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Mesoscale Discussion 1381
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MD 1381 graphic

   Mesoscale Discussion 1381
   NWS Storm Prediction Center Norman OK
   0220 PM CDT Fri Jul 21 2017

   Areas affected...Portions of the Dakotas and western/northern MN

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

   Valid 211920Z - 212145Z

   Probability of Watch Issuance...95 percent

   SUMMARY...Severe Thunderstorm Watch issuance will be forthcoming
   across portions of the region.

   DISCUSSION...Convection will likely continue to increase in coverage
   and intensity across portions of the region during the next few
   hours, with a risk for severe storms continuing into the evening.
   Regimes for convective development will likely focus around the
   following.

   (1) Updrafts at the leading edge of a persistent convective cluster
   spreading across central ND -- that is slowly becoming 
   surface-based -- may intensify as they advance into a region of
   slowly destabilizing air farther east/southeast. It is probable that
   previous, persistent, anvil-related subsidence from late yesterday
   has reinforced capping across much of central, south-central, and
   southeastern ND ahead of this activity. Related static stability
   gains aloft and boundary-layer drying have potentially resulted in a
   relative minimum in vertical mixing, theta-e, and instability ahead
   of this activity. Regardless, a continued gradual uptick in this
   central ND convection may occur as it spreads eastward and
   southeastward during the next several hours amidst MLCAPE increasing
   to 1000-2000 J/kg and 30-40 kt of effective shear, with severe
   hail/wind possible.

   (2) Initial updrafts are focusing southward into/across parts of
   north-central and central SD near a north-south warm-thermal trough
   and along the edges of the theta-e minimum previously addressed.
   This may also be a focused region for convective development into
   SD, with satellite imagery already indicating agitating cumulus
   fields amid weakly baroclinic troughs across the region. This
   activity will subsequently spread eastward and east-southeastward
   into the evening hours, with similar buoyancy/shear as previously
   mentioned supporting severe hail/wind potential.

   (3) Warm-advection-related ascent across northeast ND and northwest
   MN may be a focused zone of potentially severe thunderstorms --
   including supercells -- with severe hail and wind. A more sheltered
   boundary layer in these areas -- maintaining lower 70s dewpoints and
   stronger low-level buoyancy and areas of more backed surface winds
   enhancing effective SRH -- may support some tornado potential. This
   would especially be the case where pre-existing vertical vorticity
   is enhanced along the warm frontal zone. The supercell potential in
   this regime is more uncertain, though convection from the two
   previously mentioned regimes may eventually spread into this area by
   evening.

   ..Cohen/Coniglio/Dial.. 07/21/2017

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...DLH...FGF...ABR...BIS...

   LAT...LON   47040026 47909895 48689642 48389456 47159447 45519692
               44579906 44580062 45810093 47040026 

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Page last modified: July 21, 2017
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