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Mesoscale Discussion 1343
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MD 1343 graphic

   Mesoscale Discussion 1343
   NWS Storm Prediction Center Norman OK
   0236 AM CDT Wed Jul 19 2017

   Areas affected...Parts of central and eastern Montana into adjacent
   western Dakotas

   Concerning...Severe potential...Watch unlikely 

   Valid 190736Z - 190930Z

   Probability of Watch Issuance...20 percent

   SUMMARY...Increasing thunderstorm development with at least some
   risk for severe hail/wind is expected through daybreak.  The need
   for a watch is not currently anticipated, but trends will continue
   to be monitored.

   DISCUSSION...Widely scattered to scattered thunderstorm development
   is underway across the mountains of western into central Montana,
   and across the plains of eastern Montana.  This may be predominately
   supported by ascent associated with lower/mid tropospheric warm
   advection, downstream of an anticyclonically curved mid-level jet
   streak (40-50 kt around 500 mb) now rounding the crest of short wave
   ridging across the northern intermountain region and northern
   Rockies.  This impulse and downstream forcing for ascent are
   forecast to rapidly spread across and east of the northern Rockies
   through daybreak (and beyond).  As this encounters increasing (but
   still perhaps seasonably modest) low-level moisture across eastern
   Montana into portions of the western Dakotas, a further increase in
   thunderstorm development seems probable.

   Although some weakening and veering of 30 kt southeasterly 850 flow
   now present across the western Dakotas into eastern Montana is
   expected by daybreak, deep layer shear will still be strong and
   potentially supportive of organized storm development, including
   supercells.  The degree of instability, particularly boundary layer
   based, is the primary uncertainty concerning the extent of any
   severe weather potential.  Modestly steep mid-level lapse rates may
   allow for sufficient CAPE to support some risk for severe hail in
   stronger storms.  Any damaging wind potential will probably largely
   hinge on substantive upscale convective growth.  At the present
   time, it seems most probable that both of these risks may remain
   somewhat limited in scope/isolated or widely scattered in nature
   through at least daybreak, and perhaps well beyond.

   ..Kerr/Thompson.. 07/19/2017

   ...Please see for graphic product...


   LAT...LON   46520819 47350887 47760953 48440782 48420454 47730247
               46980171 45850231 45660335 46250516 46360618 46520819 

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