SPC MD 538

MD 0538 CONCERNING SEVERE POTENTIAL...WATCH LIKELY FOR SOUTHERN OK...NORTH TX...SOUTHWEST AR...FAR NORTHWEST LA

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 

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