Mesoscale Discussion 0538
   NWS Storm Prediction Center Norman OK
   0309 PM CDT Fri Apr 21 2017

   Areas affected...Southern OK...North TX...Southwest AR...Far
   Northwest LA

   Concerning...Severe potential...Watch likely 

   Valid 212009Z - 212215Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Severe thunderstorm development is expected later this
   afternoon. Very large hail, locally damaging winds, and a few
   tornadoes will be possible. Watch issuance is anticipated in the
   next 1-2 hours.

   DISCUSSION...At 20Z, an outflow-reinforced frontal boundary has
   become nearly stationary across north TX, southeast OK, and southern
   AR. Visible imagery indicates slowly deepening cumulus along and
   south of the boundary, though wave features noted in the cloud field
   are indicative that some convective inhibition remains. Ongoing
   elevated convection across central OK is being supported by ascent
   associated with a rather strong but low-amplitude midlevel shortwave
   moving into southwest KS. This ascent will likely only glance the
   region along and south of the boundary, but, in conjunction with
   continued boundary-layer heating, will help to support convective
   initiation later this afternoon into the early evening as the
   remaining CINH is eroded. 

   Along and south of the boundary, steep midlevel lapse rates atop a
   warm/moist boundary layer are supporting moderate-to-strong
   buoyancy, with MLCAPE of 1500-2500 J/kg noted in recent
   mesoanalyses. Effective shear of 45-55 kts is more than sufficient
   for organized storm structures, with initial discrete supercell
   development expected before consolidation into more of a linear mode
   along the boundary becomes more likely into the evening. Any
   discrete supercell that develops in the warm sector will be capable
   of all severe hazards. The tornado threat will be maximized with any
   supercells that interact with the frontal boundary, especially in
   areas where some heating/recovery has occurred north of the
   boundary. 

   While the timing of deep convective development remains somewhat
   uncertain, tornado watch issuance is likely when initiation appears
   imminent, which will likely be sometime in the 2030-22Z timeframe.

   ..Dean/Goss.. 04/21/2017

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

   ATTN...WFO...LZK...SHV...TSA...FWD...OUN...SJT...

   LAT...LON   33439933 33759936 34459721 34709555 34759477 34419378
               33799340 33049350 32529418 32399524 32209622 32209740
               32469853 33069914 33439933