139
ACUS11 KWNS 120104
SWOMCD
SPC MCD 120104
INZ000-ILZ000-WIZ000-IAZ000-120330-

Mesoscale Discussion 1941
NWS Storm Prediction Center Norman OK
0804 PM CDT Tue Aug 11 2026

Areas affected...Eastern Iowa into northern Illinois and far
northwest Indiana

Concerning...Severe potential...Watch possible

Valid 120104Z - 120330Z

Probability of Watch Issuance...60 percent

SUMMARY...Strong to severe thunderstorms may materialize between
02-05 UTC across northern Illinois into northern Indiana. These
storms will pose a risk of large to very large hail and severe
gusts. Trends will be monitored, and watch issuance may be needed.

DISCUSSION...The northwestern edge of the cold pool associated with
the earlier  passage of an MCS is becoming increasingly diffuse
across northern IL based on recently surface observations with
temperature warming back into the upper 70s and briefly into the 80s
before the onset of nocturnal cooling. Aloft, westerly 700-500 mb
flow has advected steep lapse rates eastward back into the region,
which were well sampled by the recent DVN and ILN 00 UTC RAOBs. As a
result, the thermodynamic environment is steadily destabilizing and
becoming increasingly supportive of intense convection. (These
soundings also sampled extreme buoyancy between 4000-5000 J/kg
MLCAPE and weak inhibition around -50 J/kg.)

Over the past few hours, a subtle disturbance has been tracked
across IA, evident by weak mid-level ascent. This feature has been
supporting weak attempts at convection across southern MN and
western WI, and will likely promote additional attempts within the
destabilizing air mass across far eastern IA into northern IL later
this evening. Latest guidance suggests that convective initiation is
likely between 02-05 UTC, and will likely be focused along and north
of the residual outflow boundary. Initially discrete to
semi-discrete cells may mature into supercells before storm
interactions within the zone of ascent promote clustering and
upscale growth. Large to very large hail will be possible given the
ample buoyancy and around 40 knots of effective shear if initially
discrete modes can be adequately sustained. Strong to severe winds
also appear possible given the shallow nature of the residual cold
pool across northern IL. It remains unclear exactly how long
discrete storm modes will persist and how far downstream into IN
convection will develop. Regardless, trends will be monitored, and
watch issuance is probable given the favorable convective
environment.

..Moore/Mosier.. 08/12/2026

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

ATTN...WFO...IWX...IND...LOT...ILX...MKX...DVN...ARX...

LAT...LON   40168790 41669142 41919167 42299174 42649163 42819136
            42919083 42859053 41848731 41518698 41158685 40798685
            40478695 40258713 40148731 40088754 40168790

MOST PROBABLE PEAK TORNADO INTENSITY...UP TO 90 MPH
MOST PROBABLE PEAK WIND GUST...55-70 MPH
MOST PROBABLE PEAK HAIL SIZE...1.50-2.50 IN