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Mesoscale Discussion 807
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   Mesoscale Discussion 0807
   NWS Storm Prediction Center Norman OK
   0354 PM CDT Sun May 21 2017

   Areas affected...Deep South TX

   Concerning...Severe potential...Watch possible 

   Valid 212054Z - 212230Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Isolated severe thunderstorms, with a threat for large
   hail and damaging winds, will spread into portions of Deep South
   Texas over the next several hours.

   DISCUSSION...Deep convection has gradually advected east of the
   highest terrain of northern Mexico.  A large cluster of
   thunderstorms has now propagated toward the Rio Grande Valley and a
   few robust storms will soon move east of the international border. 
   The strongest storm, over Webb County, likely contains severe hail
   and is propagating slowly east just north of a stationary front. 
   For some time the eastern-most convection has struggled to maintain
   intensity as it shifts east of the border; however, boundary-layer
   heating across Deep South TX has contributed to significant buoyancy
   as temperatures are near 90F just south of the front.  There is some
   concern the storms over Webb County, and adjacent convection west of
   the river, may propagate into a small corridor of
   Webb/Zapata/Duval/Jim Hogg counties.  Given the limited extent of
   influence it's not clear whether a WW will be issued.  Even so,
   large hail and damaging winds could accompany this activity.

   ..Darrow/Weiss.. 05/21/2017

   ...Please see for graphic product...


   LAT...LON   28259967 28069879 27439835 26609862 26599941 27630028

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