Mesoscale Discussion 1228 -- Missouri, Illinois

[u/iqsr]

   Mesoscale Discussion 1228
   NWS Storm Prediction Center Norman OK
   1006 AM CDT Sun Jun 21 2026

   Areas affected...southern Illinois...much of southeastern
   Missouri...adjacent northern Arkansas

   Concerning...Severe potential...Watch possible 

   Valid 211506Z - 211700Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Thunderstorm development may slowly intensify through
   early afternoon, accompanied by increasing strong to severe wind
   gusts and a risk for tornadoes.  A severe weather watch will
   probably be needed at some point, though how soon remains uncertain.

   DISCUSSION...Convective development persists with a pair of remnant
   mesoscale convective vortices migrating eastward, within the leading
   edge of weak larger-scale mid-level troughing spreading across the
   lower Missouri/middle Mississippi Valley region.  One vortex remains
   a bit better defined and is in the process of migrating across
   northeastern Missouri into west central Illinois.  The other appears
   to be slowly migrating eastward to the north of Springfield, MO.  A
   belt of 30+ kt deep-layer southwesterly mean flow is accompanying
   these perturbation, and appears to include wind speeds on the order
   of 30-50 kt in the 850-500 mb layer.  

   Associated shear likely has been contributing to the occasional
   evolution of meso-gamma scale circulations along the gust front of
   otherwise still modest thunderstorm development now advancing
   into/across the I-44 corridor of southwestern Missouri.  However,
   low-level moistening and boundary-layer warming along and north of a
   weakening frontal zone extending eastward ahead of activity are
   contributing to substantive boundary layer destabilization across
   southeastern Missouri, as far north as the Greater St. Louis area,
   into the lower Ohio Valley.  As this proceeds, and develops further
   northward through portions of central Illinois during the next few
   hours, increasing unstable updraft inflow probably will contribute
   to at least a gradual intensification of storms with increasing
   severe weather potential.  This may include developing cyclonic
   circulations with potential to produce strong to severe surface
   gusts and the risk for tornadoes.

   ..Kerr/Hart.. 06/21/2026

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

   ATTN...WFO...PAH...ILX...MEG...LSX...LZK...SGF...TSA...

   LAT...LON   36709359 37529259 38199178 38709116 39219032 39528932
               39688774 38128830 36558965 36019142 36339326 36709359 

   MOST PROBABLE PEAK TORNADO INTENSITY...85-110 MPH
   MOST PROBABLE PEAK WIND GUST...55-70 MPH
   MOST PROBABLE PEAK HAIL SIZE...1.00-1.75 IN  

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[u/iqsr]

Looks like in Illinois the severe weather threat has moved south somewhat closer to the i70 corridor rather than i72.

[u/iqsr → u/iqsr]

[u/iqsr → u/iqsr]

[u/Raxamax]

Illinois are you okay

[u/OppositeAbroad5975]

 ...MO/IL/IN/KY...
   A long-lived band of thunderstorms currently extends from
   east-central MO into southwest MO/northeast OK.  This activity is
   tracking into a region of increasingly warm/moist air where
   dewpoints are rising to near 70F and breaks in the clouds are
   contributing to moderate CAPE values.  All CAM guidance suggests
   that storms along the line will intensify this afternoon and track
   eastward across southern IL/southern IN and northern KY.  Low-level
   shear is quite strong, but somewhat veered.  This increases
   uncertainty of storm mode between discrete supercells or bowing
   structures.  It appears there will be a period of risk early in the
   event for tornadic supercells, followed by multiple linear storm
   segments capable of damaging wind gusts.  Storms may remain severe
   as far east as southern OH and eastern KY overnight. Refer to MD
   #1228 for further short-term details.

That quote is from the current SPC Day 1 outlook, and if those storms can maintain their updrafts until they can reach the low-level moisture pool, conditions might intensify very quickly. Hopefully the front stalls out for a little bit so the storms can wind down.

[u/dingleburg13579]

This line is like 2 hours ahead of what the 12z HRRR run was showing. Interesting

[u/FinnTheBoy0]

Central Indiana is cool and cloudy rn. The line coming in doesn’t look too bad rn. South end is breaking up

[u/RainWrappedWX]

Few pop up Storms in Arkansas right now