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Mesoscale Discussion 93
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   Mesoscale Discussion 0093
   NWS Storm Prediction Center Norman OK
   0234 PM CST Sat Jan 21 2017

   Areas affected...Portions of southern MS...LA

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

   Valid 212034Z - 212300Z

   Probability of Watch Issuance...80 percent

   SUMMARY...The risk for severe thunderstorms capable of large hail,
   damaging winds, and tornadoes will be increasing through the late
   afternoon and into the evening. The issuance of a Tornado Watch is
   likely within the next few hours.

   DISCUSSION...The isallobaric response to 2-hour pressure falls on
   the order of 2-4 mb across the lower MS Valley will be the continued
   northward advance of a warm front analyzed approximately west-east
   across southern LA. The air mass south of this boundary is
   characterized by surface dewpoints in the middle 60s to around 70F,
   supporting moderate to locally strong buoyancy -- aided by steep
   midlevel lapse rates sampled by the 18Z Slidell sounding (around 8
   C/km in the 700-500-mb layer). As this air mass continues to spread
   northward within a modest warm-advection regime well in advance of
   stronger deep ascent farther west, diurnally reduced capping and
   modest ascent will likely support an increase in surface-based
   thunderstorm development -- especially during the late afternoon and
   evening as more vigorous ascent approaches. Initial shallow
   convective towers are already evolving along the warm frontal zone
   across parts of southern LA, in proximity to pockets of stronger
   insolation, per visible imagery.

   While there is some uncertainty regarding the timing of more robust
   convective development given the aforementioned capping, effective
   shear around 45-60 kt will likely support supercell structures.
   Initially, large hail will be likely, though elongating/increasingly
   curved low-level hodographs into the evening hours suggest that the
   tornado risk will likely increase. Damaging winds will also be
   possible in conjunction with small-scale, congealing convective

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


   LAT...LON   31358876 31878999 31919152 31439294 31209331 30679310
               30109167 30188977 30568868 31008843 31358876 

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