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Mesoscale Discussion 1387
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   Mesoscale Discussion 1387
   NWS Storm Prediction Center Norman OK
   0835 PM CDT Fri Jul 21 2017

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

   Concerning...Severe Thunderstorm Watch 429...

   Valid 220135Z - 220330Z

   The severe weather threat for Severe Thunderstorm Watch 429
   continues.

   SUMMARY...Short-term severe threat is focusing across the eastern
   half of WW429.

   DISCUSSION...Numerous strong thunderstorms have developed within the
   southern-southwestern flank of an old MCS from southern WI into
   northern IL. This activity is maturing near/north of an early-day
   outflow boundary that remains draped south of Lake MI across
   northern IL. Several supercell structures are noted within this
   activity but further storm mergers should lead to an expanding
   cluster and new MCS. This MCS should mature over northern
   IL/southern WI then propagate across southern Lake MI into parts of
   lower MI and northern IN. Damaging winds are the greatest threat,
   although a tornado can not be ruled out with supercells near the
   boundary.

   Farther west across IA, there is some concern that a strengthening
   southwesterly LLJ will initiate new convection atop reinforced
   outflow late this evening. If this occurs large hail could become a
   concern with elevated storms across this region.

   ..Darrow.. 07/22/2017

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

   ATTN...WFO...IWX...GRR...IND...LOT...ILX...MKX...DVN...ARX...
   DMX...

   LAT...LON   42839365 42368620 40648620 41109367 42839365 

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