SPC AC 301630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1030 AM CST Wed Dec 30 2020

   Valid 301630Z - 311200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS EAST-CENTRAL TO
   DEEP SOUTH TX...

   ...SUMMARY...
   Isolated severe hail and damaging winds, along with a brief tornado
   will be possible through tonight across parts of east-central to
   Deep South Texas.

   ...East-central to Deep South TX...
   No appreciable change appears needed to the cat 1/MRGL risk area,
   although an adjustment has been made to expand the underlying
   wind/tornado probabilities for this afternoon's convective threat in
   east-central/southeast TX.

   A shortwave trough over western Chihuahua will gradually progress
   east-southeast towards the Coahuila/Durango border area through 12Z
   tomorrow. In the wake of a northern stream shortwave trough, a
   surface cold front will move southeast across eastern to south TX
   through the period. The movement of the front will slow later
   tonight in response to weak cyclogenesis near the lower TX, in the
   zone of ascent downstream from the northern Mexico trough. This
   pattern will also maintain a broad southerly low-level jet across
   the TX coast that should diurnally weaken through sunset before
   strengthening again late tonight, with a continued inland transport
   of mid to upper 60s boundary-layer dewpoints.

   The severe threat will probably occur within two phases, one this
   afternoon across a portion of east-central/southeast TX and the
   other overnight towards the middle TX coast. A pre-frontal
   confluence arc of convection is ongoing just ahead of the impinging
   cold front across central TX. A few cloud breaks ahead of this arc
   should yield thin MLCAPE increasing to between 500-1000 J/kg. While
   guidance generally suggests low-level flow will subside somewhat
   this afternoon (with the RAP appearing more aggressive than the
   NAM/ECMWF), modest hodograph curvature might prove sufficient for a
   lower-end supercell or two capable of producing all hazards. The
   undercutting nature of the front will further limit longevity of any
   surface-based supercell. After a potential lull in the severe threat
   this evening, the risk for a severe storm or two should slowly
   increase after 06Z tonight as deep-layer and low-level shear
   gradually strengthen in the confined warm sector near the lower to
   middle TX coast. The bulk of a greater tornado threat should still
   await until after 12Z in the D2 period.

   ..Grams/Dean.. 12/30/2020

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