SPC AC 191256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0656 AM CST Sat Dec 19 2020

   Valid 191300Z - 201200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY ACROSS
   PARTS OF THE MIDDLE/UPPER TEXAS COAST...

   ...SUMMARY...
   Isolated strong to severe thunderstorms may occur today across parts
   of the middle/upper Texas Coast.

   ...Synopsis...
   A generally progressive upper-air pattern will prevail through the
   period, with broadly cyclonic flow spreading eastward from the
   central to eastern CONUS.  The most important feature aloft will be
   a positively tilted trough -- initially evident in moisture-channel
   imagery from northern WI across eastern NE to far west TX and
   adjoining northern MX.  This perturbation will maintain positive
   tilt while moving eastward and gradually weakening.  By 00Z the
   trough should extend from Lower MI across the Ozarks to south-
   central TX.  By 12Z tomorrow, it should reach southeastern ON,
   eastern KY and AL.

   Surface analysis showed weak lows over TX between MWL-SPS and
   between DRT-LRD, connected by a cold front.  The current south TX
   low, or a new one, should move northeastward just of the mid/upper
   TX Coast this afternoon, turning eastward along or just south of the
   LA Coast tonight.  Mesoscale convective effects may modulate the low
   track southward and/or somewhat faster.  The trailing cold front
   will move across the TX Coast and northwestern Gulf, while extending
   southwest of the low.  A weaker inland version of the front, and a
   collocated surface trough, will extend northeastward from the low. 

   ...Mid/upper TX coastal plain...
   Scattered thunderstorms are expected to develop, increase in
   coverage and spread across the outlook area from mid/late morning
   into the afternoon, offering the potential for isolated strong-
   severe gusts and marginally severe hail.  A tornado cannot be ruled
   out as well. 

   The bulk of convection should occur in the form of a primary band
   developing in a zone of relatively maximized low-level lift near the
   front.  With weak CINH in place, isolated thunderstorms also are
   possible farther east near the immediate coast and over the Gulf. 
   This evolution should occur as the zone of lift encounters a
   boundary layer steadily destabilizing from a combination of:
   1.  Warm/moist advection of a still-modifying marine layer from the
   Gulf, and 
   2.  Muted diurnal heating, amidst considerable cloud cover. 

   Although the CAPE-shear parameter space will be optimized offshore,
   deep shear will increase over the outlook area as the mid/upper
   trough approaches, with 40-50-kt effective-shear magnitudes becoming
   common.  Well-curved but marginally sized hodographs are expected,
   supporting 0-1-km SRH in the 100-200 J/kg range.  Modest low/middle
   level lapse rates will keep peak preconvective MLCAPE around 500-800
   J/kg inland near the coast, decreasing substantially northward.  A
   few high-res mesoscale progs indicate that weak surface-based
   buoyancy (i.e., MLCAPE 200-500 J/kg) briefly may extend as far
   northward as parts of the HOU metro, immediately before the main
   area of convection moves through.  As such, the marginal line has
   been adjusted somewhat northward in this area.  

   Late this afternoon into evening, the convective band should shift
   eastward over the Gulf, Golden Triangle region of southeast TX, and
   portions of southwestern LA.  This activity should outpace the
   eastward extent of inland surface-based destabilization east of the
   HOU area.  By then, the most intense convection (including supercell
   potential) will be relegated to the Gulf -- where the most well-
   modified and unstable inflow air will reside -- posing some threat
   to the various offshore interests.

   ..Edwards/Smith.. 12/19/2020

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