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Mesoscale Discussion 1420
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   Mesoscale Discussion 1420
   NWS Storm Prediction Center Norman OK
   0241 PM CDT Tue Jul 25 2017

   Areas affected...Portions of SD...northern NE

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

   Valid 251941Z - 252145Z

   Probability of Watch Issuance...95 percent

   SUMMARY...Severe thunderstorm development is expected around the
   2030Z-2130Z time frame. Severe Thunderstorm Watch issuance will be

   DISCUSSION...Initial boundary-layer cumulus development is noted in
   Cherry County NE, along a northeast-southwest frontal segment that
   extends into northeast SD. Despite weak large-scale ascent, hot
   surface temperatures into the upper 90s to near 104F have permitted
   the erosion of antecedent capping near and south the front.
   Sufficient frontal ascent will support deep convective development
   within the next couple of hours. Initial development may be focused
   in northwest/north-central NE, downshear from a 1008-mb,
   thermally-enhanced frontal cyclone analyzed south-southeast of
   Chadron, with activity also developing and spreading into SD into
   the late afternoon and evening.

   The 18Z Aberdeen SD RAOB sampled steep mid-level lapse rates, and
   modifications to the low-level thermodynamic profile to account for
   observations near/south of the boundary indicate MLCAPE around
   2000-3500 J/kg -- aided by surface dewpoints holding in the 60s in
   many areas. Sufficient mid-level flow will exist -- especially with
   northward extent across the region (around 40 kt sampled by the
   aforementioned sounding) -- for convective organization/clusters.
   Severe hail and wind will be likely. Intense cold pools will likely
   amalgamate as convective coverage increases into the evening,
   fostering localized pockets of convective upscale growth. This will
   locally bolster the severe-wind risk, with the severe-hail risk
   continuing amidst steep lapse rates.

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

   ...Please see for graphic product...


   LAT...LON   43420196 44340012 45259798 45269657 44219659 43539706
               42989831 42360045 42210166 42240246 42740268 43420196 

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