Beryl to regain tropical storm status, but otherwise, the Atlantic is quiet
It's been a while since I wrote my last blog entry, and since then, the Atlantic as seen their second named storm of the season, Beryl. To see two named storms before the official start of the hurricane season is truly historic. In fact, it hasn't occurred in over 100 years. To make things even more remarkable, Beryl made landfall near Jacksonville Beach, Florida during the pre-dawn hours of Monday, with winds of 70 mph; this makes Beryl the strongest pre-June, USA-landfalling tropical cyclone on record. Since then, Beryl has moved northward and northeastward in response to a trough to its north. As of the latest advisory from the National Hurricane Center, maximum sustained winds were near 30 mph with a minimum barometric pressure of 1005 millibars. Satellite imagery reveals a well-defined center of circulation with well-defined banding located primarily over the Atlantic.
Figure 1. Afternoon visible satellite imagery of Tropical Depression Beryl.
The forecast for Beryl
Beryl is expected to continue towards the northeast over the next few days as it is embedded within the westerlies around the the base of an eastward-moving shortwave trough located over the Central USA. This should bring Beryl off the coast of South Carolina or North Carolina, over the warm Gulf Stream waters, by tomorrow morning. Despite moderate vertical wind shear, most of the models re-intensify it into a tropical storm; the official NHC forecast closely follows this. A second landfall on the southeastern coast of North Carolina is possible tomorrow afternoon as the cyclone continues moving northeast. However, with moderate wind shear, it appears that most tropical storm-force winds will be located on the eastern quadrant, or the offshore quadrant, of Beryl. Regardless, much of the eastern half of North Carolina should expect torrential rainfall, gusty winds, and brief tornadoes...especially tomorrow morning when parameters are at their greatest. The latest National Hurricane Center track forecast continues the tropical storm out to sea as we head into Thursday, gradually transitioning into an extratropical storm before dissipating on Friday. I agree with this track.
INIT 29/2100Z 25 KT 30 MPH...INLAND
12H 30/0600Z 25 KT 30 MPH...INLAND
24H 30/1800Z 35 KT 40 MPH...OVER WATER
36H 31/0600Z 45 KT 50 MPH
48H 31/1800Z 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 01/1800Z 45 KT 50 MPH...POST-TROP/EXTRATROP
Hurricane Bud to make landfall on the Mexican coastline
Hurricane Bud, the first hurricane of the 2012 Pacific hurricane season, is very nearly a major hurricane (if it isn't already one) this evening. Satellite imagery reveals a ring of cold cloud tops in excess of -70 °C surrounding a well-defined eye feature that has been partly cloudy for most of the day. The latest National Hurricane Center advisory listed Bud as a 110 mph Category 2 hurricane, just one mile per hour short of major hurricane status. However, according to the latest ATCF file update, Bud has winds of 115 mph. The minimum barometric pressure as reported by recon earlier this afternoon was 962 millibars, and the system was moving towards the northeast at 9 mph in response to a trough in the Midwest USA. This puts Bud on a track right towards the Mexican coastline.
A Hurricane Warning is in effect for the coast of Mexico from Manzanillo to Cabo Corrientes. A Hurricane Watch is in effect for the coast of Mexico from Punta San Telmo to east of Manzanillo.
A Tropical Storm Warning is in effect for the coast of Mexico from Punta San Telmo to east of Manzanillo, and a Tropical Storm Watch is in effect for the coast of Mexico from Cabo Corrientes to San Blas.
Figure 1. Afternoon visible satellite imagery of Hurricane Bud.
The forecast for Bud
As aforementioned, Bud has began the forecast northeast turn towards the coast of Mexico in response to a strong trough across the Midwest USA. A track towards the northeast is expected to continue for the next 2 days, which should send Bud into the Mexican coastline during the afternoon tomorrow. Thankfully, a sharp increase in dry air and a sharp decline in Sea Surface Temperatures should act to weaken Bud substantially by landfall; right now, the NHC is forecasting a Category 1 hit. Regardless of the strength, very heavy rainfall and gusty winds should be expected along the Mexican coastline. Afterwards, a ridge of high pressure is expected to build across the East USA. This should act to send Bud back into the East Pacific waters, though as a significantly weaker system. In fact, it is possible that Bud could emerge as a tropical depression and dissipate into a remnant low thereafter. Regeneration is not expected.
INIT 25/0000Z 95 KT 110 MPH
12H 25/1200Z 100 KT 115 MPH
24H 26/0000Z 90 KT 105 MPH
36H 26/1200Z 55 KT 65 MPH...INLAND
48H 27/0000Z 40 KT 45 MPH...INLAND
72H 28/0000Z 30 KT 35 MPH...OVER WATER
96H 29/0000Z 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 30/0000Z 20 KT 25 MPH...DISSIPATED
Updated: 04:38 PM GMT on iunie 14, 2012
A A A
Gone with the wind shear; TD #Two-E a threat to Mexico; Watching the Atlantic
In a quick manner, the first tropical storm of the 2012 Atlantic hurricane season, Alberto, formed during the afternoon hours of Saturday from a warm-core low pressure area that formed over the Gulf Stream underneath an upper-level trough split. Just as quick, Alberto is on a weakening trend this afternoon and likely not a tropical storm any longer after attaining a peak intensity of 60 mph with a barometric pressure of 995 millibars. This is due to strong wind shear out of the west-southwest. No further strengthening is forecast from Alberto despite the fact that Sea Surface Temperatures lie near 27 °C (81 °F), above the values needed to sustain a tropical storm. The system has begun an eastward movement, and should turn northeast as a trough deepens to the cyclone's west. Alberto is such a compact storm and remains far enough offshore that it should not affect any land areas.
INIT 22/0000Z 35 KT 40 MPH
12H 22/1200Z 30 KT 35 MPH
24H 23/0000Z 30 KT 35 MPH
36H 23/1200Z 25 KT 30 MPH...POST-TROP/EXTRATROP
48H 24/0000Z 25 KT 30 MPH...DISSIPATED
Figure 1. Afternoon visible satellite imagery of a weakening Tropical Storm Alberto.
Tropical Depression Two-E destined to become a Mexico threat
The second tropical depression of the 2012 Pacific hurricane season, Two-E, is likely to become a significant threat to Mexico over the coming days. The system has taken some time to organize today due to its large size and connections to the monsoon trough, but recent visible satellite loops reveal a sharp increase in organization. The latest National Hurricane Center advisory centered Tropical Depression Two-E at 9.6 °N 101.0 °W with maximum sustained winds of 30 knots (35 mph) and a minimum barometric pressure of 1005 millibars. The depression is currently on a west-northwest motion but is expected to turn northwest, north, and northeast over the coming days as a strong trough passes over the US Central Plains and draws Two-E northward.
An objective analysis from the Cooperative Institute for Meteorological Satellite Studies (CIMSS) reveals Tropical Depression Two-E is in a very favorable environment for further intensification characterized by Relative Humidity values above 70% and wind shear between 5-10 knots. The atmosphere should continue being favorable for strengthening as the week progresses, and Tropical Depression Two-E is poised to become "Tropical Storm Bud" later tonight and "Hurricane Bud" by Wednesday morning. Some of the global models indicate that the system could undergo a period of rapid intensification, and in fact, the latest SHIPS model forecast gave TD #Two-E a 56% chance of undergoing a 25 kt rapid intensification over the next four days. As the trough of low pressure pulls the cyclone northward, an increase in wind shear will likely cause weakening before landfall on the Mexican coastline late Thursday night or Friday. Once inland, Two-E should weaken quickly over the highly mountainous terrain.
INIT 22/0000Z 30 KT 35 MPH
12H 22/1200Z 45 KT 45 MPH
24H 23/0000Z 60 KT 60 MPH
36H 23/1200Z 70 KT 75 MPH
48H 24/0000Z 75 KT 85 MPH
72H 25/0000Z 95 KT 105 MPH
96H 26/0000Z 70 KT 80 MPH
120H 27/0000Z 60 KT 70 MPH...INLAND
Figure 2. Afternoon visible satellite imagery of a strengthening Tropical Depression Two-E.
Watching the Atlantic
An area of disturbed weather associated with a weak low pressure area in the Gulf of Honduras is producing heavy shower and thunderstorm activity across surrounding areas of the Caribbean Sea and southern Gulf of Mexico. Satellite loop analysis reveals potential broad cyclonic turning in the lower levels of the atmosphere, but immediate development is not expected as wind shear lies between 20-40 knots. Despite this, a few of the global models attempt to spin this up into a decent low pressure area as the week progresses. This is likely due to a brief letup of wind shear across the region, and organization of this low pressure is certainly possible. However, a combination of land interaction and increased wind shear afterwards means that anything organized will quickly become disrupted. I give this disturbance a Low, ~0% chance, of becoming a tropical cyclone in the next 48 hours.
The Western Caribbean will need to be watched over the next 10 days as a strong high pressure builds across the Eastern USA, causing heights there to rise and heights across the Gulf and Caribbean to lower. The GFS and ECMWF show the potential for mischief to occur as wind shear lowers across the region, but not all models show this solution and any development would be monsoonal, or slow, in nature.
The final area that will need to be watched as we head into the weekend is off the East Coast of the USA. A few of the global models show the potential for tropical or subtropical development off the coastline as a strong trough digs into the Western USA, causing a southwest to northeast orientation of the high pressure across the East USA. This would allow for an upper level disturbance to get "stuck" off the coast of North Carolina and South Carolina over the Gulf Stream, in a very similar manner to the synoptic pattern that spawned Tropical Storm Alberto.
Figure 3. Afternoon black/white infrared imagery of the West Atlantic.
Updated: 12:59 AM GMT on mai 22, 2012
A A A
Aletta holding onto life; 92E slowly organizing; Watching the Atlantic for trouble
Tropical Depression Aletta is clinging to life this evening despite the presence of moderate wind shear and dry air. In fact, an objective analysis from the Cooperative Institute for Meteorological Satellite Studies (CIMSS) reveals high south-southwesterly wind shear in excess of 25-30 knots affecting the system. In addition, water vapor satellite images show Aletta is located on the eastern periphery of a large area of very dry air. However, the system has continually firing deep convection in excess of -60 °C atop a well-defined center of circulation. It is uncertain how long convection will continue to fire, but Advanced Dvorak Technique numbers show that Aletta may very well be on the fringes of tropical storm intensity.
Figure 1. Late evening infrared imagery of Tropical Depression Aletta.
Forecast for Aletta
Tropical Depression Aletta has began a northward turn due to the effects of a weakness passing to its north across the US Central Plains. As this weakness continues eastward, Aletta should continue a northward motion and eventually turn towards the northeast. The latest SHIPS model continues with high wind shear in excess of 20 knots throughout the rest of the forecast period and a very dry atmosphere characterized by 700-500 mb Relative Humidity values below 50%. These conditions would typically act to weaken a system rapidly, but given how Aletta continues to build deep convection in such an environment argues for a more gradual weakening trend. The official National Hurricane Center forecast dissipates the cyclone in roughly 36 hours or so; I believe it will take slightly longer than that. Some of the global models are predicting that after Aletta degenerates into a remnant area of low pressure it could be absorbed within a larger circulation (Invest 92E). Considering how weak the low pressure area would likely be, a Fujiwhara effect is not feasible.
FORECAST MAX WINDS
INIT 18/0000Z 30 KT 35 MPH
12 HR 18/1200Z 30 KT 35 MPH
24 HR 19/0000Z 25 KT 30 MPH
36 HR 19/1200Z 25 KT 30 MPH
48 HR 20/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
72 HR 20/1200Z 20 KT 25 MPH...POST-TROP/REMNT LOW
96 HR 21/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
120 HR 21/1200Z...DISSIPATED
Figure 2. Rainbow imagery of Invest 92E.
Invest 92E slowly organizing
Invest 92E is gradually becoming better organized in the East Pacific this evening. Disregarding the recent dissipation of deep convection, which is likely due to the effects of D-MIN, the last visible satellite images of the day revealed a slightly better defined area of low pressure as opposed to its appearance earlier this morning. An objective analysis from CIMSS reveals high wind shear of 20-30 knots. However, given its satellite appearance, this may have analyzed a good deal too high. Relative Humidity values lie in excess of 75% which is very favorable for continued organization.
The forecast for 92E
As aforementioned, 92E lies within an environment favorable for continued development. In fact, a majority of the models show the invest attaining tropical storm status within the next four days. The latest National Hurricane Center Tropical Weather Outlook gave 92E a Medium chance, 40%, of becoming a tropical cyclone within the next 48 hours (5 PM PDT Saturday). All things considered, I believe the disturbance has a slightly lower chance, 30%, of becoming at least a tropical depression by Saturday evening. While difficult to forecast, the latest SHIPS model forecast gives 92E a respectable 26% probability of undergoing rapid intensification of at least 30 mph. I wouldn't rule it out at all either after the weekend.
The long term track of 92E is unknown. Many of the global models, such as the ECMWF and GFS, continue 92E west-northwest/northwestward over the next few days. However, as a strong weakness builds over the US Central Plains, a turn towards the north and then northeast is predicted. How far west the disturbance gets before turning determines whether or not it makes landfall on the Mexican coastline the first time. If 92E does not manage to get very far before feeling the effects of the weakness, the chances for landfall are heightened. On the flip side, if the disturbance can get very far west, its chances of making landfall are reduced as a ridge of high pressure will likely build in behind the trough. Regardless, residents living along the Mexican coastline need to be on high alert for 92E as many of the models, GFS and ECMWF included, intensify "Bud" into a hurricane before landfall.
FORECAST MAX WINDS
INIT 18/0000Z 25 KT 30 MPH
12 HR 18/12000Z 25 KT 30 MPH
24 HR 19/0000Z 25 KT 30 MPH
36 HR 19/1200Z 25 KT 30 MPH
48 HR 20/0000Z 25 KT 30 MPH
72 HR 20/1200Z 25 KT 30 MPH
96 HR 21/0000Z 30 KT 35 MPH
120 HR 21/1200Z 30 KT 35 MPH
Watching the Atlantic
The global models continue to hint on the development of a hybrid low pressure area off the coast of the Carolinas over the weekend as an upper level low becomes stacked with a low pressure area at the surface over the Gulf Stream. It doesn't appear particularly likely that a strong system would develop, but a weak area of low pressure that brings heavy rainfall and gusty winds along the coasts of South Carolina, North Carolina, and Virginia specifically is definitely feasible. I am giving this area a low chance, 10%, of becoming a tropical cyclone over the next 48 hours.
After several weeks of waiting on the GFS model's solution to verify, an area of low pressure has finally developed in the Gulf of Honduras. Little to no convection obscures this low pressure area due to high wind shear in excess of 20 knots and the effects of D-MIN. Unfavorable conditions are expected to continue for the next few days as an upper level low stays parked along the northern Gulf Coast. It is possible that wind shear could lower by the middle of next week as an anticyclone moves across Central America, but land interaction will likely be a big inhibitor for anything tropical that tries to form. As an upper level trough passes to the north of the disturbance, a northeastward track will likely be induced, tracking the low and associated heavy rainfall across Florida and the Bahamas. Due to strong westerlies, anything that exited the Caribbean would likely be sheared to part quickly. I am giving a low chance, ~0%, of this low pressure becoming a tropical cyclone over the next 48 hours.
Updated: 11:41 AM GMT on mai 21, 2012
A A A
Tropical Storm Aletta peaks in intensity; Invest 92E forms, likely to become Bud
Tropical Storm Aletta, the first named storm of the 2012 Pacific hurricane season, has peaked and is quickly deteriorating in organization this afternoon as evidenced by visible satellite imagery. The tropical cyclones consists of an intense area of convection with substantial outflow noted on the northern side. No spiral bands exist with Aletta, likely due to the combination of moderate easterly wind shear and very dry air to the system's west. Subjective Dvorak Satellite Classifications remain unchanged at T2.5 from both TAFB and SAB, so it appears that the tropical storm will remain as such. Maximum sustained winds as of the 8 AM PDT advisory are 40 mph with a minimum barometric pressure of 1004 millibars.
Figure 1. Afternoon infrared satellite imagery of Tropical Storm Aletta.
The forecast for Tropical Storm Aletta
As aforementioned, Aletta lies in a region of moderate wind shear and dry air. This should act to quickly weaken the system over the next 24-48 hours. The official National Hurricane Center forecast has Aletta degenerating into a post-tropical/remnant area of low pressure in 72 hours; I believe that, with increased wind shear and a dry environment, the cyclone will probably become post-tropical in 36 hours or so. Aletta is expected to continue west to west-northwestward over the next 12 hours or so under the influence of a strong mid-level ridge of high pressure, but a turn towards the north and eventually northeast is likely as a weakness develops to the north of the system. It is possible that the remnants of Aletta could eventually bring rainfall to the Baja of California. My forecast for Tropical Storm Aletta is as follows:
...FORECAST MAX WINDS...
INIT 16/2000Z 35 KT 40 MPH...TROPICAL STORM
12 HR 17/0800Z 30 KT 35 MPH...TROPICAL DEPRESSION
24 HR 17/2000Z 25 KT 30 MPH...TROPICAL DEPRESSION
36 HR 18/0800Z 25 KT 25 MPH...POST TROP/REMNT LOW
48 HR 18/2000Z 20 KT 25 MPH...POST TROP/REMNT LOW
72 HR 19/0800Z 15 KT 20 MPH...DISSIPATED
96 HR 19/2000Z 15 KT 20 MPH....DISSIPATED
120 HR 20/0800Z 15 KT 20 MPH...DISSIPATED
Invest 92E forms
A westward moving area of low pressure in the East Pacific a few hundred miles east of Tropical Storm Aletta, Invest 92E, has developed and is poised to become "Bud" over the next few days. Visible satellite loops reveal a distinctive low-level spin associated with the disturbance, and Subjective Dvorak Satellite Classifications are T1.0 from both SAB and TAFB. The National Hurricane Center is currently giving 92E a Medium chance, 30%, of developing into a tropical cyclone over the next 48 hours. Given its satellite appearance and current level of organization, I am giving 92E a higher chance, 50%, of developing into a tropical cyclone over the next two days.
Figure 2. Afternoon infrared satellite imagery of Invest 92E.
The forecast for 92E
Invest 92E lies in an environment very conducive for tropical development, characterized by wind shear around 10 knots and Sea Surface Temperatures in excess of 28 °C. In addition, very little dry air is present due to the upward pulse of the MJO. This should allow the invest to gradually strengthen over the next few days, attaining tropical depression status in 36 hours or so and tropical storm status in 48 hours or so. The disturbance is expected to track northwest over the next 24 hours before briefly stalling out as a weakness passing to the north of the system. This should turn the system back towards the east before a ridge of high pressure builds across the west, forcing the cyclone towards the southeast. Yet another weakness should emerge by 120 hours out, pulling the system to the north towards the Mexican coastline, which is depicted by two of the global models, the GFS and the ECMWF. For this reason, residents living along the Mexican coastline need to pay close attention to Invest 92E and be on high alert, as the steering pattern is reminiscent of those that develop during the early and late portions of the Pacific hurricane season which are known to allow tropical cyclone hits along the Mexican coastline. My forecast for the invest is as follows:
...FORECAST MAX WINDS...
INIT 16/2000Z 25 KT 30 MPH...INVEST
12 HR 17/0800Z 25 KT 30 MPH...INVEST
24 HR 17/2000Z 25 KT 30 MPH...INVEST
36 HR 18/0800Z 30 KT 35 MPH...TROPICAL DEPRESSION
48 HR 18/2000Z 35 KT 40 MPH...TROPICAL STORM
72 HR 19/0800Z 40 KT 45 MPH...TROPICAL STORM
96 HR 19/2000Z 50 KT 60 MPH...TROPICAL STORM
120 HR 20/0800Z 60 KT 70 MPH...TROPICAL STORM
Updated: 08:33 PM GMT on mai 16, 2012
A A A
Tropical Storm Aletta and the first day of the 2012 Pacific hurricane season
Tropical Storm Aletta, the first named of the 2012 Pacific hurricane season, has formed in the East Pacific on what is ironically the first day of the season in that basin. As of the 8 AM PDT advisory, maximum sustained winds are estimated at 40 mph and the minimum barometric pressure is down to 1004 millibars. Visible satellite imagery reveals a recent deep burst of convection atop Aletta's well-defined circulation, and Subjective Dvorak Satellite Classifications are T3.0 from both SAB and TAFB. For this reason, I believe Aletta is a good deal stronger than earlier this morning, and likely has maximum sustained winds near 50 mph.
Figure 1. Afternoon visible satellite imagery of Tropical Storm Aletta.
The forecast for Tropical Storm Aletta
Aletta is quickly running out of time to strengthen. Water Vapor Satellite loops reveal that a large mass of dry air lies to the north and west of the tropical storm. This, in combination with increased wind shear due to a strong upper-level trough passing to the north, should act to weaken the system as it moves west into increasingly cooler waters. The official forecast calls for gradual weakening after 24 hours, with dissipation of the tropical storm expected by 120 hours. A westward track is expected as the cyclone remains south of a strong mid-level ridge of high pressure.
2012 Pacific hurricane season begins today
The 2012 Pacific hurricane season is now underway, and it is expected to be a busy one--well above average in fact. Sea Surface Temperatures in the East Pacific lie well above normal, which should act to keep the upward pulse of the MJO in the basin a great majority of time. With increased thunderstorm activity, outflow produced from those storms will act to increase wind shear across the Atlantic but provide favorable conditions for tropical development in the East Pacific. In addition, the latest ENSO update from the Climate Prediction Center shows Sea Surface Temperatures in Nino region 3.4 up to 0.0 °C. This will act to further produce favorable conditions for development in the East Pacific. Thankfully, a majority of the tropical cyclones that form in the Pacific tend to move westward away from the Mexico coastline. Once in a while however, especially in the early and late portion of the season, deep troughs can act to steer the cyclones into the coast.
Figure 2. The list of tropical cyclone names that will be used during the 2012 Pacific hurricane season and their respective pronunciations.
Updated: 08:23 PM GMT on mai 15, 2012
A A A
Tropical Depression One-E forms, no threat to Mexico
Nature is jumping the gun a little early this year, as the first cyclone of the 2012 Pacific hurricane season, Tropical Depression One-E, has developed a day before the official start of the season. This development is from an area of disturbed weather, previously dubbed Invest 90E, that formed about 550 miles south-southwest of Acapulco, Mexico two days ago. An OceanSat pass from last night revealed a well-defined center and maximum sustained winds of 25-30 mph, but convection was ill-defined and relatively shallow at the time. Visible satellite imagery from this morning combined with Satellite estimates, however, revealed that convection was deep enough and sufficiently defined to renumber Invest 90E to Tropical Depression One-E.
Figure 1. Afternoon satellite imagery of Tropical Depression One-E.
The forecast for 01-E
Tropical Depression One-E lies atop warm Sea Surface Temperatures in excess of 29 °C and light wind shear between 10-15 knots, both of which are conducive for further intensification. These thermodynamics should remain at least marginally favorable over the next 24-36 hours which may allow the depression to briefly attain tropical storm status. If that were to occur, it would be given the name "Aletta". After that, a steep increase in wind shear is forecast by the global models as a strong upper-level trough passes near the system. This should lead to dissipation by 84-96 hours out as the depression moves west into increasingly cool waters around the southeastern side of a well-defined ridge of high pressure. This ridge should act to keep 01-E a far distance from the Mexico coastline.
Updated: 08:41 PM GMT on mai 14, 2012
A A A
2012 Atlantic hurricane season forecast
The 2012 Atlantic hurricane season is expected to be near or slightly above the long-term average with 12 named storms, 6 hurricanes, and 3 major hurricanes. Significant factors contributing to this decrease in tropical cyclone activity, compared to previous years, include the dissipation of La Niña in the equatorial Pacific Ocean, above normal Sea Level Pressures, rainfall patterns in the Atlantic and above normal Sea Surface Temperatures.
Figure 1. My predicted numbers compared to other forecasting agencies.
- Entire U.S. coastline – 51% (average for last century is 52%)
- U.S. East Coast – 23% (average for last century is 31%)
- U.S. Gulf Coast – 41% (average for last century is 30%)
- Expected near average major hurricane landfall risk in the Caribbean Sea
- El Niño-Southern Oscillation & Vertical Wind Shear Relationship
- Rainfall Patterns over West Africa & Tropical Atlantic Relationship
- North Atlantic Oscillation & Mean Sea Level Pressure Relationship
- Sea Surface Temperatures
EL NIÑO-SOUTHERN OSCILLATION/VERTICAL WIND SHEAR
El Nino-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five year. During El Niño, high Sea Surface Temperatures across the East Pacific generates intense thunderstorm activity. As a result, outflow produced by those thunderstorms enhances westerly wind shear across the Gulf of Mexico, Caribbean Sea, and Western Atlantic. This provides generally favorable conditions for tropical cyclogenesis in the Pacific. Oppositely, during Neutrals and weak to moderate La Niña, above average Sea Surface Temperatures in the Atlantic generate intense thunderstorm activity that produces significant outflow aloft, providing high vertical wind shear across the Pacific. This provides generally favorable conditions for tropical cyclogenesis in the Atlantic.
RAINFALL PATTERNS OVER WEST SAHEL/TROPICAL ATLANTIC
West Africa is the birthplace for most Atlantic tropical cyclones. In addition, it is also the origin of the West African Dust outbreaks known as the Sahara Air Layer. Wetter than normal conditions across the Sahel argue for wetter and more convective tropical waves, and also prevent major SAL outbreaks during the season. Drier than normal conditions produce hotter tropical waves that weaken as they move into the East Atlantic due to an unfavorable temperature gradient. This also means many major SAL outbreaks are likely during the season.
A wetter atmosphere across the Atlantic is typically indicative of Madden-Julian Oscillation, or MJO, in its upward phase. During this phase, tropical cyclogenesis is enhanced as instability remains high and convection is encouraged. A drier atmosphere is typically indicative of a downward phase of the MJO, which reduces vertical instability and suppresses convection. Uncommonly, tropical cyclones do form during the downward phase of the MJO.
NORTH ATLANTIC OSCILLATION/MEAN SEA LEVEL PRESSURE
There are two phases of the North Atlantic Oscillation: positive and negative. A positive North Atlantic Oscillation (NAO) implies a stronger subtropical ridge in the Atlantic and deeper than normal Icelandic low. The negative NAO phase indicates a weaker than normal subtropical ridge and shallower than normal Icelandic low. The positive phase of the NAO reduces Sea Surface Temperatures in the Atlantic due to increased evaporational cooling, while the negative phase of the NAO increases Sea Surface Temperature due to decreased evaporational cooling.
The negative NAO typically enhances tropical cyclone activity in the Atlantic due to weaker trade winds, while a positive NAO typically suppresses tropical cyclone activity due to stronger than normal trade winds that can not only act to shear the system, but prevent the disturbance from attaining a well-defined center.
The positive NAO correlates with above average Mean Sea Level Pressure (MSLP) which is unfavorable for tropical cyclogenesis, while a negative NAO correlates with below average MSLP which enhances tropical cyclogenesis.
SEA SURFACE TEMPERATURES
Sea Surface Temperatures have an enormous effect on the peak intensities of tropical cyclones and usually determine whether a system is tropical or subtropical. Values below 26 °C aren’t typically warm enough for tropical cyclones, and cyclones typically develop as subtropical cyclones. Values above that are conducive for tropical cyclone development.
Expected in 2012
In order to retrieve analogue years for a particular hurricane season, I compare different historical ENSO cycles that resembled weather patterns similar to that of the winter and early spring of 2012. Using that technique, I came up with quite a few analogue years: 1951, 1957, 1965, 2002, and to a lesser extent, 2004. The average of all aforementioned hurricane seasons give us 12.75 named storms, 5.6 hurricanes, and 3.2 hurricanes. I have taken the lower values of all these numbers due to the uncertainty pertaining to whether or not El Niño will surface during the season.
In short, the motto for this hurricane season is that there will be less named storms overall, but there is an increased chance for USA landfalls. An early start to the hurricane season appears likely, and an early end to the hurricane season is plausible, especially if El Niño surfaces in the equatorial Pacific. I am expecting 12 named storms, of which 6 will strengthen into hurricanes, and of which 3 will intensify into major hurricanes, or hurricanes with maximum sustained winds greater than 111 mph.
Thanks for reading,
Updated: 11:48 PM GMT on mai 14, 2012
A A A