Latest SPC Mesoscale Discussion continues significant severe thunderstorm threat for northern Illinois and the Chicago Metro area through midnight

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Mesoscale Discussion 1385
   NWS Storm Prediction Center Norman OK
   0628 PM CDT Fri Jul 21 2017

   Areas affected...Eastern IA...Southern WI...Northern IL...Northwest

   Concerning...Severe Thunderstorm Watch 429...

   Valid 212328Z - 220100Z

   The severe weather threat for Severe Thunderstorm Watch 429

   SUMMARY...Severe threat continues across most of WW429.

   DISCUSSION...Remnants of long-lived MCS, that evolved over the
   central Plains before dawn, have progressed into the upper Great
   Lakes region. Considerable amount of redevelopment is occurring in
   proximity to the remnant center with the majority of strong updrafts
   evolving along the southwestern flank of the mid-level debris shield
   from eastern IA into extreme southwest WI. Early-day convection has
   contributed to a well-defined outflow boundary that is draped across
   the south side of the Chicago metro, westward into IA where a few
   supercells have evolved. This new development will likely continue
   to grow upscale with storm mergers likely contributing to another
   mature MCS over southern WI/northern IL later this evening. Large
   hail and damaging winds remain possible with most storms this
   evening and a tornado can not be ruled out with supercells that
   utilize more favorable shear near the outflow boundary.


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