Mesoscale Discussion 0240
   NWS Storm Prediction Center Norman OK
   0737 PM CDT Tue Mar 23 2021

   Areas affected...Parts of northwest/west central Illinois into east
   central Missouri

   Concerning...Severe potential...Watch unlikely 

   Valid 240037Z - 240200Z

   CORRECTED WORDING FIST PARAGRAPH

   Probability of Watch Issuance...5 percent

   SUMMARY...A couple of low-topped supercells may pose a risk for
   small hail, locally strong wind gusts and perhaps some risk for a
   brief, weak tornado another hour or two before diminishing fairly
   quickly after sunset.

   DISCUSSION...There has been a recent increase in low/mid-level
   rotation evident in intensifying convective cells to northwest of
   St. Louis, MO.  This is generally just to the south of the narrow
   squall line supported by an arcing band of stronger forcing for
   ascent associated with the mid-level vorticity center progressing
   into/across the central Missouri/Iowa border area.

   Warming associated with subsidence evident in water vapor imagery to
   the south is still suppressing thunderstorm development, but breaks
   in cloud cover within the moist warm sector have contributed to a
   corridor of weak mixed-layer CAPE up to 250 J/kg.  For surface
   parcels CAPE may be has high as 500 J/kg, allowing for a brief flare
   up of stronger convection contributing to the increased rotation,
   supported by moderately large, clockwise curved low-level hodographs
   beneath a 30-40+ kt southerly 850 mb jet.  It is possible that this
   could continue across the river into parts of northwest and west
   central Illinois, accompanied by a risk for small hail and locally
   strong surface gusts, with perhaps some potential for a brief, weak
   tornado.  However, this potential is expected to diminish as the
   intensity of the convection wanes fairly rapidly after sunset.

   ..Kerr/Guyer.. 03/24/2021

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

   ATTN...WFO...ILX...LSX...DVN...

   LAT...LON   39539098 39819104 40189111 40269109 40649094 39919026
               39069028 38659060 39539098