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Mesoscale Discussion 800
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   Mesoscale Discussion 0800
   NWS Storm Prediction Center Norman OK
   1256 PM CDT Sat May 20 2017

   Areas affected...Portions of south-central TX

   Concerning...Severe Thunderstorm Watch 253...

   Valid 201756Z - 201930Z

   The severe weather threat for Severe Thunderstorm Watch 253

   SUMMARY...An isolated large hail and damaging wind threat should
   persist across Severe Thunderstorm Watch 253 this afternoon.

   DISCUSSION...A supercell producing sporadic large hail has recently
   moved into Frio County TX, and it remains the primary severe threat
   across south-central TX in the short term. A slowly southward-moving
   surface cold front extends from near Austin to the vicinity of Del
   Rio TX per latest surface observations and visible satellite
   imagery. Even though mid-level heights are forecast to rise slightly
   across south-central TX this afternoon, additional showers and
   thunderstorms are forming mainly along and south of the front,
   possibly in association with a ribbon of vorticity emanating from
   the higher terrain of northern Mexico. The thermodynamic environment
   will remain favorable for sustained convection this afternoon, as
   rich low-level moisture evident in the 12Z sounding from CRP/latest
   surface observations and steep mid-level lapse rates are
   contributing to MLCAPE of generally 1500-2500 J/kg along and south
   of the front. Although low-level winds are not overly strong per
   recent VWPs from CRP, they strongly veer to westerly and strengthen
   with height at mid/upper levels. 40-55 kt of resultant effective
   bulk shear will easily support supercell structures with both a
   large hail and damaging wind threat. Recent runs of the operational
   and experimental HRRR suggest a slow southeastward movement of
   ongoing/additional convection across the Severe Thunderstorm Watch
   this afternoon, with some potential for clustering with time.

   ..Gleason.. 05/20/2017

   ...Please see for graphic product...


   LAT...LON   28180028 29200084 29610073 29629692 28569702 28149756
               28059789 28039941 28180028 

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