Severe Thunderstorm Warning 356, NWS Lincoln, IL

Area Forecast Discussion
National Weather Service Lincoln IL
1243 PM CDT Thu Aug 13 2026

.KEY MESSAGES...

- Midday Thunderstorm Complex: A cluster of thunderstorms tracks
  across Iowa will likely impact parts of the forecast area,
  with increasing concerns for areas near the Illinois River
  valley. Isolated severe thunderstorms and efficient, heavy
  rainfall are a concern.

- Heat Concerns South: South of the impending thunderstorm
  debris, hot and humid conditions yield heat indices near 105F.
  A Heat Advisory is active through early evening for southeast
  Illinois, and will need to be expanded northward this weekend.

- Overnight Flood Watch: repeated thunderstorms will heighten
  flash flooding risks. A Flood Watch is in effect for counties
  along/north of I-74 through Friday morning.

&&

.UPDATE...
Issued at 935 AM CDT Thu Aug 13 2026

MCS continues to track across southeast Iowa this morning, with
cold cloud tops near -75C. Convective line has maintained itself
and warnings continue to be issued in that area. Ahead of the
system in central Illinois, conditions are fairly stable due to
the extensive low cloudiness and earlier fog across the
northern CWA. However, a NW-SE CAPE gradient may allow for a
path toward the Illinois River Valley in west central Illinois.
None of the reflectivity guidance in the morning CAM's feature
such a scenario at the moment, though various RAP CAPE profiles
do show the gradient aspect. Overall, little change was made for
today's PoP's in that area.

Looking ahead to tonight, the next MCS still appears to set up
near the warm front across northern Illinois this evening,
gradually sinking toward the I-74 corridor toward midnight. Some
minor PoP adjustments were done to increase rain chances near
and north of I-74. No changes were made to the Flood Watch at
this time.

&&

.DISCUSSION...
Issued at 441 AM CDT Thu Aug 13 2026

.Morning Dense Fog...

A strong radiational cooling regime early this morning has
fostered patchy dense fog development across central Illinois.
Despite high-level anvil debris streaming overhead near/north
of the I-74 corridor around midday, bringing a threat for
upstream convection, lingering near-surface moisture and weak
boundary- layer flow (winds < 10 kt through 3 kft) will keep fog
grounded past daybreak. Shallow warm air advection (WAA)
overrunning the effective surface boundary currently draped just
south of the forecast area will also aid in maintaining low
stratus/fog well into the mid-morning hours. A Dense Fog
Advisory is in effect through 9 AM CDT for locations roughly
north of a Quincy to Terre Haute line.

.Midday Convective Complex & Heavy Rain Potential...

Attention then shifts to the afternoon as the first of multiple
convective rounds approaches from the west. Early morning radar
trends reveal a MCC positioned over western IA. Short- term
high- resolution guidance remains in solid consensus that this
complex will track near and north of the I-74 corridor by
midday, riding along the nose of a receding low-level jet (LLJ),
a convectively-augmented shortwave trough, and the northern
edge of the instability gradient.

Severe potential with this initial wave remains conditional.
While cold pool dynamics and potential Rear Inflow Jet (RIJ)
organization bear watching, poor lapse rates, lingering morning
fog/stratus, and a persistent low-level temperature inversion
could keep this incoming activity elevated and predominantly
sub-severe.

Regardless of severe risk, this initial round will be an
efficient heavy rainfall producer. Atmospheric soundings display
classic tropical-like characteristics: (a) anomalous PWAT
values (2-2.2 inches); (b) low- to mid-level RH exceeding 80%;
(c) a deep warm cloud layer (> 4km); and (d) a long, skinny
CAPE profile.

Latest HREF 6-hour Probability-Matched Mean (PMM) QPF targets
0.5 to 1.5 inches across the northern forecast area with this
initial midday push.

.Heat & Humidity Across the South...

South of the convective footprint and the effective surface
boundary (primarily south of a Quincy to Terre Haute line),
strong surface heating and rich moisture will persist. Highs
near 90 paired with dewpoints in the mid 70s will yield heat
index values between 100-105F. A Heat Advisory is in effect
through early evening for portions of southeast Illinois.

.Overnight Flash Flood Threat...

The threat for flash flooding escalates significantly tonight
across central and northern Illinois. The effective surface
boundary will begin to retreat northward as a sharpening warm
front. An intensifying nocturnal LLJ impinging directly onto
this elevated baroclinic zone will fuel repeated convective
initiation and training thunderstorm complexes near and north of
the I-74 corridor.

With antecedent conditions potentially primed by the midday
convective round, susceptibility to rapid runoff could be high.
HREF QPF PMM guidance indicates an additional 1-2 inches
overnight, with mesoscale training corridors capable of
producing localized totals exceeding 4 inches. Given the high-
efficiency airmass and high potential for training, a Flood
Watch has been issued for counties along and north of I-74
through Friday morning.

.This Weekend Through Mid-next Week...

The surface warm front is projected to surge north of the area
Friday night into Saturday, re-introducing triple- digit heat
indices and scattered elevated warm-advection convection to
start the weekend. The Heat Advisory will most likely need to be
expanded northward for Friday and Saturday.

A stronger, more potent shortwave trough is then forecast to
dig out of the Northern Plains Sunday into Monday, driving a
sharper surface cold front through the Mississippi Valley. While
showers and thunderstorms may accompany the frontal passage,
deterministic models and global ensembles show remarkably strong
consensus for a deeper continental polar (cP) airmass to push
into the central U.S. early next week. This will usher in a
period of below-normal temperatures, lower dewpoints, and an
extended break from the active convective pattern by Monday and
Tuesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1243 PM CDT Thu Aug 13 2026

Cluster of severe storms just passed KPIA and will likely make
it to KBMI shortly after 18z if they hold together. Another
round of storms is expected to develop across northern Illinois
this evening and sink southward, though arrival at the northern
terminals (KPIA/KBMI) would probably be closer toward 07-09z or
so. These would be accompanied by deteriorating ceilings from
north to south overnight, with MVFR conditions most likely from
KPIA-KCMI. Southern extent is more questionable, but heavier
showers may result in some brief visibility drops into that
range.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday morning for ILZ027>031-037-038-045-
046.
Heat Advisory until 8 PM CDT Sunday for ILZ066>068-071>073.

&&

KEY MESSAGES...Geelhart
UPDATE...Geelhart
DISCUSSION...MJA
AVIATION...Geelhart

Hazardous Weather Outlook
National Weather Service Lincoln IL
500 AM CDT Thu Aug 13 2026

ILZ027>031-036>038-040>057-061>063-066>068-071>073-141000-
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-
500 AM CDT Thu Aug 13 2026

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

.DAY ONE...Today and tonight.

Heat indices of 95 to 105 are expected this afternoon south of I-72.

A thunderstorm complex will move through central Illinois from late
morning through afternoon. Damaging winds gusts are the primary
threat, though a couple tornadoes cannot be ruled out. Additional
storms are expected late tonight into the overnight hours with
damaging winds and large hail possible with any stronger storms.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Hot and humid conditions will expand back northward through late
week. Peak afternoon heat indices will range from 95 to 105 Friday
through Sunday areawide.

Daily chances for thunderstorms exist through the weekend. Some of
these storms could be severe and produce heavy rain through at least
Saturday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed later this morning into afternoon.

Expected storm motion is toward the east at 40-45 mph.

NMA

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Preliminary Local Storm Report
National Weather Service Lincoln IL
150 PM CDT Thu Aug 13 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0108 PM     Tstm Wnd Dmg     3 W Normal              40.51N 89.04W
08/13/2026                   McLean             IL   Broadcast Media

            Portions of roof damaged at Rivian plant.

&&

Preliminary Local Storm Report
National Weather Service Lincoln IL
154 PM CDT Thu Aug 13 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0124 PM     Tstm Wnd Dmg     4 E Bloomington         40.48N 88.92W
08/13/2026                   McLean             IL   Broadcast Media

            Power lines down east of Bloomington
            Airport, along Towanda Barnes Rd.

&&