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Mesoscale Discussion 1171
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   Mesoscale Discussion 1171
   NWS Storm Prediction Center Norman OK
   0328 PM CDT Wed Jun 28 2017

   Areas affected...Eastern WY...Western SD...Eastern MT

   Concerning...Severe potential...Watch unlikely 

   Valid 282028Z - 282230Z

   Probability of Watch Issuance...20 percent

   SUMMARY...Isolated severe thunderstorms are possible this afternoon,
   with a threat of hail and locally damaging wind.

   DISCUSSION...Widely scattered thunderstorms have developed from
   eastern MT/WY into western SD, in conjunction with diurnal heating
   and in advance of a low-amplitude upper trough moving through the
   interior Northwest. 

   The strongest convection within the MCD area has thus far been
   across northeast MT, in the vicinity of a weak surface boundary
   where winds are favorably backed out of the east, resulting in
   effective shear of 35-45 kt and effective helicity of 100-150 m2/s2.
   In conjunction with SBCAPE of 500-1000 J/kg, the wind profile in
   this area will continue to support organized convection in the short
   term, including transient supercell structures with a threat of
   large hail and locally damaging wind. Strong drying/mixing across
   far eastern MT and western ND should limit the severe threat with
   eastward extent across this area. 

   Further south, additional clusters of thunderstorms are moving
   across southeast MT and eastern WY, and an isolated strong cell has
   recently developed south of Rapid City. While wind profiles are not
   quite as favorable compared to northeast MT, effective shear of
   25-35 kt in conjunction with SBCAPE of 500-1250 J/kg will support
   the potential for modestly organized clusters and perhaps a
   supercell or two, capable of large hail and locally damaging wind. 

   In general, the magnitude and coverage of the severe threat across
   these areas are expected to remain too low for watch issuance.

   ..Dean/Guyer.. 06/28/2017

   ...Please see for graphic product...


   LAT...LON   42970449 43020476 43200505 43760506 44570517 46620596
               47940612 48090449 45770401 44050259 43140254 43100273
               43050316 43030410 42970449 

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