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Mesoscale Discussion 1342
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   Mesoscale Discussion 1342
   NWS Storm Prediction Center Norman OK
   0758 PM CDT Tue Jul 18 2017

   Areas affected...Portions of western SD/NE

   Concerning...Severe potential...Watch unlikely 

   Valid 190058Z - 190300Z

   Probability of Watch Issuance...20 percent

   SUMMARY...An isolated supercell producing large hail and strong to
   damaging winds will likely continue this evening. Watch issuance
   remains unlikely due to the very isolated severe threat.

   DISCUSSION...Isolated thunderstorms developed this afternoon within
   a post-frontal upslope flow regime across the higher terrain of
   southern MT and eastern WY, and have subsequently moved
   east-southeastward into western SD/NE this evening. 00Z sounding
   from Rapid City SD shows steep low and mid-level lapse rates, which
   combined with modest low-level moisture, is resulting in weak to
   locally moderate instability, with MLCAPE generally 500-1500 J/kg.
   Although flow below 700 mb remains weak, winds do veer and
   strengthen with height, particularly above 500 mb, which is
   resulting in 40-50 kt of effective bulk shear, sufficient to support
   supercell structures. Recent radar trends suggest the northern
   supercell (across northwestern SD as of 0055Z) remains capable of
   producing large hail, perhaps some greater than 2 inches in
   diameter, along with strong to damaging winds. An isolated supercell
   moving across eastern WY into western NE has since weakened as it
   moved into an area with more convective inhibition. The northern
   storm has the potential to remain severe over the next few hours as
   it moves southeastward across western SD along an instability
   gradient. A modestly strengthening southeasterly low-level jet may
   also support storm intensity this evening. Given that the overall
   severe threat is expected to remain very isolated, watch issuance is

   ..Gleason/Edwards.. 07/19/2017

   ...Please see for graphic product...


   LAT...LON   45220374 45210292 44920191 44410149 43060100 42090179
               41700267 41860368 42190397 45000395 45220374 

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