Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 161729
SWODY2
SPC AC 161728

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1128 AM CST Sat Dec 16 2017

Valid 171200Z - 181200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL GULF COAST STATES AND LOWER MISSISSIPPI VALLEY...

...SUMMARY...
A few damaging wind gusts and perhaps an isolated/brief tornado
threat may occur on Sunday with thunderstorms across parts of the
central Gulf Coast region, including the lower Mississippi Valley.

...Synopsis...
Southwesterly mid/upper-level flow will extend across the southern
Plains and Southeast on Sunday. Within this flow, a shortwave trough
will likely be located across parts of the lower/mid MS Valley
Sunday morning. This shortwave trough will continue quickly
northeastward across the OH/TN Valleys to the Mid-Atlantic by Sunday
evening. A 35-50 kt south-southwesterly low-level jet associated
with the shortwave trough will be present across the central Gulf
Coast/lower MS Valley region at the start of the period, and will
likely shift northeastward across AL/GA through Sunday afternoon. At
the surface, a weak low should develop east-northeastward across LA
and southern MS through the day, while a warm front extends eastward
from the low across southern LA/MS/AL.

Farther west, a closed upper low over northwestern Mexico and
southern Arizona should move little through the period. Isolated
thunderstorms may occur Sunday afternoon/evening beneath this upper
low owing to cold mid-level temperatures supporting modestly steep
mid-level lapse rates and resultant weak instability.

...Central Gulf Coast/Lower MS Valley...
Scattered thunderstorms will likely be ongoing at the start of the
period from south-central LA into parts of southern MS in
association with lift attendant to the previously mentioned
low-level jet and mid-level shortwave trough. The best overlap of
weak instability and strong effective bulk shear will likely occur
early in the period (Sunday morning through early afternoon) along
and south of the warm front that will be draped east-west across
southern LA/MS. Surface dewpoints in the low to mid 60s will likely
be present in the warm sector, and may be sufficient to support
surface-based convection, particularly across the immediate coastal
areas of LA. Given the strength of low-level winds, isolated strong
to damaging winds may occur with the more robust convection Sunday
morning and early afternoon. An isolated/brief tornado also cannot
be ruled out with strongly veering/strengthening of low-level winds
in the 0-3 km layer. This marginal severe weather threat should
diminish by Sunday evening as large-scale lift with the shortwave
trough becomes increasingly removed from the Southeast, low-level
winds veer to southwesterly and weaken, and the loss of diurnal
heating reduces already weak instability.

..Gleason.. 12/16/2017

$$



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