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Mesoscale Discussion 1421
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   Mesoscale Discussion 1421
   NWS Storm Prediction Center Norman OK
   0321 PM CDT Tue Jul 25 2017

   Areas affected...Portions of central/southern MN...far western WI

   Concerning...Severe potential...Watch likely 

   Valid 252021Z - 252245Z

   Probability of Watch Issuance...80 percent

   SUMMARY...The severe-thunderstorm risk will be increasing through
   late afternoon and then continue into the evening. Severe hail and
   wind, and perhaps some limited tornado risk, are expected to
   accompany storms. Present indications are that Watch issuance will
   be likely.

   DISCUSSION...A quasi-stationary frontal zone arcs from far eastern
   SD into southwest and south-central MN into north-central IA. While
   substantial MLCIN for mixed-layer parcels continues, modest surface
   heating amid upper 60s to lower 70s surface dewpoints is promoting
   strong destabilization amidst steep mid-level lapse rates. This is
   especially the case over the warm side of the frontal zone, where
   MLCAPE upwards of 3500 J/kg exists based on modifications to the 18Z
   Aberdeen SD RAOB.

   Elevated convection along the southern periphery of a subtle
   mid-level impulse crossing the northern High Plains has shown some
   signs of intensification in far eastern SD near the frontal zone.
   While likely remaining elevated in the short term, the moist and
   diurnally heated, destabilizing boundary layer may contribute to
   some intensification of this activity as it spreads into MN, with
   isolated severe potential.

   Subsequently, storms will likely develop across the discussion area
   through the late afternoon and into the evening hours, in response
   to strengthening warm advection and persistent frontal ascent. The
   aforementioned buoyancy, combined with 30-45 kt of effective shear,
   will likely support severe hail and wind. Increasing convective
   coverage into the evening should yield localized upscale convective
   growth with corridors of greater severe-wind risk, while the
   severe-hail risk continues.

   Some limited tornado risk could exist with any supercell structures
   residing along the warmer side of the frontal zone, where the
   combination of 150-250 m2/s2 of effective SRH and surface-based
   effective inflow layers will exist. However, this risk may be
   curtailed by strong outflow as convection increases in coverage and
   cold pools amalgamate, with a storm mode becoming predominated by
   clusters.

   Uncertainty exists regarding the timing of more robust uptick in the
   severe potential. This is the result of somewhat modest deep ascent
   associated with the lack of a more salient mid-level perturbation,
   and presence of ample antecedent capping. Regardless, severe
   potential is presently forecast to be ongoing by early evening.

   ..Cohen/Grams.. 07/25/2017

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

   ATTN...WFO...MPX...FGF...FSD...ABR...

   LAT...LON   44079615 45199639 45799589 45769367 45409262 44719262
               44039367 44079615 

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Page last modified: July 25, 2017
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