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

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0740 AM CDT Sun Jun 21 2026

Valid 211300Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
CENTRAL PLAINS INTO THE MID MISSISSIPPI AND LOWER OHIO VALLEYS...

...SUMMARY...
Scattered severe thunderstorms should occur from the central High
Plains into the mid Mississippi and lower Ohio Valleys, mainly this
afternoon into tonight. Large to very large hail, severe/damaging
winds (some 75+ mph), and tornadoes will all be possible. Some
potential for strong tornadoes may develop across parts of Missouri,
Illinois, and Indiana during the afternoon and evening.

...Central Plains to the Mid Mississippi and Lower Ohio Valleys...
An MCS with a convectively augmented parent MCV extends from parts
of central MO into southeast KS/northern OK this morning. The
airmass downstream of this MCS across MO is not particularly
unstable, but greater low-level moisture and related instability is
present across northeast OK and vicinity. Current expectations are
for the MCS/MCV to continue eastward across the mid MS Valley and
Ozarks through the morning while posing an isolated threat for
severe/damaging winds. With time, some re-invigoration of the MCS
appears possible into the lower OH Valley as destabilization occurs
with daytime heating. Trailing outflow from the morning convection
will serve as a focus for additional thunderstorms later today as a
compact/enhanced low-level jet moves eastward from the Ozarks into
the mid MS/lower OH Valleys.

There still appears to be potential for more robust thunderstorm
development this afternoon across these areas along and south of the
outflow boundary/front. Moderate instability will likely develop in
the presence of 30-40 kt of deep-layer shear, which will be
sufficient to support supercells. Enhanced low-level shear will also
be present owing to the eastward-migrating low-level jet. This will
foster a risk for tornadoes with any sustained supercells, and a
strong tornado appears possible. Considered greater tornado
probabilities and a categorical upgrade in a narrow corridor across
parts of MO/IL/IN, but there remains too much uncertainty regarding
sufficient destabilization in the wake of the morning activity. The
severe/damaging wind threat will also increase, with the potential
for multiple clusters to form and track eastward into the lower OH
Valley through the evening and early overnight hours.

Convective development should also occur farther west along/near the
composite outflow boundary/front across the Ozarks into southern
KS/northern OK by peak afternoon heating. Strong instability and
moderate deep-layer shear will likely support updraft organization
and a mix of supercells and multicell clusters. Large to very large
hail should be the primary threat initially, but deep-layer shear
vectors aligned largely parallel to the boundary should foster
convective mergers and an increasing threat for severe/damaging
winds with multiple clusters that should form and spread
east-southeastward across OK and the Ozarks through the evening. At
least an isolated severe wind threat may persist overnight with
southward extent across the southern Plains into AR given the large
degree of buoyancy forecast.

A mid-level shortwave trough over WY this morning will move
east-southeastward towards the central High Plains by this
afternoon. Enhanced westerly flow aloft attendant to this shortwave
trough will overspread the central High Plains through the day. A
convectively reinforced front extends from northwest OK into the
central High Plains, and modest low-level upslope flow is forecast
to the north of this boundary today. Recent short-term guidance
continues to suggest that moderate instability will develop with
filtered daytime heating in the presence of steep mid-level lapse
rates and strong effective bulk shear. Isolated supercells that can
develop in this regime across eastern CO into western NE/KS should
pose mainly a large to isolated very large (2+ inches) hail risk
initially, before some clustering/upscale growth possibly occurs
this evening. Scattered severe/damaging winds would become an
increasing concern if this mode transition occurs, and isolated
significant gusts (75+ mph) appear possible.

..Gleason/Broyles.. 06/21/2026

Day 1 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
1013 AM CDT Sun Jun 21 2026

Valid 211700Z - 221200Z

The existing Elevated area over the southwest was expanded slightly
to include the San Luis Valley and nearby areas of southern CO.
Conditions there will be similar to the drawn area over much of
western and northern NM with westerly winds near 10-15 mph combined
with afternoon RHs of 5-15% atop very dry dead fuels. Other areas
remain on track from the previous forecast with no additional
updates necessary.

..Stearns.. 06/21/2026

.PREV DISCUSSION... /ISSUED 0131 AM CDT Sun Jun 21 2026/

...Synopsis...
Zonal mid-level flow will prevail over the CONUS today, with
multiple embedded impulses poised to traverse this flow. Across the
Intermountain West, one such mid-level impulse will overspread the
central Rockies, encouraging surface low development along the
southern High Plains. Dry westerly flow will ensue across the desert
Southwest, with 15-20 mph sustained westerly surface winds and 5-15
percent RH overspreading dry fuels from far eastern Arizona into
central New Mexico, warranting the continuation of Elevated
highlights. Dry and windy conditions will also channel within the
Snake River Plain by afternoon peak heating in southern Idaho. Here,
15-20 mph sustained westerly surface winds will coincide with 15
percent RH and dry fuels, with Elevated highlights maintained here
as well.

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