U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211628 
Storm Prediction Center ac 211627 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1027 am CST Tue Feb 21 2017 

Valid 211630z - 221200z 

..no severe thunderstorm areas forecast... 

the risk for severe thunderstorms appears negligible across the 
U.S., Today through tonight. Isolated thunderstorms are possible 
from the lower Mississippi Valley region eastward along the Gulf 
coastal plain, and from northern California to parts of Wyoming and 
the northern High Plains. 

..lower Mississippi Valley and eastern Gulf Coast... 
Scattered thunderstorms are expected along with and in advance of an 
upper low that will move eastward along the Gulf Coast this 
afternoon and tonight. Cold temperatures aloft will support some 
small hail across portions of southern la this afternoon as modest 
heating/destabilization occurs, though low tropopause heights in the 
vicinity of the upper cyclone will limit the depth of convection and 
preclude any substantial severe hail risk. 

..northern California/southern or into the Great Basin/northern rockies... 
As an upper trough approaches the Pacific coast, generally weak 
convection is expected across a broad area from the northern 
California/southern or coast eastward into the Great Basin and northern 
rockies, with isolated thunderstorms possible where convection 
becomes sufficiently deep. Somewhat greater instability across 
portions of the northern California coast into the northern Sacramento 
Valley may support some thunderstorms capable of small hail and 
locally gusty winds, though generally weak effective shear 
magnitudes should limit storm organization. 

..portions of the northern High Plains... 
A shortwave trough over portions of the northwest will move quickly 
into the northern High Plains this afternoon and evening. Ahead of 
this feature, weak but sufficient instability will support 
fast-moving clusters of convection later this afternoon and tonight 
from central/eastern Montana into ND. Relatively strong low/midlevel flow 
and effective shear may support locally gusty winds and perhaps some 
small hail with the strongest of these clusters. 

.Dean/guyer.. 02/21/2017 


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