Alert archive / Illinois / Flash Flood Warnings in 2025

Flash Flood Warning 23, NWS Chicago, IL

August 18, 2025Boone County, Winnebago County, Illinois

NWS Chicago, IL issued this flash flood warning at Aug 18, 2025, 6:52 PM CDT for Boone County, Winnebago County, Illinois. It was in force for 3 hr. No storm report in the archive falls inside the area while it was in force.

The area

The counties it named.

The facts

Issued: Aug 18, 2025, 6:52 PM CDT (23:52 UTC)
Expired: Aug 18, 2025, 9:30 PM CDT
In force: 3 hr
Issued by: NWS Chicago, IL
Area: Storm-based polygon
Flood severity: None

Counties Boone County, IL, Winnebago County, IL

The radar

The Milwaukee radar (KMKX), 80 km from the middle of the area, began a volume at 23:53 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

What forecasters said that day

The local office's own words before this flash flood warning went out, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Chicago, IL wrote its last area forecast discussion before this flash flood warning went out at Aug 18, 6:37 PM CDT, 15 minutes earlier.

Area forecast discussion, Aug 18, 6:37 PM CDT, 15 minutes before

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Hazardous weather outlook, Aug 18, 3:43 PM CDT, 3 hours 9 minutes before

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

In the hours before

Special weather statement, Aug 18, 2:12 PM CDT, 4 hours 40 minutes before

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

Special weather statement, Aug 18, 2:53 PM CDT, 3 hours 59 minutes before

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Local storm report, Aug 18, 5:07 PM CDT, 1 hour 45 minutes before

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Local storm report, Aug 18, 5:57 PM CDT, 55 minutes before

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Special weather statement, Aug 18, 6:25 PM CDT, 27 minutes before

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Local storm report, Aug 18, 6:42 PM CDT, 10 minutes before

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The office that day

Everything else NWS Chicago, IL issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Aug 17, 7:17 PM CDT	Flash Flood Warning	LOT	LaSalle	3 hr
Aug 18, 1:14 PM CDT	Special Weather Statement	LOT	Winnebago	46 min
Aug 18, 1:45 PM CDT	Special Weather Statement	LOT	Boone	30 min
Aug 18, 2:12 PM CDT	Special Weather Statement	LOT	McHenry	33 min
Aug 18, 2:21 PM CDT	Flood Watch	LOT	Boone, Cook, DeKalb, DuPage, Grundy, Kane and 9 more	13 hr
Aug 18, 2:53 PM CDT	Special Weather Statement	LOT	DeKalb	37 min
Aug 18, 3:17 PM CDT	Severe Thunderstorm Warning	LOT	DeKalb	43 min
Aug 18, 3:39 PM CDT	Severe Thunderstorm Warning	LOT	DeKalb, Kane, Lee, Ogle	51 min
Aug 18, 3:49 PM CDT	Areal Flood Advisory	LOT	DeKalb	3 hr
Aug 18, 4:04 PM CDT	Tornado Warning	LOT	Kane	26 min
Aug 18, 4:09 PM CDT	Severe Thunderstorm Warning Considerable damage threat	LOT	Cook, DeKalb, DuPage, Kane	81 min
Aug 18, 4:17 PM CDT	Areal Flood Advisory	LOT	DeKalb, Lee	71 min
Aug 18, 4:21 PM CDT	Areal Flood Advisory	LOT	Kane	3 hr
Aug 18, 4:28 PM CDT	Severe Thunderstorm Warning	LOT	DeKalb, Kane, Kendall	47 min
Aug 18, 4:29 PM CDT	Special Marine Warning	LOT		2 hr
Aug 18, 4:57 PM CDT	Areal Flood Advisory	LOT	Kane	42 min
Aug 18, 5:07 PM CDT	Areal Flood Advisory	LOT	DuPage	32 min
Aug 18, 5:26 PM CDT	Flash Flood Warning	LOT	DeKalb, Lee	3 hr
Aug 18, 5:37 PM CDT	Areal Flood Advisory	LOT	Cook, DuPage, Kane, Kendall, Will	4 hr
Aug 18, 5:47 PM CDT	Flash Flood Warning	LOT	DuPage	3 hr
Aug 18, 6:00 PM CDT	Severe Thunderstorm Warning	LOT	Ogle	30 min
Aug 18, 6:01 PM CDT	Flash Flood Warning Considerable damage threat	LOT	DeKalb, Lee	3 hr
Aug 18, 6:17 PM CDT	Areal Flood Advisory	LOT	LaSalle	3 hr
Aug 18, 6:25 PM CDT	Special Weather Statement	LOT	Ogle	20 min
Aug 18, 6:30 PM CDT	Areal Flood Advisory	LOT	Cook, Will	3 hr

Nearby

Flash Flood Warnings in Illinois, 2025 Every flash flood warning in 2025

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; radar from the NEXRAD Level II archive.