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Mesoscale Discussion 52
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   Mesoscale Discussion 0052
   NWS Storm Prediction Center Norman OK
   1257 PM CST Sun Jan 15 2017

   Areas affected...Portions of southwest TX

   Concerning...Severe potential...Severe Thunderstorm Watch likely 

   Valid 151857Z - 152130Z

   Probability of Watch Issuance...80 percent

   SUMMARY...The risk for severe thunderstorms will increase and spread
   across the region during the next several hours. While there is some
   uncertainty regarding the timing of more substantial increase in
   severe risk, the issuance of a Severe Thunderstorm Watch is likely
   within the next few hours.

   DISCUSSION...Arcs of convection are spreading eastward/northeastward
   across portions of West TX, within a region of strong DCVA preceding
   an upper cyclone emerging over the TX Trans-Pecos region, and at the
   leading edge of an attendant midlevel speed maximum/dry slot.
   Antecedent continental polar air ahead of this activity continues to
   modify, with a warm front advancing northward across portions of the
   Edwards Plateau and vicinity. Two-hour surface pressure falls around
   2.0-3.5 mb highlight the ongoing warm advection accompanying a 30-40
   kt low-level jet, indicated by area VAD wind profiles. This will
   support additional northward motion of the boundary during the next
   several hours, especially as the upper cyclone emerges over the
   Plains. As a result, higher theta-e air south of the boundary --
   presently characterized by dewpoints in the middle 60s -- will
   continue to build northward. With modest diurnal heating of this air
   mass, inflow for convection along the southern end of the
   aforementioned arcs of convection should gradually become
   surface-based during the next few hours.

   Present indications are that the severe risk -- aided by 45-55 kt of
   effective shear combining with 500-1250 J/kg of MLCAPE near and
   south of the boundary -- will increase in the 20Z-22Z time frame,
   and develop eastward/northeastward. A quasi-linear mode may tend to
   support primarily a severe-wind risk. However, sufficiently steep
   midlevel lapse rates may promote an isolated severe-hail risk --
   particularly with line-embedded/leading supercell structures. Ample
   low-level shear could support a tornado or two, though the
   undercutting nature of line-related outflow, combined with the
   ongoing incipient stages of boundary-layer destabilization, cast
   doubt on a more substantial tornado risk.

   ..Cohen/Guyer.. 01/15/2017


   LAT...LON   29940188 31120166 31820058 31859969 31259928 30159939
               29500099 29940188 

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