SPC AC 311238

   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

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z