Tornado Warning 14, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
357 PM CDT Mon Aug 18 2025

.KEY MESSAGES...

- Several rounds of showers and storms are likely this
  afternoon and evening. Some storms may be severe with damaging
  winds and torrential downpours.

- Additional showers and storms may develop overnight, primarily
  along and south of I-80. The main threat with any additional
  storms would be torrential downpours.

- A pattern change toward cooler and less humid conditions is
  expected by midweek. Even cooler and less humid conditions may
  arrive over the weekend.

&&

.DISCUSSION...
Issued at 239 PM CDT Mon Aug 18 2025

Through Tuesday:

Two rounds of thunderstorms late this afternoon and evening
still appear to be in store, though the gap that was expected
between these may be more short lived than previously
anticipated. The initial line of storms stretching south out of
WI has been very slow to expand across northern IL but the last
few radar scans finally show more organization, and even the
potential development of a rear inflow jet in the storms along
the I-39 corridor. In the wake of the somewhat disorganized
earlier afternoon activity, more widespread and longer lived
showers and storms are expected to develop upstream and move
through this evening.

Plenty of boundary layer support for convective development is
already in place with widespread dewpoints in the mid to upper
70s across the CWA, and any convergent outflow from new cells
should also help to trigger new storms in the near term. Support
for severe storms remains somewhat marginal and would mostly be
driven by thermodynamics given that the more favorable shear
axis is farther to the north over WI. Rainfall rates up to 2
inches per hour are certainly possible.

The second round of activity, initially expected to trail the
first by an hour or two, is also showing signs of developing and
expanding in coverage across eastern IA into northwest IL along
with better background synoptic and mesoscale support from an
upper trough and more pronounced shear. The additional concern
with this second round would include heavy rain and the
possibility for flooding. This would arise from a combination of
seasonably high PWATs, the relatively slow expected cell
motions, and the overall extent of expected coverage.

After these two rounds move out, we continue to monitor for the
chance overnight into early tomorrow of a final round of
showers and thunderstorms as the main upper trough, developing
surface low, and trailing frontal zone push through. There also
appears to be some support from a low level jet toward the
predawn hours ahead of the upper trough. With the high
precipitable water values and repeated rounds of rain, a flood
watch for flash flooding is in effect through late tonight.

After highs today in the mid 80s, temperatures tomorrow may be
similar to today's, especially ahead of the front, but given the
morning convection and possible lingering cloud cover along
with dewpoints a few degrees lower, heat indices should pose
less of a concern.

Lenning

Tuesday Night through Monday:

High pressure moving overhead in the wake of the frontal passage
should bring an end to the hot, humid, and active pattern we
have been experiencing these past few days. Temperatures return
to a more seasonable level and humidity levels also fall. The
main concern from mid- to late-week would be potential wave
growth on Lake Michigan and an elevated swim risk with onshore
flow.

Ensemble model guidance is advertising a strong upper-level low
barreling toward the Great Lakes this weekend, which should
usher in a seasonably cool and relatively dry airmass. With
meager moisture return ahead of the principal cold front on
Saturday, we currently favor a dry frontal passage. Falling
850mb temperatures toward the upper single digits (+7 to +9C)
will support highs in the mid to upper 70s and overnight lows in
the 50s. Perhaps even more noticeable will be the lower humidity
levels, as dew points will be poised to fall into the 50s. The
cool and dry pattern should continue into early next week before
a period of warmer, and potentially stormy, conditions return
to close the month of August.

Lenning/Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1245 PM CDT Mon Aug 18 2025

For the afternoon and evening we are looking for two rounds of
thunderstorm activity, one with the initial line of storms
approaching the RFD area at forecast issuance time, and then
more widespread and longer lived activity expected to develop
upstream and move through in the wake of the first line.

It is actually not completely certain that the initial line
heading toward RFD will hang together or build south enough to
reach the Chicago area terminals, though the convective
inhibition that has been in place for most of the day appears to
be eroding. Thus it would seem that convergence along the
pronounced line of cumulus stretching south out of Wisconsin
would be enough to trigger expanded development. There is also a
decent possibility of random pop up convection out ahead of
this line. Thus the strategy was to extrapolate this first line
through RFD, DPA, and ORD/MDW and add a small buffer around the
most likely arrival times.

The second round of activity would be expected to trail the
first by an hour or two, assuming it actually manages to
develop at all. Upstream across Iowa there is a broad region of
agitated cumulus along with hints of a MCV circulation showing
signs of trying to intensify. Boundary layer dewpoints in the
mid to upper 70s will certainly provide sufficient moisture,
and this is also the area with better synoptic and mesoscale
support in terms of an upper trough and more pronounced shear.

After these two rounds move out, there is one more chance
overnight into early tomorrow for a final round as the main
upper trough and low level frontal zone push through.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 3 PM CDT this afternoon for
     ILZ006-ILZ103-ILZ104.

     Flood Watch until 4 AM CDT for
     ILZ003-ILX004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108

IN...Flood Watch until 4 AM CDT for INZ001-INZ002
LM...None.

&&

Visit us at weather.gov/chicago

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
343 PM CDT Mon Aug 18 2025

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-192045-
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-
343 PM CDT Mon Aug 18 2025 /443 PM EDT Mon Aug 18 2025/

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 70 mph.
     Level 1 of 5 Large Hail Risk...up to quarter size.
     Level 1 of 5 Tornado Risk.
  Significant Flooding Risk.
  Limited Excessive Heat Risk.

DISCUSSION...

Scattered thunderstorms will continue through this evening. Some
thunderstorms may be severe with damaging winds the primary
severe weather threat.

Very heavy rain is expected with these thunderstorms, which may
lead to localized flash flooding, especially in urban areas.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Tuesday...
  Limited Flooding Risk.
  Limited Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

Spotter reports will be needed through this evening.

GENERAL STORM MOTION OF THE DAY:

Moving toward the east at 20 to 25 mph.

LMZ740>745-192045-
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-
343 PM CDT Mon Aug 18 2025

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 1 of 5 Hail Risk...up to quarter size.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Tuesday...
  Hazardous conditions for small craft are likely.
  Limited Thunderstorm Risk.
Wednesday...
  Hazardous conditions for small craft are likely.

Visit us at www.weather.gov/chicago

Special Weather Statement
National Weather Service Chicago/Romeoville
114 PM CDT Mon Aug 18 2025

ILZ003-004-181900-
Boone IL-Winnebago IL-
114 PM CDT Mon Aug 18 2025

...STRONG THUNDERSTORMS WILL IMPACT NORTHEASTERN WINNEBAGO AND
NORTHWESTERN BOONE COUNTIES THROUGH 200 PM CDT...

At 113 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from South Beloit to near Winnebago. Movement was
east at 25 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Rockton, Loves Park, Candlewick Lake, Machesney Park, South Beloit,
Roscoe, Caledonia, Timberlane, Poplar Grove, Beloit, and Rockford.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Severe Thunderstorm Watch remains in effect until 600 PM CDT for
north central Illinois.

&&

LAT...LON 4249 8873 4230 8892 4230 8922 4250 8907
TIME...MOT...LOC 1813Z 269DEG 20KT 4249 8906 4230 8922

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

DK

Special Weather Statement
National Weather Service Chicago/Romeoville
145 PM CDT Mon Aug 18 2025

ILZ004-005-181915-
Boone IL-McHenry IL-
145 PM CDT Mon Aug 18 2025

...A STRONG THUNDERSTORM WILL IMPACT NORTHWESTERN MCHENRY AND
NORTHERN BOONE COUNTIES THROUGH 215 PM CDT...

At 145 PM CDT, Doppler radar was tracking a strong thunderstorm over
Capron, or near Poplar Grove, moving east at 30 mph.

HAZARD...Wind gusts of 50 to 55 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

Locations impacted include...
Poplar Grove, Harvard, Candlewick Lake, Timberlane, Chemung, Capron,
and Caledonia.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

A Severe Thunderstorm Watch remains in effect until 600 PM CDT for
north central and northeastern Illinois.

&&

LAT...LON 4232 8853 4233 8891 4249 8881 4249 8847
TIME...MOT...LOC 1845Z 270DEG 26KT 4239 8875

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...55 MPH

DK

Special Weather Statement
National Weather Service Chicago/Romeoville
212 PM CDT Mon Aug 18 2025

ILZ005-181945-
McHenry IL-
212 PM CDT Mon Aug 18 2025

...A STRONG THUNDERSTORM WILL IMPACT NORTHERN MCHENRY COUNTY THROUGH
245 PM CDT...

At 212 PM CDT, Doppler radar was tracking a strong thunderstorm near
Hebron, or near Harvard, moving east at 30 mph.

HAZARD...Wind gusts up to 50 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

Locations impacted include...
Greenwood, Wonder Lake, Spring Grove, McCullom Lake, Woodstock,
Hebron, Holiday Hills, McHenry, Lakemoor, Bull Valley, Johnsburg,
Ringwood, Richmond, Pistakee Highlands, Prairie Grove, Island Lake,
and Fox Lake.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

A Severe Thunderstorm Watch remains in effect until 600 PM CDT for
northeastern Illinois.

&&

LAT...LON 4228 8820 4230 8861 4250 8858 4249 8820
TIME...MOT...LOC 1912Z 271DEG 26KT 4241 8849

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...50 MPH

DK

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