Flash Flood Warning 18, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 PM CDT Wed Jun 24 2026

.KEY MESSAGES...

- Scattered thunderstorms expected this afternoon and evening,
  a few may be severe with large hail and damaging winds.

- Another chance of showers and possibly a few thunderstorms
  during the day Friday, with another chance of showers and
  thunderstorms Saturday night perhaps into early Sunday
  morning.

- First heat wave of the summer expected early next week with
  the potential for highs in the 90s and peak afternoon heat
  indices over 100 degrees.

&&

.DISCUSSION...
Issued at 146 PM CDT Wed Jun 24 2026

Shortwave trough over the western Great Lakes along with
associated weak sfc front from northwest Wisconsin south into
eastern Iowa should encourage scattered thunderstorm development
this afternoon. These storms should move southeastward across
our CWA late this afternoon into this evening. Jet streak is
evident on water vapor imagery nosing into SE MN/SW WI early
this afternoon. Guidance depicts a 40-50kt 500mb speed max
spreading across southern WI and northern IL this afternoon
and evening. This will result in a increase in deep layer shear
into the 40kt range.

Rain associated with lead shortwave should continue to exit the
area early this afternoon with at least partial clear already
spreading into our far western CWA. This should allow for
moderate instability to develop across the area with MLCAPES of
1000-1500 J/kg. The moderate instability coupled with fairly
strong deep layer shear should support supercells with a
potential for large hail and perhaps damaging winds. Weak mid
level lapse rates may limit the magnitude of the hail/wind
threat, but the long straight hodographs would support splitting
supercells and a hail threat.

This front should clear the CWA late this evening bringing an
end to the showers/storms by late evening. In the wake of the
front, looks for dry weather Thursday and most of Thursday
night. There is a signal in most guidance for another low
amplitude shortwave to potentially becoming convectively
enhanced before moving toward the mid Mississippi Valley Friday.
Instability should be pushed well south of our CWA, so if this
MCV takes a more northerly track it would likely be in a
weakening state as it encounters more stable and drier low level
air mass over our area Friday. Conversely, it is possible that
the MCV may tend to propagate more southward toward the
instability gradient. In collaboration with neighboring offices,
lowered NBM pops for Friday some and kept slight thunder
chances confined to areas south of the Kankakee/Illinois Rivers.
Cloud cover should keep temps cooler Friday, but if it does
rain then highs would need to be lowered even more, especially
southern CWA where better rain chances should be.

Over the weekend, an upper level ridge is progged to begin
developing over the middle and lower Mississippi Valley region.
A warm front is progged to lift northward Saturday night into
Sunday. Some threat of showers and thunderstorms will exist
ahead of this front and ahead of the EML advecting eastward with
the developing upper ridge. Medium range guidance is in good
agreement in developing a fairly strong (595-596dm) 500mb ridge
over the Mississippi Valley early next week.

Sunday, there's likely a small chance of showers/storms in the
morning, then probably dry in the afternoon. With the warm front
moving through the area, a weak lake breeze is possible along
the IL shore perhaps offering a bit cooler conditions right
along the lake. Elsewhere, highs should climb into the upper 80s
to around 90 with higher dewpoints likely pushing afternoon heat
indices well into the 90s to near 100.

Monday through Wednesday, current forecast has highs in the low
to mid 90s with dewpoints in the 70s resulting in heat indices
solidly over 100 degrees. Based on the current model guidance,
these values are reasonable, but with the heart of the upper
ridge progged to be centered south toward the lower Ohio and
mid-lower Mississippi Valley it is possible that ring of fire
convection currently progged to be mostly to our north could
sneak farther south or send outflow south into parts of our area
offering some reprieve from the heat. Harder to forecast that
this far out, but something to watch as a possible spoiler to
what otherwise looks like the first formidable heat wave of the
summer.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 635 PM CDT Wed Jun 24 2026

Forecast concerns include...

Scattered thunderstorms this evening.
Wind shift/lake breeze to northeast Thursday evening.

A line of strong/severe thunderstorms will move across the
Chicago terminals in the next 1-2 hours. Another line of
thunderstorms across southwest WI and far northwest IL may move
across the terminals during the mid/late evening. Have
maintained prob thunder mention for this potential for now.

Prevailing southeast winds are expected to continue this evening
and may become more easterly behind the first round of storms.
Winds are expected to shift light westerly overnight, then west/
northwest to 10kt Thursday morning. A lake breeze/cold front is
expected to shift winds to the northeast late Thursday afternoon
into early Thursday evening for the Chicago terminals. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

Visit us at weather.gov/chicago

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
217 PM CDT Wed Jun 24 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-251930-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
217 PM CDT Wed Jun 24 2026 /317 PM EDT Wed Jun 24 2026/

This Hazardous Weather Outlook is for portions of north central
Illinois...northeast Illinois and northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Level 2 of 5 Damaging Wind Risk...up to 60 mph.
     Level 2 of 5 Large Hail Risk...up to ping pong ball size.

DISCUSSION...

Scattered thunderstorms are expected this afternoon and evening.
A couple of the storms could become severe with large hail and
damaging winds the primary threat.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Friday...
  Limited Thunderstorm Risk south of I-80.
Saturday night and Sunday morning...
  Limited Thunderstorm Risk.
Monday...
  Elevated Excessive Heat Risk.
Tuesday...
  Limited Thunderstorm Risk.
  Elevated Excessive Heat Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed through this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the southeast 30 mph, isolated storms may move
  northeast up to 40 mph.

LMZ740>745-251930-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
217 PM CDT Wed Jun 24 2026

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Level 2 of 5 Thunderstorm Wind Gust Risk...up to 50 knots.
     Level 2 of 5 Hail Risk...up to ping pong ball size.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Saturday night into Sunday morning...
  Limited Thunderstorm Risk.
Tuesday...
  Limited Thunderstorm Risk.

- Izzi

Visit us at www.weather.gov/chicago

Preliminary Local Storm Report
National Weather Service Chicago IL
734 PM CDT Wed Jun 24 2026

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

0715 PM     Tstm Wnd Gst     3 SE Grand Ridge        41.21N 88.78W
06/24/2026  M53 MPH          La Salle           IL   Public

            Private weather station reported 53 mph gust
            southeast of Grand Ridge. Time approximate.

&&

WSL

Preliminary Local Storm Report
National Weather Service Chicago IL
742 PM CDT Wed Jun 24 2026

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

0725 PM     Flood            River Grove             41.93N 87.84W
06/24/2026                   Cook               IL   Public

            Video shared to social media shows a few
            inches of standing water on Grand Avenue
            near Forest Avenue.

&&

WSL

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