U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 202002 
Storm Prediction Center ac 202000 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0200 PM CST Mon Feb 20 2017 

Valid 202000z - 211200z 

..there is a marginal risk of severe thunderstorms across parts of 

Thunderstorms will move across parts Louisiana into the lower 
Mississippi Valley this afternoon through tonight. An isolated 
damaging gust will be possible. 

The only change to the outlook is to move the marginal threat line 
eastward to near the Texas and Louisiana state-line behind a band of 
thunderstorms moving into southwestern Louisiana. A marginal severe 
threat will continue with this convection as it moves across 
southern Louisiana where a moist airmass is in place. Afternoon 
forecast soundings across southern Louisiana show strong deep-layer 
shear and enough instability for a marginal wind damage threat. 

.Broyles.. 02/20/2017 

Previous discussion... /issued 1030 am CST Mon Feb 20 2017/ 

..southeast Texas to Louisiana... 
A north/south-oriented band of composite-outflow-related convection 
will continue to slowly progress eastward across far southeast Texas 
into Louisiana through the afternoon and evening. While 
line-preceding cloud cover remains semi-thick late this morning, 
some pockets of stronger heating may occur ahead of the line this 
afternoon and allow for at least a modest intensification of storms 
along and immediately ahead of the line. Furthermore, low-level 
winds may modestly strengthen by early evening as these storms shift 
eastward across Louisiana. These factors suggest that a few storms 
could produce locally damaging winds and/or a brief tornado this 
afternoon into tonight, but the overall risk is currently expected 
to remain relatively marginal and isolated in nature. 

..coastal northern California... 
A modestly amplifying shortwave trough and strong mid/high-level jet 
will continue to approach northern California through this evening. 
Although temperatures will be cool and buoyancy will be very weak 
over inland areas, steepening lapse rates in the presence of strong 
deep-layer southwesterly winds could allow a few stronger storms to 
approach coastal northern California this evening or early in the 


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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