U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 220542 
Storm Prediction Center ac 220540 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1140 PM CST Tue Feb 21 2017 

Valid 221200z - 231200z 

..no severe thunderstorm areas forecast... 

the risk for severe thunderstorms appears negligible across the 
U.S., Today through tonight. 

Within one branch of split westerlies emanating from the 
mid-latitude Pacific, large-scale troughing is expected to continue 
developing inland of the Pacific coast, through much of the 
intermountain west, rockies and High Plains during this period. A 
series of significant shorter wavelength perturbations embedded 
within the base this trough will remain progressive, including one 
forecast to pivot across the Great Basin late this evening through 
the overnight hours. Models suggest that this feature could support 
the initiation of significant surface cyclogenesis to the Lee of the 
Colorado rockies by 12z Thursday. 

A preceding impulse, already emerging from the larger scale 
troughing, is currently supporting frontal wave development across 
parts of the northern plains. This latter feature is expected to 
track from parts of the mid Missouri Valley this morning through the 
Great Lakes region by late tonight, with the trailing cold front 
advancing southeastward across the upper Mississippi Valley and 
north Central Plains. 

Despite these developments, cooling/drying across the Gulf Coast 
into the Gulf of Mexico, on northerly flow in the wake of a 
lower/mid-tropospheric cyclone now digging into the eastern Gulf, 
will preclude a substantive inland return flow of moisture through 
this period and beyond. This cyclonic circulation is forecast to 
continue slowly southeastward across parts of southern Florida, the 
Keys and the Florida Straits by daybreak Thursday, and may be 
accompanied by additional surface cyclogenesis near/south of the 
Keys into the Bahamas. Even in the warm sector of this feature, 
however (which may remain south/southeast of the keys), rather 
modest boundary layer moistening seems likely to contribute to 
limited convective potential. One or two rounds of thunderstorm 
activity may impact the Florida Keys and portions of the Florida 
Peninsula today through tonight, but the potential for severe storms 
still appears negligible at this time. 

Otherwise, areas of isolated to widely scattered thunderstorm 
activity will be possible with the short wave impulses within and 
emerging from the large-scale upper troughing across the west. 
However, convective potential, in general, is expected to remain low 
across most areas. 

.Kerr.. 02/22/2017 


Mesoscale Discussion

acus11 kwns 202038 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202037 

Mesoscale discussion 0198 
National Weather Service Storm Prediction Center Norman OK 
0237 PM CST Mon Feb 20 2017 

Areas affected...parts of Louisiana and into far southwest 

Concerning...severe potential...watch unlikely 

Valid 202037z - 202230z 

Probability of watch issuance...20 percent 

Summary...a slow increase in storm intensity is ongoing, along with 
low-probability risk for an isolated damaging wind gust or two, or a 
brief tornado. At this time, it appears that risk will remain 
sufficiently limited to preclude the need for ww issuance. 

Discussion...latest surface analysis shows persistent southeasterly 
flow off the north-central Gulf of Mexico, contributing to both 
onshore advection of upper 60s to low 70s dewpoints, as well as some 
enhancement to the low-level shear profile. As a semi-organized 
band of storms continues crossing western Louisiana, a weakly 
heating/destabilizing airmass (characterized by around 750 j/kg 
mixed-layer cape) is contributing to some increase in updraft 
strength -- both within the line, and with isolated showers 
developing ahead of the main band of storms. 

While potential for additional destabilization is limited, low-level 
and deep-layer shear are both sufficient to support a few stronger 
cells, and at least weak/mid-level rotation. Indeed, a few cells 
immediately ahead of the main convective band have exhibited weak 
rotation per area WSR-88D data, and this will likely continue over 
the next several hours. Overall risk, however, appears likely at 
this time to remain marginal, and thus likely not requiring watch 

.Goss/guyer.. 02/20/2017 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 29749036 29169024 29079132 29409283 30819321 32059330 
32479261 32109152 30889136 29749036