SPC 1200Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1132 PM CST Tue Dec 22 2020

Valid 231200Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
SOUTHERN LOUISIANA AND FAR SOUTHERN MISSISSIPPI...

...SUMMARY...
A few severe thunderstorms are expected from late afternoon through
tonight from the upper Texas Coast across much of southern Louisiana
and Mississippi, with isolated activity from southern Alabama into
the western Florida Panhandle by Thursday morning.

...Synopsis...
A large, positive-tilt upper trough will progress across the Plains
today, and will take on a neutral orientation as it approaches the
MS Valley by 12Z Thursday. Ahead of this trough, a broad belt of
moderate southwesterlies aloft will develop across much of the
eastern CONUS, with steady height falls/cooling aloft. The strongest
lift is forecast to occur after 00Z from the Sabine River across MS
and into AL and TN, where strengthening low-level convergence will
increasing divergence aloft. 

At the surface, the primary surface low will develop over WI and
Upper MI during the day, with a cold front across the MS Valley and
upper TX Coast. A secondary low is forecast to develop overnight
across MS and AL, coincident with the aforementioned large-scale
ascent.

Surface dewpoints will increase into the mid to upper 60s F along
the TX Coast by 00Z, with mid 60s F across southern LA and coastal
MS/AL/FL Panhandle. This front will provide the primary focus for
severe weather, including damaging gusts, and a couple tornadoes.

...Southeast TX, southern LA, MS, AL, western Florida Panhandle...
Scattered, non-severe storms are possible during the afternoon from
southeast TX into LA preceding the cold front where heating and
moisture advection will result in an uncapped air mass. Storms are
expected to intensify after 00Z, near the TX/LA border, becoming
numerous along the front overnight. 

The most favorable combination of surface-based effective parcels as
well as low-level SRH should develop across southern LA during the
late evening, then across far southern MS, AL and into the western
FL Panhandle by 12Z Thursday. The main threat is expected to be
damaging gusts with the linear structures along the front, but any
isolated or embedded cells may produce a tornado as effective SRH
increases to around 300 m2/s2, while maintaining modest midlevel
lapse rates which will boost updraft intensity.

Elsewhere, while lift will be strong across other parts of MS and
AL, forecast soundings indicate the surface air mass may remain too
cool for a severe threat.

..Jewell/Gleason.. 12/23/2020

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