U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 220058 
Storm Prediction Center ac 220057 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0657 PM CST Tue Feb 21 2017 

Valid 220100z - 221200z 

..no severe thunderstorm areas forecast... 

the risk for severe storms appears negligible across the U.S. 
Through tonight. 

Despite the presence of seasonably mild conditions across much of 
the U.S., Under the prominent influence of split belts of westerlies 
emanating from the mid-latitude Pacific, thermodynamic profiles 
across most areas remain largely unsupportive of appreciable 
convective potential. However, within broader scale upper troughing 
gradually shifting across and inland of the Pacific coast, through 
the intermountain west and rockies, forcing for ascent and 
destabilization associated with a number of smaller scale 
perturbations have been contributing to a few areas of generally 
weak thunderstorm activity. The most significant of these 
perturbations have been progressing east northeastward along a 
strong, but weakening cyclonic mid-level jet, across northern 
California through the northern Great Basin and northern rockies. 

East of The Rockies, a lower/mid tropospheric closed low within the 
southern-most branch of mid-latitude westerlies has been supporting 
some thunderstorm activity across the lower Mississippi Valley late 
this afternoon. With the unstable warm sector of the associated 
occluded surface low confined to the eastern Gulf of Mexico, this 
has largely been aided by daytime heating beneath the mid-level cool 
pool, and does not seem likely to persist beyond the 01-03z time 

..northern plains... 
One of the impulses emerging from the mid-latitude Pacific now 
appears to be in the process of progressing east of the northern 
rockies, into the crest of low amplitude ridging across the northern 
plains. Mid-level moisture return to an eastward developing zone of 
stronger lower/mid tropospheric warm advection appears to be 
contributing to the maintenance of ongoing thunderstorm activity 
across the western Dakotas. This activity seems likely to persist 
and spread eastward across the Dakotas through the 03-06z time, and 
perhaps continue into portions of the upper Midwest overnight. 

..north California/southern Oregon/northwestern Nevada... 
Objective analysis suggests that weak to modest boundary layer 
destabilization has taken place across the region, beneath cold 
mid-level temperatures along/north of the stronger mid-level jet 
axis. Before this environment stabilizes appreciably with the loss 
of daytime heating this evening, forcing associated with another 
inland migrating short wave impulse may still contribute to an 
increase in scattered thunderstorm activity. 

..Florida Peninsula... 
Latest guidance suggests that vigorous warm sector thunderstorm 
activity may at least approach Gulf coastal areas of the Florida 
Peninsula by 12z Wednesday. However, destabilization appears 
unlikely to become supportive of appreciable severe weather 
potential, given the weak to modest nature of the wind fields and 
vertical shear to the east of the approaching mid-level closed 

.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... 


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