Mesoscale Discussion 0684
   NWS Storm Prediction Center Norman OK
   0332 AM CDT Wed May 19 2021

   Areas affected...portions of south Texas

   Concerning...Severe potential...Watch possible 

   Valid 190832Z - 191030Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Thunderstorms may develop east of the Rio Grande early
   this morning, bringing a threat for damaging gusts and hail.

   DISCUSSION...At 08z, an outflow boundary stretched westward from
   just north of Corpus Christi to just north of Laredo. South of this
   boundary, surface dewpoints are generally in the low to mid 70s F
   beneath very steep midlevel lapse rates (per 06z BRO RAOB). This is
   contributing to strong instability, with MLCAPE values greater than
   2500 J/kg present. A thunderstorm cluster has been developing near
   the higher terrain across the Rio Grande in Mexico. This activity
   will gradually spread east over the few several hours. 

   While effective shear is somewhat marginal across south Texas, a
   modest southerly low level jet coupled with strong instability
   should be sufficient for some organized cluster to track along/east
   of the Rio Grande this morning. Some CAMs depict an organized bow
   moving across this area, though timing of this feature remains
   somewhat uncertain as current convection is developing earlier than
   what has been indicated in forecast guidance. Should an organized
   cluster emerge, damaging gusts (and perhaps hail with any
   semi-discrete cells given very steep midlevel lapse rates) will be
   possible through the morning hours. Convective trends will be
   monitored and a severe thunderstorm watch may be needed in the next
   couple of hours.

   ..Leitman/Hart.. 05/19/2021

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...CRP...EWX...BRO...

   LAT...LON   26539933 26909965 28090041 28300043 28460034 28629979
               28669943 28599866 28309806 28149790 27799774 27119776
               26469799 26049830 26099873 26219892 26539933