SPC AC 231250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0650 AM CST Wed Dec 23 2020

   Valid 231300Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE CENTRAL
   GULF COAST REGION...

   ...SUMMARY...
   A threat for damaging gusts and a few tornadoes will shift eastward
   across the central Gulf Coast region, this afternoon through
   tonight.

   ...Synopsis...
   The dominant upper-air feature over the CONUS will remain a high-
   amplitude, progressive, synoptic-scale trough -- initially apparent
   in moisture-channel imagery from MB south-southwestward across the
   central High Plains to Chihuahua.  Reinforced by several shortwaves
   and vorticity maxima, the associated cyclonic-flow field will shift
   westward and expand through the period, covering most of the land
   between the Rockies and Appalachians by 00Z.  By then, the major
   trough should extend from MN-NM, where it will be reshaped by a
   strong, basal shortwave perturbation now digging southeastward over
   UT.  By 12Z, the synoptic trough should extend from the upper
   Mississippi Valley across MO to central/southwest TX.

   At the surface, a primary low was analyzed over southern MN.  A cold
   front extended from there over western IA, eastern KS, western OK,
   the TX Panhandle, and central NM.  The low should reach northern WI
   or western Upper MI by 00Z and begin occluding, with cold front
   extending across southern IL, northeastern AR, and east to south-
   central TX.  By 12Z, with the occluded low over eastern Lake
   Superior, the cold front should be sweeping eastward to
   southeastward near an axis from DTW-JKL-MOB and across the
   northwestern Gulf.  A secondary surface low may develop along the
   cold front tonight, over parts of MS and AL, as the basal mid/upper-
   level shortwave perturbation pivots eastward over central TX. 

   ...Central Gulf Coast and vicinity...
   Widely scattered to scattered thunderstorms should form by mid/late
   afternoon and expand in coverage this evening, in two primary early
   episodes:
   1.  A cold-frontal band across parts of AR and northeast/east TX,
   shifting eastward across AR/LA. This activity initially will offer a
   marginal severe threat as it impinges on a boundary layer somewhat
   destabilized by both muted diurnal heating and synoptically forced
   theta-e advection.  
   2.  A nearly front-parallel but warm-sector low-level convergence
   zone from southeast TX to northern LA, likewise shifting eastward
   but not as quickly as the front.  As such, the first should overtake
   the second tonight, probably across portions of MS to southern LA,
   forming a single primary thunderstorm band.

   The combined, primary thunderstorm band, in turn, should shift
   eastward along or just ahead of the front over southeastern LA, MS,
   and portions of AL and the western FL Panhandle by 12Z.  Late
   tonight, a few thunderstorms also may move ashore in the western FL
   Panhandle within a newer prefrontal convergence zone developing over
   the adjoining Gulf.  

   Damaging gusts will be possible throughout the outlook area, along
   with at least a marginal supercellular and/or QLCS tornado threat
   for most of the distance inland.  The greatest juxtaposition of a
   favorably modified moist boundary layer with strengthening ambient
   shear and low-level hodographs will be across the "slight" area this
   evening and tonight.  Surface dew points in the low/mid 60s F --
   apparent in the partly modified warm sector over the northwestern
   Gulf now, should spread inland as far north as southern AR this
   afternoon and southern portions of MS/AL tonight, offsetting
   marginal lapse rates enough to yield around 300-800 J/kg MLCAPE. 
   Meanwhile the approaching trough and its mass response will lengthen
   the deep-shear vector and enlarge hodographs, with effective
   shear/SRH values around 45-50 kt and 150-300 J/kg respectively,
   where sustained surface-based inflow appears most probable.  

   Already modest boundary-layer lapse rates and low-level CAPE will
   diminish with northward/inland extent across the central Gulf Coast
   region, but isolated severe gusts and/or a tornado cannot be ruled
   out even near the north edges of the "marginal" area where MLCAPE
   less than 200 J/kg is expected.  The blended severe regime should
   reach the western FL Panhandle by 12Z, then continue into day-2 from
   there eastward and southeastward across parts of FL/GA.

   ..Edwards/Smith.. 12/23/2020

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