Mesoscale Discussion 1341
   NWS Storm Prediction Center Norman OK
   0300 PM CDT Sat Jul 24 2021

   Areas affected...Parts of southern Lake Michigan...northern Illinois
   and adjacent northwestern Indiana...into parts of southeastern Iowa

   Concerning...Severe potential...Watch possible 

   Valid 242000Z - 242200Z

   Probability of Watch Issuance...60 percent

   SUMMARY...Widely scattered to scattered strong thunderstorm
   development appears increasingly possible through 5-7 PM CDT,
   particularly around the Greater Chicago metropolitan area, posing a
   risk for a few strong downbursts.

   DISCUSSION...Deepening convective development is ongoing along a
   zone of boundary-layer confluence near/south of a slowly southward
   advancing convective outflow boundary, with thunderstorms beginning
   to initiate near/northwest of the Greater Chicago metropolitan area.

   This coincides with a low-level thermal ridge axis, along which
   strong daytime heating has contributed to steepening near surface
   lapse rates and mixed-layer CAPE on the order of 2000+ J/kg. This is
   also near the southwestern periphery of larger-scale mid-level
   troughing shifting across and to the east of the Great Lakes
   vicinity, where 30-40 kt northwesterly flow around 500 mb is
   contributing to strong deep-layer shear.

   Although forcing for ascent, particularly in mid/upper levels, may
   only allow for widely scattered thunderstorm development to the
   west-southwest of southern Lake Michigan late this afternoon, much
   of the Greater Chicago metro may be impacted.  This could include an
   upscale growing line or clusters of storms with the potential to
   produce at least a few strong downbursts.

   ..Kerr/Hart.. 07/24/2021

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

   ATTN...WFO...IWX...LOT...ILX...DVN...

   LAT...LON   41619143 42308965 42558661 41638688 41228927 40569141
               41619143