Mesoscale Discussion 1789
   NWS Storm Prediction Center Norman OK
   0701 PM CDT Tue Sep 28 2021

   Areas affected...Portions of south Texas

   Concerning...Severe potential...Watch unlikely 

   Valid 290001Z - 290130Z

   Probability of Watch Issuance...5 percent

   SUMMARY...Isolated strong/severe storms ongoing near the Mexican
   border and far south Texas may produce a few severe wind or hail
   reports over the next several hours. Storm coverage is expected to
   gradually decrease with the loss of diurnal heating suggesting a
   watch is not needed.

   DISCUSSION...On the southern fringes of a weak upper low observed
   from regional WV imagery across the southern Rockies, isolated to
   widely scattered thunderstorms are ongoing across portions of
   northern Mexico and southern Texas. Surface obs and SPC mesoanalysis
   show these storms are ongoing in a warm, moist and unstable
   environment with 2000-2500 J/kg of MLCAPE and 2-6km lapse rates of
   7-8 C/km. Vertical shear remains weak with only 25-30 kts of
   deep-layer shear in place owing to relatively weak mid-level flow.
   The favorable buoyancy/shear profiles should be sufficient for a few
   organized multicells and perhaps a weak supercell.

   As ongoing storms move to the east northeast with time, occasional
   strengthening is possible, with a risk for damaging downburst winds
   or hail. Recent radar trends have shown this with two lead storms
   over Zavala and Frio counties over the last half hour. The
   downstream environment should remain favorable for a lower-end
   severe risk for at least a couple more hours. After sunset,
   increasing nocturnal inhibition and the movement of the main
   synoptic ascent from the upper low away from convection should
   result in a gradual downtrend in coverage/intensity. Storms may
   linger for a few hours after dark across the southern parts of the
   discussion area, closer to the coast, where a greater reservoir of
   warm/unstable air exists. However, weak vertical shear suggests
   little organization potential and overall confidence in convective
   evolution is low. Given the limited spatial/temporal threat window
   for wind and hail, a weather watch is unlikely.

   ..Lyons/Grams.. 09/29/2021

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

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

   LAT...LON   29080079 29460052 29710014 29899944 29899854 29269720
               29039714 28729709 28359717 27989743 27769818 27769828
               27829910 27879942 27889997 29080079