U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 210055 
Storm Prediction Center ac 210053 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0653 PM CST Mon Feb 20 2017 

Valid 210100z - 211200z 

..there is a marginal risk of severe thunderstorms tonight near 
southeast Louisiana coastal areas... 

A couple of strong thunderstorms are possible near southeastern 
Louisiana coastal areas tonight, perhaps accompanied by some severe 
weather potential. 

Much of North America remains under the influence of split branches 
of westerlies emanating from the mid-latitude Pacific. Despite the 
presence of relatively mild conditions across much of the U.S. In 
association with this regime, thermodynamic profiles are generally 
stable across most areas, with low or negligible convective 

..Mississippi Valley/northwestern Gulf Coast region... 
A narrow plume of higher moisture content air has returned northward 
across northwestern Gulf coastal areas through the upper Midwest, 
ahead of sheared/elongated troughing within the westerlies. This 
troughing within both the northern and southern streams has largely 
been in phase as it progresses across the plains. But the northern 
stream troughing is forecast to increasingly pivot northeastward and 
eastward more rapidly than the southern stream troughing, as a 
strong upstream mid/upper jet streak continues to nose inland across 
northern California and the northern Great Basin. Within the 
southern stream troughing, models suggest that modest lower/mid 
tropospheric cyclogenesis may occur tonight across parts of eastern 
Texas into the lower Mississippi Valley. 

Isolated to widely scattered thunderstorm activity has been 
occurring early this evening, within a narrow convective band, 
generally along the leading edge of remnant composite outflow 
generated by extensive convection last night. It appears that a 
risk for thunderstorms may linger across eastern Arkansas through 
portions of northeastern Missouri/west central Illinois another 
couple of hours (into the 02-04z time frame). However, highest 
thunderstorm probabilities probably will become increasingly 
confined to portions of The Ark-la-tex and lower Mississippi Valley, 
southward into the Gulf of Mexico, aided by large-scale forcing for 
ascent associated with the evolving southern branch perturbation. 

Inland of immediate coastal areas, most thunderstorm activity seems 
likely to remain rooted above at least a shallow surface-based 
stable layer, which is expected to mitigate any appreciable severe 
weather potential. However, the environment over the open waters of 
the north central Gulf of Mexico appears at least marginally 
supportive of organized convection, including supercells. And 
convection capable of producing gusty winds and/or a tornado may not 
be out of the question near southeast Louisiana coastal areas 

..northern California... 
Cooling aloft associated with an inland migrating upper impulse may 
be accompanied by some risk for thunderstorm activity across coastal 
areas into northern portions of the Sacramento Valley by the 05-07z 
time frame. In the vicinity of a 100+ kt 500 mb jet streak, 
low-level and deep layer shear probably will become supportive of 
organized severe storm development, perhaps including supercells, 
given sufficient boundary layer destabilization. Across and inland 
of coastal areas, this remains too uncertain for an outlook of 
appreciable severe probabilities. 

.Kerr.. 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... 


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