SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0638 AM CST Thu Dec 31 2020

Valid 311300Z - 011200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS TODAY INTO
EARLY TONIGHT FROM THE UPPER TX COAST TO SOUTHWESTERN MS...

...SUMMARY...
A few tornadoes, damaging winds, and isolated large hail will be
possible today along the upper Texas coast, and through tonight
across parts of Louisiana, Mississippi, and Alabama.

...Upper TX coast today to MS/AL overnight...
A midlevel closed low south of the Big Bend will eject northeastward
toward the Ozarks by early Friday, in response to an upstream
shortwave trough digging south-southeastward from northern CA toward
northwestern Mexico.  At the surface, a 1004 mb low as of 12z will
develop inland across the upper TX coastal plain today along a
quasi-stationary front, and the low will continue
north-northeastward toward the Ark-La-Tex tonight.  The warm sector
(with boundary-layer dewpoints in the mid-upper 60s) will spread
inland to the east of the cyclone and contribute MLCAPE of 500-1000
J/kg today across extreme southeast TX/southwest LA, and into
tonight across southern MS and southwestern AL.

Thunderstorm development this morning will be focused along the
front near the developing surface cyclone along the upper TX coastal
plain.  Convection will spread northeastward through tonight as the
cold front surges eastward across LA/MS to the south and southeast
of the cyclone.  Effective bulk shear of 45-60 kt will be favorable
for organized severe storms on the nose of a 70-90 kt midlevel jet,
and low-level shear/hodograph curvature (effective SRH of 200-300
m2/s2) will be maximized in the northern portions of the warm sector
in conjunction with a southerly 40-50 kt low-level jet.  

The combination of linear forcing for ascent along the cold front
and substantial cross-boundary flow and deep-layer shear vectors
will support a somewhat broken band of supercells and line segments.
 Embedded supercells will also be possible in a loose band along a
secondary moisture plume from the southeastern Gulf of Mexico toward
southwestern AL by tonight.  Surface-based buoyancy and the strong
vertical shear will support the potential for a few tornadic
supercells, damaging gusts, and isolated marginally severe hail. 
However, the magnitude of the tornado threat could be tempered some
by rather modest low-level lapse rates inland, and less-than-ideal
phasing with the ejecting midlevel trough and surface cyclone
glancing the northwest side of the surface warm sector.

..Thompson/Gleason.. 12/31/2020

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