Storm Prediction Center Mesoscale Discussion 1171

   Mesoscale Discussion 1171
   NWS Storm Prediction Center Norman OK
   0650 PM CDT Wed Jun 17 2026

   Areas affected...Parts of south-central Illinois into southwest
   Indiana

   Concerning...Tornado Watch 342...344...

   Valid 172350Z - 180145Z

   The severe weather threat for Tornado Watch 342, 344 continues.

   SUMMARY...Large hail, damaging winds, and the potential for a few
   tornadoes will continue spreading east-southeastward this evening --
   within Tornado Watches 342/344.

   DISCUSSION...A couple supercells are tracking east-southeastward
   along the southern flank of a larger convective cluster in
   east-central/southeast IL -- with additional
   development/intensification farther west in south-central IL.
   Despite the presence of very strong low/deep-layer flow/shear
   sampled by the IND VWP (60 kt low-level jet and 650-700 m2/s2 0-1 km
   SRH), these storms are tracking through a corridor of convectively
   processed air from earlier storms. However, the strong low-level
   mass response accompanying an approaching midlevel wave may support
   additional boundary-layer recovery toward the inflow of these storms
   during the next few hours. If this can occur, the robust low-level
   shear/streamwise vorticity will support a few tornadoes (some of
   which could be strong-intense) with any longer-lived supercells.
   Very large hail and severe wind gusts (75+ mph) will also be
   possible with these storms as they continue east-southeastward.

   ..Weinman.. 06/17/2026

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

   ATTN...WFO...IND...ILX...LSX...

   LAT...LON   39128678 38888686 38788721 38928769 39158879 39188973
               39368996 39648986 39718954 39718833 39598770 39458719
               39338688 39128678 

   MOST PROBABLE PEAK TORNADO INTENSITY...120-145 MPH
   MOST PROBABLE PEAK WIND GUST...65-80 MPH
   MOST PROBABLE PEAK HAIL SIZE...2.00-3.50 IN