the wave.

The wave should move through the Lesser Antilles Islands on Thursday morning. At this point, if I had travel plans in the Caribbean, I wouldn't change them, since any development of this system is likely to be slow. However, I would check the situation frequently, as surprises are common. The Hurricane Hunters are on call to investigate the wave on Wednesday at 2pm EDT.


Figure 1. Preliminary model tracks for the mid-Atlantic tropical wave.

Elsewhere in the tropics
A tropical wave moving through the Bahamas and South Florida is under 20 knots of wind shear and is not expected to develop.

I'll be back Wednesday morning with an update.

Jeff Masters

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Nothing new in the tropics today

By: JeffMasters, 03:01 PM GMT on august 08, 2006

A tropical wave moving westward at 15-20 mph over the mid-Atlantic near 13N 47W, about 800 miles east of the Lesser Antilles Islands, continues to have the potential to develop into a tropical depression. The wave has a well defined surface circulation, and is currently under about 10-15 knots of wind shear, which is low enough to allow some slow development. However, there is quite a bit of dry air surrounding the storm, and thunderstorm activity is limited. It appears unlikely the wave will develop today. However, the long-range outlook for this wave is a little more favorable than it appeared yesterday. The upper level low that was expected to bring hostile wind shear over the wave later this week has weakened and is no longer expected to be a major influence. There is still expected to be 10-20 knots of wind shear along the path of the wave the remainder of the week, which is just low enough that a depression may be able to form. Dry air will continue to be a problem for the wave, and will likely keep development slow. The GFDL model predicts that the wave will develop by Thursday into a weak tropical storm, and move through the central Lesser Antilles Islands and into the Caribbean Sea. None of the other computer models develop the wave. I expect that the GFDL has the right idea, and this wave will develop into a tropical depression Wednesday or Thursday. The Hurricane Hunters are on call to investigate the wave on Wednesday at 2pm EDT.


Figure 1. Preliminary model tracks for the mid-Atlantic tropical wave.

Elsewhere in the tropics
A tropical wave moving through the Bahamas is under 20-30 knots of wind shear and is not expected to develop. An area of intense thunderstorms off the coast of North Carolina is also under high wind shear, and is not expected to develop. Long range models are showing the possibility of development off the Carolina coast this weekend, but anything developing here would likely be a threat only to Bermuda.

I'll be back this afternoon with an update, and an analysis of the NOAA updated seasonal hurricane forecast, due out later this morning.

Jeff Masters

Updated: 03:13 PM GMT on august 08, 2006

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Tropical wave has potential to develop into tropical depression

By: JeffMasters, 02:09 PM GMT on august 07, 2006

A tropical wave over the mid-Atlantic near 12N 42W, about 1300 miles east of Barbados, has the potential to develop into a tropical depression. However, the long-term survival of this system is questionable. The wave has a well defined surface circulation, and is currently under about 10-15 knots of wind shear, due to upper level winds from the east. As the wave moves westward over the next 36 hours, it should encounter an area of lower wind shear where these upper level winds will relax, and I believe a tropical depression will probably develop by Tuesday night. However, as the wave continues westward at 15-20 mph, it is forecast to encounter strong upper-level westerly winds associated with the counter-clockwise flow of air around an upper-level low pressure system northeast of Puerto Rico (Figure 1). This strong wind shear should weaken or destroy the wave. This is shown well by the GFDL model, which intensifies the wave into a weak tropical storm by Wednesday, then dissipates it later in the week. None of the other computer models develop the wave at all.


Figure 1. Water Vapor satellite image from 8:45am EDT August 7. Note the substantial dry air (brown colors) to the northwest of the tropical wave we're watching, and the upper level low to the northwest of the wave that is expected to bring strong westerly wind shear later this week.

Intense thunderstorm activity associated with the wave is mostly on the west side of the circulation, due to the strong upper-level winds from the east pushing all the convection over to that side. There is a substantial amount of dry air to the northwest of the wave, as seen in the water vapor image from this morning (Figure 1.) This dry air has resulted in a decrease in the intensity and areal coverage of the wave's thunderstorm activity this morning. If we do get the expected decrease in wind shear later today and tomorrow, this should allow the thunderstorm activity to build back in over the circulation center.


Figure 2. Preliminary model tracks for the mid-Atlantic tropical wave.

For today, it does not appear that this tropical wave is going to be a major threat. I'll be back with an update on Tuesday. Elsewhere in the tropical Atlantic, there are no threat areas to talk about.

Jeff Masters

Updated: 03:06 PM GMT on august 07, 2006

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African disturbance a threat to become the next tropical storm

By: JeffMasters, 03:04 PM GMT on august 06, 2006

A tropical wave over the mid-Atlantic near 12N 35W, about 700 miles southwest of the Cape Verdes Islands, has the potential to develop into a tropical depression in the next few days. The wave has a very large surface circulation. This circulation has an unusual elongated oval shape, oriented east-west, as one can see in satellite loops and the latest QuikSCAT wind pattern. Surface winds around the circulation were generally an unimpressive 15-20 mph in the 4am EDT QuikSCAT pass, and the wave has only limited thunderstorm activity. Wind shear is a favorable 5-10 knots, and sea surface temperatures are 27-28 C, plenty warm enough to support development. There is some African dust and dry air to the northwest, but not nearly as much as we saw with Chris last week. Conditions are much more favorable for development than we saw for Chris, and I expect we will see a tropical depression form by Tuesday night.

The computer models take the wave towards the west or west-northwest, bringing it to the Lesser Antilles Islands by Friday or Saturday. The GFDL models intensifies it to a Category 1 hurricane by Friday. Some of the other models are less impressed with the wave; the NOGAPS model doesn't develop it at all.


Figure 1. Preliminary model tracks for the mid-Atlantic tropical wave.

Chris looks pretty dead
Satellite imagery this morning shows very little is left of Tropical Depression Chris, just a few scattered thunderstorms near western Cuba stretching northward into the Gulf of Mexico. This activity is expected to move westward through the Gulf of Mexico and wind up near the Texas/Mexico border by Tuesday. Although wind shear is a relatively friendly 5-15 knots over the remains of Chris, there is so little left of it that redevelopment probably does not have time to occur before it moves ashore.

Figure 2. Preliminary model tracks for the remains of Chris.

Jeff Masters

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The curse of Chris comes through

By: JeffMasters, 11:13 PM GMT on august 05, 2006

Hello all, Jeff Masters back again from a few days on Lake Superior. That's always a great way to beat the heat, but not so much during this incredibly hot summer of 2006. I have never been able to do what I did today--dive to 20 foot depth in Lake Superior and not be bone chilled. The water temperature in the lake--and all the Great Lakes--are at record levels this summer. I was able to spend up to 30 minutes at a stretch in the ridiculously warm 70+ degree water in some of the bays of this usually 45-55 degree lake. In any case, I'm ready and rested to take on the peak part of hurricane season, now that the preliminaries are over with.

The "curse" of Chris comes through
As I mentioned in my Wednesday blog, storms named Chris have been notorious for their lack of oomph, and I was counting on this to be the case again this year when I decided to take a few days off this week. Well, I got away with it, but don't think I wasn't concerned. I did call in to check and see if Chris of 2006 was going to bash Florida, and was ready to cut short my trip. My friend Dr. Chris Landsea of NHC was also concerned, and was ready to start boarding up the house again. But, the curse of Chris came through again, and the storm is dead. Will it rise again? Tune in tomorrow, and I'll have an analysis of this, plus all the other news from the tropics.

Jeff Masters

Updated: 11:16 PM GMT on august 05, 2006

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Chris is not looking impresive today

By: JeffMasters, 09:22 PM GMT on august 04, 2006

Hi everybody, I'm Rob Carver filling in for Dr. Jeff Masters again
this afternoon. Well, TD Chris fell short of regaining tropical storm
status this afternoon, according to the 500PM EDT update from NHC. In
a nutshell, Chris's circulation is rather broad and weak, but it's
still a closed circulation (meaning that the low-level winds form a
loop around the center). It's still not the most impressive storm
from the satellite's perspective as this animation
shows, but there are still thunderstorms associated with this storm.
Two upper-level lows continue to bracket Chris,
making reorganization and redevelopment a tricky prospect before it
starts to graze the northern coast of Cuba.

However, once Chris clears Cuba and makes it into the Gulf of Mexico,
intensification is a real prospect with the warm sea-surface
temperatures, and the 12Z GFS is predicting the wind shear
associated with the western upper-level low will decrease over the
next few days. NHC is forecasting this as well, predicting Chris will
regain tropical storm status 4 days from now. However, this is all
contingent upon Chris maintaining a closed circulation as it passes
through the Florida Straits. In any event, the majority of the models
and the official forecast have Chris moving steadily WNW into the Gulf
of Mexico after passing Cuba. Needless to say, if you live in
southern FL, the Keys, or Cuba, you should keep an eye on this storm.


Forecast Map of Chris

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Is this the end of Chris?

By: JeffMasters, 04:27 PM GMT on august 04, 2006

A hurricane hunter aircraft found a flight level wind of only 29 knots this morning. Thus Chris has been officially downgraded to a tropical depression. You can see in the visible floater loop that Chris is pretty much devoid of any thunderstorm development and you can also see the northeasterly shear that has torn Chris apart.
The system remains between two upper-level low pressure systems and the shear is not expected to relax. The official forecast has Chris regaining tropical storm status, but if that strong wind shear continues, this will not happen. If Chris makes it to the Florida straits, it may have a chance of intensification as it meets the favorable environment in the Gulf of Mexico. Then again, all it needs is for the shear to relax, like it seemed it got a a bit of time yesterday afternoon, and it may regain the tropical storm status that it once had. There may be a pocket of low shear to the storm's north so keep a wandering eye on that.

The high pressure ridge over the eastern United States will keep Chris on a general west-northwestern course over the next few days. The official track has it scraping northern Cuba Saturday night/Sunday morning.

Updated: 04:29 PM GMT on august 04, 2006

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Chris is still a tropical storm...

By: JeffMasters, 09:50 PM GMT on august 03, 2006

Hi, this is Rob Carver, filling in for Jeff Masters this afternoon.
TS Chris, while weak, is still barely a tropical storm in NHC's eyes.
The hurricane hunters didn't find any winds of tropical storm strength,
but NHC felt it was still possible there were some strong enough winds
in the circulation to maintain it's status. However, the forecast still
calls for Chris to weaken to a tropical depression within the next 12
hours.

TS Chris continues to be very weak and disorganized with the bulk
of its thunderstorms away from the center of circulation. Below is
a satellite-derived estimate of the rainfall rate, which shows this.



Also, radar imagery from San Juan, PR shows no significant changes in the
intensity or organization of the thunderstorms on the SE side of Chris.



Right now, the wind shear around Chris is about 10 knots which is
decreasing which would be favorable for Chris. However, there is
an area of significant shear (about 40 knots) SE of Florida which
would be not be favorable for an already weak storm.

Model guidance is fairly consistent in bringing Chris or it's remnant
south of Florida and into the Gulf of Mexico. Some models, such as the
GFS have Chris dissipating relatively soon.




The August update of the long-range hurricane forecast issued by
Dr. William Gray's group at Colorado State University has been updated here.
The forecast has been adjusted downwards slightly, to account
for less favorable conditions for hurricanes in the Atlantic basin.

The new forecast calls for 15 named storms, which is two less than the May 31 forecast of 17 storms. Gray's group also predicts 7 hurricanes and 3 Category 3 or greater hurricanes for this season, which is two fewer than the May 31 forecast.

Gray's group cites four main reasons for this shift. First,
the sea-level pressure over the tropical Atlantic has risen,
which is less favorable for the formation of tropical cyclones.
Second, the speed of the trade winds has increased, enhancing the
vertical wind shear which tends to weaken tropical cyclones. Third,
the sea-surface temperatures (SSTs) of the tropical Atlantic has
fallen, which lowers the amount of energy availible for storms.
Finally, the SSTs over the eastern equatorial Pacific are starting
to rise. This would tend to cause changes in the upper-level winds
which would generate more wind-shear in the tropical Atlantic.
Of course, we will have to wait and see how accurate this forecast
is. The next scheduled update is September 1.

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The Curse of Chris Continues?

By: JeffMasters, 04:34 PM GMT on august 03, 2006

Hello everybody, this is Shaun taking over for Dr. Masters the next few days. You may also hear from another meteorologist here named Rob.

What a difference a day makes, huh? Just yesterday, Dr. Masters was talking about Chris almost being hurricane strength and the NHC had it eventually gaining hurricane status as well. Now the official forecast has it turning into a tropical depression (if it hasn't done so already) and drifting aimlessly west-northwestward towards Cuba.


Official NHC Forecast.

The only real convection noted from Chris is well to the southeast of the storm's center near Puerto Rico. An upper-level system dropped southward overnight and really did a number on the system. This caused an increase in shear to 15-20 knots and dry air that really weakened the storm. The hurricane hunter found a weak storm that may have turned into a tropical depression already. Nonetheless, the convection is expected to hammer Puerto Rico and continue towards Hispaniola later today.



There is some chance for re-development if the wind shear slacks off and the dry air can be cut off. But we will want to see if there is any evidence of that later in the day. The steering ridge system's north will continue to keep the storm to the south on a west-northwestern course towards the southern Bahamas and eventually Cuba.

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Tropical Depression Chris?

By: JeffMasters, 02:59 PM GMT on august 03, 2006

Here's a quick update on still Tropical Storm Chris.

From the National Hurricane Center discussion:
"The combination of dry low- to mid-level air and shear...associated with an upper-level cyclone that dropped southward into Chris...has taken its toll on the tropical cyclone. Chris is devoid of deep convection within about 75 N mi radius of the center."

Again in the 2006 Hurricane Season, shear rules. The GFS model continues to forecast shear over the westerly path is which Chris is moving.

With that said, eyes will be glued to this cyclone. If shear relaxes a bit, the storm could intensify. With the current shear trend, the storm might completely dissipate.

Hopefully the rule of King Shear will continue, making the residents of Florida and the Islands breathe a bit easier.

Figure 1. The long range radar from San Juan, Puerto Rico showing the showers and thunderstorms of Chris.


-John Celenza in for Jeff Masters

Updated: 03:00 PM GMT on august 03, 2006

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Chris near hurricane strength

By: JeffMasters, 12:21 PM GMT on august 02, 2006

Chris continues to become better organized, and may become a hurricane later today. The latest hurricane hunter eye report at 7:09am EDT found flight level winds of 67 knots, which translates to about 60-65 mph at the surface. There are upper level lows to Chris' east and west, and Chris is embedded in a low shear zone of 10-15 knots between these two lows. The lows are helping enhance the upper-level outflow from the storm. The satellite presentation of Chris has improved substantially, and we now see a more symmetrical storm that is not sucking in so much of the dry air surrounding it. Radar from San Juan, Puerto Rico, shows an eye-like feature developing (Figure 1). Chris is over warm 29-30 C water that is favorable for intensification. The key question, as always, is wind shear. Chris is a small storm that is very vulnerable to wind shear. Any movement of Chris towards either of the upper level lows surrounding it will bring hostile wind shear that will weaken the storm. However, the current model forecasts call for Chris to maintain its position exactly between these lows, and for the shear to drop to 5-10 knots. The most likely ranges for Chris' intensity on Sunday when it is expected to be near Florida range from weak tropical storm (45 mph) to strong Category 2 hurricane (110 mph).


Figure 1.Latest long-range radar image from Jan Juan, Puerto Rico.


Path of Chris
The recent record heat wave over the Eastern U.S. means that the Bermuda High is extending further west than usual, creating a blocking ridge of high pressure that will prevent Chris from recurving to the north in the next five days. The GFS, NOGAPS, UKMET, Canadian, and European models all agree on a west-northwest track taking Chris north of the Dominican Republic and Cuba, into the Bahama Islands, then into Florida or just south of Florida by Sunday. The lone outlier model is the usually reliable GFDL, which takes Chris into the Dominican Republic. For now, NHC is discounting the GFDL. I would not cancel any travel plans for Puerto Rico or the Dominican Republic this week, but those of you planning on visiting the Bahamas may want to rethink those plans.

It now appears likely that Chris will enter the Gulf of Mexico early next week and be a threat to the Gulf Coast. There is a trough of low pressure that will be moving across the Eastern U.S. on Monday that may turn Chris more to the north; high pressure is then forecast to build in on Tuesday and force Chris back to the west-northwest. Given this forecast, there is no region of the Gulf Coast that can assume Chris will miss them.

The "curse" of Chris
This is not the first time a tropical storm named Chris has come. There were storms named Chris in 1982, 1988, 1994, and 2000. Each time, Chris has been an insignificant storm that either never made it to hurricane strength, or in one case, barely made it to hurricane strength but stayed out to sea and never had a nice photogenic appearance. However, the 2000 incarnation of Chris did set a record--shortest amount of time as a tropical storm. Chris in 2000 lasted just one advisory before wind shear tore it apart. I've happily needled my friend Dr. Chris Landsea, Science and Operations Officer at the National Hurricane Center, every six years about this "curse" of Chris. He's always pretty cheerful about it, saying it was a good curse to have. Well, I'm hoping that the "curse" of storms named Chris continues this year, and I can happily tell Chris he'll have to wait until 2012 to get that nice-looking eye that a storm named Chris has never had!

Elsewhere in the tropics
Nothing else is happening in the tropical Atlantic, although most of the computer models are predicting a new tropical depression may develop off the coast of Africa this weekend.

This will probably be my last blog on Chris for several days, as I promised my family I would take off at least two days this August. This is the time that it worked out to be. Hurricanes are not respecters of the best-laid plans of humans! I'll be back to blogging on Saturday, and wunderground.com meteorologist Shaun Tanner will update my blog in my absence. He's on the west coast, so these updates will not come as early in the morning.

Jeff Masters

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Chris is stronger

By: JeffMasters, 08:06 PM GMT on august 01, 2006

The Hurricane Hunters found much stronger winds than expected in Chris this afternoon. The 2:50 pm EDT eye report indicated a central pressure of 1007 mb, down 2 mb from the most recent advisory. Most surprising were the winds in the southeast quadrant, which were in the 55-60 mph range. Radar out of Guadeloupe shows this intense band thunderstorms rather nicely. The northern side of Chris is still devoid of thunderstorms due to the dry air and wind shear. The storm's appearance on satellite imagery is improved from this morning, and shows the beginnings of a Central Dense Overcast (CDO) feature typical of strong tropical storms. Wind shear has dropped another 5 knots this afternoon, down to 10-20 knots, so some continued strengthening is possible. The latest set of model runs are quite divergent on what Chris might do, and I think we really need to wait until the next set of model runs is in before we can rely on the computer models. Unfortunately, tonight's flight of the NOAA jet was cancelled, so we'll have to wait until Wednesday night for the jet to fly. Most of the computer models are still dissipating Chris by five days from now.

I'll be back with an update in the morning with the latest.

Jeff Masters

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Tropical Storm Chris; August hurricane outlook, part II

By: JeffMasters, 11:37 AM GMT on august 01, 2006

It's August in the tropics, the first of the peak months of hurricane season. Befitting the arrival of August comes the arrival of Tropical Storm Chris, which formed this morning just east of the Leeward Islands. The formation of Chris came in defiance of significant adversity--wind shear was 20-25 knots last night when the storm formed into a tropical depression, and is still a rather hefty 15-20 knots. Considerable dry air lies to Chris' north, and strong upper-level winds from the north are acting to push this dry air into Chris' core, keeping the storm from intensifying much. Radar out of Martinique shows a decent band of thunderstorm to the storm's southeast and east, but no thunderstorm activity on the northwest side where shear and dry air are impacting the storm. We don't have a recent QuikSCAT pass to gauge the winds, but two passes by a satellite equipped with a microwave sensor came up with an estimate of 40 mph surface winds. The Hurricane Hunters are scheduled to visit at 2pm EDT this afternoon, so we'll know more then.

This shear and dry air will continue to affect Chris over the next two days. The shear is forecast to gradually weaken, which may allow some slow intensification. Chris must tightrope walk a very narrow path between two upper-level cold lows in order to strengthen significantly. One of these cold lows is just north of the Bahamas, and the other is northeast of Chris. These lows are forecast to move slowly west-northwest in tandem with Chris, and if Chris can stay exactly between them, low enough wind shear exists to potentially allow some strengthening. Any deviation from this scenario will put Chris under hostile wind shear, which will act to limit intensification or even dissipate the storm.

Last night's computer model runs did not start out with a very good initial picture of the current strength of Chris, and dissipated the storm within 72 hours. We need to wait until the next set of model runs based on this morning's 8am EDT (12Z) data are available before taking much stock in both the track and intensity forecasts of the models. The NOAA jet is scheduled to fly the storm tonight, so the best model data for Chris will be available Wednesday morning.

Chris will bring heavy rains and high winds to the Leeward Islands today, primarily to those islands lying to the south of the storm's center, where dry and and wind shear are less of a problem. Puerto Rico should get a good soaking on Wednesday, and after that, the prognosis is very uncertain. Chris could become a hurricane late in the week, but I put the chances of this at 10%. Dissipation is a more likely scenario, since there is so much wind shear around. The most likely scenario of all is that Chris will remain a tropical storm over the next five days.

August hurricane outlook
As we've seen repeatedly, sea surface temperatures are important for hurricane formation and intensification, but nothing happens unless the wind shear is low. When we look at wind shear, the standard measure is the wind at the upper atmosphere (200 mb, usually around 40,000 feet in altitude) minus the wind just above the surface (850 mb, or about 5,000 feet altitude). This difference in wind speed is plotted in Figure 1 for July 2006. The key feature to look at is the anomalies in the bottom portion of the plot. The entire tropical Atlantic was under higher than normal wind shear during July, with wind shear up to 6-8 meters per second over the Caribbean (12-16 knots). The only region that experienced below normal wind shear was off the coast of North Carolina. Not coincidentally, this is where July's only named storm formed (Beryl).


Figure 1. Observed wind shear for July 2006. The top portion shows the difference in wind between 200 mb and 850 mb pressure levels, and the bottom image shows the departure from normal.Image credit: NOAA's Climate Prediction Center.

What, then can we expect for wind shear in August? The long range shear forecast from NOAA (Figure 2) shows near normal wind shear over the Atlantic for the remainder of hurricane season. The latest 2-week outlook from the GFS model agrees, calling for near normal wind shear during the first half of August. Since wind shear is expected to be near normal, and SSTs should be near normal, we should expect a near normal level of hurricane activity for August. "Normal" should be placed in context with the above-normal level of hurricane activity we've been seeing since 1995. In the ten years since 1995, not including the El Nino year of 1997, the Atlantic has averaged 4.4 named storms, 2 hurricanes, and one intense hurricane in August. My prediction for August follows a similar line: 4-5 named storms, two hurricanes, and one intense hurricane. All this assumes that El Nino doesn't rear its head; the recent warming of ocean waters in the Equatorial Pacific along the coast of South America could be the prelude to an El Nino event. These events create atmospheric circulation patterns that greatly increase the wind shear over the Atlantic, significantly cutting down on hurricane activity. However, the El Nino forecast models are predicting a continuation of the current neutral El Nino condition through August and September, and it is uncommon to have an El Nino event begin at this time of year. I doubt El Nino will be a factor in this year's hurricane season.


Figure 2. Forecast wind shear July August through October 2006. Image credit: NOAA's Climate Prediction Center.

What will be the steering pattern for August?
For much of June and July, the jet stream made a dip over the eastern U.S., creating a persistent trough of low pressure. In concert with the jet stream, the Bermuda High has stayed further east than it did in 2005. The resulting steering pattern has been taking tropical waves through the Bahamas, then north along the East Coast and out to sea. Tropical Storm Beryl took this path as well. The long range forecast from NOAA's Climate Prediction Center calls for not as strong a trough of low pressure for the remainder of hurricane season. Instead, we should expect a near normal steering pattern, with all regions of the Atlantic under their usual risk of hurricane strikes. However, the latest 2-week GFS model forecast is calling for a continuation of the June and July steering pattern, but with a somewhat weaker trough over the Eastern U.S. Thus, I am forecasting that the entire East Coast of the U.S. will have a higher than average risk of hurricane strikes in August, and the Gulf Coast will have a lower than average risk. The highest risk area of the East Coast will probably be North Carolina and South Carolina. As far as the actual percentage risks, I'll leave that up to Dr. Bill Gray's forecast team at Colorado State, who will be putting out their updated Atlantic hurricane season forecast on Thursday, August 3.

Jeff Masters

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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