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Mesoscale Discussion 1857
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   Mesoscale Discussion 1857
   NWS Storm Prediction Center Norman OK
   0750 PM CDT Wed Oct 13 2021

   Areas affected...Parts of Deep South Texas and the adjacent lower
   Rio Grande Valley

   Concerning...Severe potential...Watch unlikely 

   Valid 140050Z - 140245Z

   Probability of Watch Issuance...20 percent

   SUMMARY...Increasing thunderstorm development near and just east of
   the Interstate 35 corridor of Deep South Texas will intensify
   further during the next few hours, and perhaps occasionally pose a
   risk for severe hail and locally strong surface gusts.  It seems
   unlikely that a severe weather watch will be needed, but trends will
   continue to be monitored.

   DISCUSSION...Warm elevated mixed-layer air has suppressed deep
   convective development into early evening across the lower Rio
   Grande Valley, but thunderstorms are now initiating around Laredo
   northward into the San Antonio area.  It appears that this is being
   supported by another mid-level perturbation emerging from the
   Mexican Plateau, which is forecast to be accompanied by substantive
   mid-level cooling overspreading the Rio Grande River vicinity
   through a corridor along and east of Interstate 35 between now and
   03-05Z.  This will coincide with a narrow plume of tropical moisture
   return (including PW of 2-2.25+ inches) from the the western Gulf of
   Mexico, which is contributing to large CAPE in the presence of
   modestly steep mid-level lapse rates.

   Given this thermodynamic environment, shear associated with a belt
   of 30-40 kt south-southwesterly flow in the 700-500 mb layer may be
   sufficient to support the evolution of a few supercell structures. 
   While modest to weak southerly lower-level flow probably will remain
   a limiting factor concerning the overall severe weather potential,
   stronger cells might still be able to occasional pose a risk for
   severe hail hail and locally strong surface gusts.

   ..Kerr/Guyer.. 10/14/2021

   ...Please see for graphic product...


   LAT...LON   27169964 28419938 29389882 30029835 29819768 28519822
               27169862 26059930 27169964 

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