ACUS01 KWNS 181253
SWODY1
SPC AC 181251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0651 AM CST Wed Dec 18 2024

Valid 181300Z - 191200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE TENNESSEE VALLEY REGION...

...SUMMARY...
Isolated damaging wind gusts and a brief tornado are possible into
early afternoon across parts of the Tennessee Valley region.

...Synopsis...
A continuing progressive pattern aloft will be dominated by two main
shortwave troughs:  a trailing perturbation now over the coastal
Pacific Northwest, and a positively tilted one apparent in moisture-
channel imagery near a DBQ-STJ-P28-DHT axis.  The latter will be the
main mid/upper-level convective influence this period, as it
elongates and tracks to near a PIT-LEX-ELD-GLS line by 00Z.  The
southern part of the trough is progged to amplify and become less
positively tilted as it crosses the Mississippi Delta region
tonight, reaching northern GA, eastern AL and the western FL
Panhandle by 12Z.

At the surface, 11Z analysis showed a cold front across parts of
northern/western KY, western TN, southern AR, and northeast/central/
southwest TX.  By 00Z, the front should extend from a low over
northern NJ southwestward, roughly along the Blue Ridge, then over
northern GA to the FL Panhandle.  By the end of the period, the cold
front should be off all the Atlantic Coast except central FL,
extending southwestward to the southern Gulf.

...TN and vicinity...
Isolated, damaging to marginally severe gusts and/or a brief tornado
are possible into the afternoon, mainly over portions of TN.  A
near-frontal band of thunderstorms was ongoing from the Arklatex
region across the Mid-South, to central KY.  Ahead of this activity,
a northward-narrowing corridor of 60s F surface dewpoints (and
related near-surface-based effective-inflow parcels) was evident,
supporting 250-750 J/kg MLCAPE from near MEM to northwest of BNA,
where the 12Z sounding still showed a stable boundary layer.  See
SPC Mesoscale Discussion 2272 for near-term info.

Weak, continuing, preconvective theta-e advection, and perhaps a few
deg F of cloud-restrained diurnal warming, will offset modest
midlevel lapse rates enough to maintain and perhaps slightly
increase surface-based buoyancy eastward across TN through early/mid
afternoon.  Although flow ahead of the QLCS that has not already
done so should veer to south-southwest or southwest, enough
hodograph enlargement will remain to support around 100-150 J/kg
0-500m SRH and 200-300 J/kg effective SRH, amid 35-45-kt
effective-shear magnitudes.  As such, the main concern will arise
from episodic, embedded BOW/LEWP formations and accompanying
mesocirculations.  The convective band should outpace the already
marginally unstable boundary layer by midafternoon.

..Edwards/Goss.. 12/18/2024

$$

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