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Mesoscale Discussion 92
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   Mesoscale Discussion 0092
   NWS Storm Prediction Center Norman OK
   0233 PM CST Sat Jan 21 2017

   Areas affected...Portions of northwest LA...northeast TX...southern
   AR...far southeast OK

   Concerning...Severe potential...Tornado Watch likely 

   Valid 212033Z - 212300Z

   Probability of Watch Issuance...95 percent

   SUMMARY...The risk for severe thunderstorms capable of producing
   tornadoes, damaging winds, and very large hail is increasing this
   afternoon, and Tornado Watch issuance is likely within the next few

   DISCUSSION...A warm front continues advancing northward from
   portions of east-central TX through southern LA, within a
   warm-advection plume preceding midlevel troughing over the
   South-Central States. Embedded within broader-scale midlevel
   cyclonic flow, water vapor loops indicate a smaller-scale impulse
   advancing northeastward toward the lower Sabine Valley. Ascent
   associated with these features is expected to support thunderstorm
   development from portions of northeast TX through west-central LA --
   particularly after 22Z -- with activity subsequently spreading
   east-northeastward/eastward into the evening hours. Initial
   boundary-layer cumulus development is already noted across portions
   of east-central TX, southeast of Corsicana.

   Ample low-level moisture south of the warm front, marked by surface
   dewpoints in the middle 60s, will continue to build northward in
   response to southerly low-level mass fluxes promoted by the
   approaching upstream trough. With a well-defined elevated mixed
   layer sampled by the 18Z Shreveport sounding featuring steep lapse
   rates in the 800-550-mb layer, moderate instability will continue to
   develop northward into the zone of developing convection. With 45-60
   kt of effective shear, and an anticipated initial discrete and
   semi-discrete convective mode, significantly severe hail will be
   possible with supercell structures. The risk for damaging winds will
   increase as convection organizes into smaller-scale clusters with
   embedded bowing/rotating convective elements. While not particularly
   strong, low-level flow will be increasing through the evening. The
   resultant enlargement of low-level hodographs will foster an
   increasing tornado risk, especially with convection evolving in
   proximity to the warm frontal zone and given the steep midlevel
   lapse rates.

   ..Cohen/Hart.. 01/21/2017


   LAT...LON   32459214 31689246 31499380 32169451 34019517 34569449
               34289285 33499230 32459214 

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