ACUS01 KWNS 182002
SWODY1
SPC AC 182000
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0200 PM CST Thu Dec 18 2025
Valid 182000Z - 191200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE MS/TN/OH VALLEYS INTO THE SOUTHEAST...AND ALSO ACROSS COASTAL
NC...
...SUMMARY...
Isolated strong to severe thunderstorms capable of producing
occasional damaging winds and perhaps a tornado or two may occur
through this evening across parts of the lower/mid Mississippi
Valley into the Tennessee and Ohio Valleys and Southeast, and late
tonight across portions of coastal North Carolina.
...MS/OH/TN Valleys into the Southeast...
The primary changes to the Marginal Risk have been to clear areas
behind the low-topped convective line and cold front, and to expand northeastward somewhat based on where recent HRRR/RRFS guidance
maintains a low-topped but well-defined frontal band with potential
for localized strong/damaging gusts.
A line of low-topped convection has developed along a cold front and
is moving eastward across parts of IL and southeast MO this
afternoon. Despite very weak to negligible buoyancy, very strong
low-level flow/shear (with 50+ kt at 1 km AGL per regional VWPs) has
supported occasional gusts of 40-55 mph and isolated wind-damage
reports along this line. Forcing related to a strong mid/upper-level
shortwave trough moving quickly eastward across the region will help
to sustain a low-topped frontal band into this evening, which may be
capable of producing sporadic strong/damaging gusts into parts of
the Ohio Valley.
Recent HRRR/RRFS runs suggest that gusty winds associated with the
frontal band may spread across parts of the Allegheny Plateau
overnight, but with essentially nil buoyancy expected in this area,
confidence is currently too low to expand wind probabilities into
this area.
Farther south into parts of TN/MS/AL, somewhat greater (though still
weak) buoyancy will be in place later this afternoon into the early
evening, as 60s F dewpoints spread northward to near the TN/MS/AL
border region. A broken band of storms may develop in advance of the
front, with potential for at least marginal supercell structures and
an attendant threat of isolated damaging wind and possibly a
tornado. Any longer-lived cells may tend to merge into the
eastward-moving frontal band with time, with at least an isolated
severe threat spreading east across TN through the evening.
...Coastal NC...
No major changes have been made to the Marginal Risk across coastal
NC. See the previous discussion below for more information.
...FL Panhandle vicinity...
Cells with occasional rotation have been noted offshore of the FL
Panhandle this afternoon, though temperatures remain relatively cool
with widespread cloudiness inland. There is some potential for a
stronger cell or two to eventually approach the coast or develop
slightly inland, but lingering near-surface stability may tend to
limit the severe threat across this area.
..Dean.. 12/18/2025
.PREV DISCUSSION... /ISSUED 1031 AM CST Thu Dec 18 2025/
...Tennessee Valley/Lower Ohio Valley into the Southeast...
A pronounced mid/upper-level shortwave trough with attendant 70-90
kt mid-level jet will advance quickly eastward today across the
Upper Midwest into the MS/OH/TN Valleys. An associated deep surface
low over northern MN this morning is forecast to continue developing northeastward into Ontario/Quebec through tonight, with a trailing
cold front forecast to sweep east-southeastward over much of the
mid/lower MS Valley into the OH/TN Valleys and Southeast. Low-level
moisture return ahead of the cold front remains quite limited,
except for parts of MS/AL where low to mid 60s surface dewpoints are
beginning to spread inland. Even so, wind fields remain quite
favorable for thunderstorm organization, with ample low-level and
deep-layer shear present. Current expectations are for a low-topped
line of convection to further develop along the cold front through
this afternoon/evening. Although instability is expected to remain
minimal with northward extent into the OH/TN Valleys, there is still
potential for strong to severe gusts that will be convectively
enhanced beyond the already strong background gradient wind field.
Some chance for a tornado may also exist across parts of MS/AL where
greater instability/low-level moisture will reside. Given latest
observational and guidance trends, a Marginal Risk has been included
where the best potential for occasional strong to damaging winds is
apparent.
...Coastal North Carolina...
Isolated strong to locally severe convection may occur late tonight
through early Friday morning as an increasingly negative-tilt
mid-level trough and a related robust strengthening of the
deep-layer wind field approach eastern NC. The gradual modification
and erosion of continental/cold air is a key uncertainty due to
persistent surface high pressure along the East Coast into western
Atlantic. Some guidance such as the 12Z NAM may be a bit too
aggressive regarding late-night near-surface-based destabilization.
Still, the main area of concern is for the immediate NC coastal
areas/Outer Banks, where somewhat greater airmass
modification/low-level moistening should occur amid strong low-level
and deep-layer shear. If surface-based thunderstorms can form and
spread northeastward across this area, they could pose a threat for
isolated severe/damaging winds and perhaps a tornado. But, it is
also possible that convectively enhanced wind gusts materialize
inland (including Piedmont areas) in the predawn hours, although
confidence in this occurring remains low.
$$
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