Tornado and Hail in Illinois, August 16, 2025

Area Forecast Discussion
National Weather Service Lincoln IL
210 PM CDT Sat Aug 16 2025

.KEY MESSAGES...

- There is a marginal risk (level 1 of 5) for severe storms late
  this evening and overnight. The greatest chance for storms (30%)
  is east of the I-55 corridor.

- Dangerous heat and humidity will be in place across central
  Illinois through the weekend and may continue for at least
  portions of the area Monday and Tuesday. A Heat Advisory for
  peak heat index values of 105-110 degrees is in effect through
  Sunday evening.

&&

.DISCUSSION...
Issued at 207 PM CDT Sat Aug 16 2025

This afternoon, a broad area of low pressure is over Nebraska and
South Dakota with a warm front extending east across northern
Iowa into southern Wisconsin. Strongly unstable conditions are in
place across most of Illinois within the warm sector with MLCAPE
values in excess of 3000 J/kg. Storms should generally favor
forming in the vicinity of the warm front today, but there are
growing concerns for storms reaching central Illinois late this
evening and overnight. 12Z HRRR/NAM Nest suggest storms will grow
upscale across WI this evening and push initially SE along the
instability gradient and with the mean 850-300mb flow, but will
also propagate southward into the low level jet. Several CAMs
indicate a bowing line of storms reaching portions of central or
east central Illinois overnight, likely in a weakening phase owing
to the poor diurnal timing and weakening deep layer shear to the
south.

Synoptic setup remains fairly steady-state throughout the day
Sunday. Central Illinois remains solidly under the upper ridge
axis and well within the warm sector with the surface warm front
well to our north. Despite this, the forecast for Sunday remains
tricky as uncertainty in the evolution of storms today and tonight
gets compounded on Sunday. The more bullish runs of the 4km NAM
Nest and ARW-FV3, which both drive a complex of storms/outflow
across much of central Illinois, have high temperatures Sunday
well below the NBM 5th percentile which would be problematic for
ongoing heat headlines. 50th percentile NBM still indicates highs
in the mid 90s Sunday afternoon which would produce Heat Index
values in the 105-110 degree range. No changes are planned for the
headlines at this time, but there appears to be growing bust
potential and the picture may become more clear later tonight.

Upper ridge will begin to very slowly retrograde back towards the
Desert Southwest through the first half of the upcoming week.
Heat and humidity will likely linger across much of the local area
Monday, but a shortwave trough moving across the Great Lakes
Tuesday should help push the frontal boundary slowly south across
central Illinois Tuesday and Wednesday. PoPs will increase north
to south across central Illinois with the slow frontal passage and
temperatures in the 90s will be replaced by more seasonable
conditions in the mid 80s in its wake. It's possible an extension
of the heat advisory may be needed for southern parts of the area
Tuesday. As the front settles to our south through the second
half of the week, high pressure will build back across the Midwest
resulting in fair weather conditions along with more seasonable
temps and slightly more comfortable humidity levels.

Deubelbeiss

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1225 PM CDT Sat Aug 16 2025

6 to 8 kt SSW winds will continue this afternoon, with cirrus and
a few diurnal cu around FL060. While VFR conditions will be
predominant throughout the forecast period, there is a small (15%)
chance of a pop up storm this afternoon, and higher chance
(20-30%) overnight as a possible cluster of storms dives south.
With overnight storm chances highest across east-central IL, CMI
was the only site with high enough confidence to warrant a PROB30
group at this juncture. However, if confidence increases in storm
coverage and timing, such may also be added to other terminals
with the next package issuance.

Bumgardner

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT Sunday for ILZ027>031-036>038-
040>057-061>063-066>068-071>073.

&&

Hazardous Weather Outlook
National Weather Service Lincoln IL
238 PM CDT Sat Aug 16 2025

ILZ027>031-036>038-040>057-061>063-066>068-071>073-172000-
Knox-Stark-Peoria-Marshall-Woodford-Fulton-Tazewell-McLean-Schuyler-
Mason-Logan-De Witt-Piatt-Champaign-Vermilion-Cass-Menard-Scott-
Morgan-Sangamon-Christian-Macon-Moultrie-Douglas-Coles-Edgar-Shelby-
Cumberland-Clark-Effingham-Jasper-Crawford-Clay-Richland-Lawrence-
238 PM CDT Sat Aug 16 2025

This Hazardous Weather Outlook is for central Illinois, east
central Illinois, southeast Illinois and west central Illinois.

.DAY ONE...Tonight.

Heat indices, peaking between 105 and 110 this afternoon, will be
slow to fall this evening, dropping only to around 75 by sunrise
Sunday morning.

There is a 15% chance of a storm this evening into tonight. Any
storm that develops will be capable of frequent lightning, heavy
rain, and locally strong winds.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Once again, the combination of heat and humidity will result in heat
indices between 100 and 110 degrees Sunday afternoon. In addition,
there will be a 15% chance for thunderstorms during the afternoon
and evening.

There is a 30% chance of thunderstorms Tuesday and Wednesday. It is
too early to determine the severe weather potential.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated tonight.

Bumgardner