Alert archive / Illinois / Flash Flood Warnings in 2025

Flash Flood Warning 28, NWS Chicago, IL

August 18, 20254 counties in Illinois

NWS Chicago, IL issued this flash flood warning at Aug 18, 2025, 7:45 PM CDT for 4 counties in Illinois. It was in force for 84 min. 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, 7:45 PM CDT (00:45 UTC)
Expired: Aug 18, 2025, 9:09 PM CDT
In force: 84 min
Issued by: NWS Chicago, IL
Area: Storm-based polygon
Flood severity: None

Counties DeKalb County, IL, Kendall County, IL, LaSalle County, IL, Will County, IL

The radar

The Chicago radar (KLOT), 36 km from the middle of the area, began a volume at 00:48 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.

The nearest sounding before it

Lincoln, IL (KILX), launched 00:00Z, 0.8 hours before the alert went out and 158 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 2571 J/kg
Mixed-layer CIN: -24 J/kg
Mixed-layer LCL: 1289 m above ground
0-6 km bulk shear: 18 kt
0-1 km storm-relative helicity: 55 m²/s²
0-3 km storm-relative helicity: 94 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

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, 1 hour 8 minutes earlier.

Area forecast discussion, Aug 18, 6:37 PM CDT, 1 hour 8 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

Hazardous weather outlook, Aug 18, 3:43 PM CDT, 4 hours 2 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

Local storm report, Aug 18, 7:03 PM CDT, 42 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

Special weather statement, Aug 18, 7:07 PM CDT, 38 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

Local storm report, Aug 18, 7:17 PM CDT, 28 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

Local storm report, Aug 18, 7:25 PM CDT, 20 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

Local storm report, Aug 18, 7:35 PM CDT, 10 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

Local storm report, Aug 18, 7:42 PM CDT, 3 minutes before

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

The office that day

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

Issued	Product	Office	Counties	Lasted
Aug 18, 7:02 PM CDT	Flash Flood Warning	LOT	Cook, Will	70 min
Aug 18, 7:02 PM CDT	Special Weather Statement	LOT	Lee	13 min
Aug 18, 7:07 PM CDT	Flash Flood Warning	LOT	LaSalle	92 min
Aug 18, 7:07 PM CDT	Special Weather Statement	LOT	Will	23 min
Aug 18, 7:12 PM CDT	Severe Thunderstorm Warning	LOT	DeKalb, LaSalle, Lee	33 min
Aug 18, 7:37 PM CDT	Flash Flood Warning	LOT	McHenry	3 hr
Aug 18, 7:38 PM CDT	Flash Flood Warning Considerable damage threat	LOT	LaSalle	3 hr
Aug 18, 7:53 PM CDT	Special Weather Statement	LOT	Kendall	22 min
Aug 18, 8:11 PM CDT	Severe Thunderstorm Warning	LOT	Will	19 min
Aug 18, 8:12 PM CDT	Flash Flood Warning	LOT	Cook, Will	2 hr
Aug 18, 8:26 PM CDT	Flash Flood Warning Considerable damage threat	LOT	Cook	3 hr
Aug 18, 8:32 PM CDT	Special Marine Warning	LOT		88 min
Aug 18, 8:35 PM CDT	Areal Flood Warning	LOT	DeKalb, Kane, Kendall, LaSalle, Lee	14 hr
Aug 18, 8:56 PM CDT	Areal Flood Advisory	LOT	Kankakee, Will	3 hr
Aug 18, 9:09 PM CDT	Areal Flood Warning	LOT	Kendall	3 hr
Aug 18, 9:20 PM CDT	Areal Flood Advisory	LOT	Cook, DuPage, Kane, Will	13 hr
Aug 18, 11:33 PM CDT	Areal Flood Warning	LOT	Cook	13 hr
Aug 18, 11:43 PM CDT	Areal Flood Advisory	LOT	Kendall	11 hr
Aug 18, 11:54 PM CDT	Special Weather Statement	LOT	Will	21 min
Aug 19, 12:03 AM CDT	Flash Flood Warning	LOT	Cook, Will	87 min
Aug 19, 12:04 AM CDT	Special Weather Statement	LOT	Will	26 min
Aug 19, 12:05 AM CDT	Special Marine Warning	LOT		115 min
Aug 19, 12:15 AM CDT	Special Weather Statement	LOT	Will	15 min
Aug 19, 12:30 AM CDT	Special Weather Statement	LOT	Will	30 min
Aug 19, 4:02 AM CDT	Small Craft Advisory	LOT		51 hr
Aug 19, 4:18 AM CDT	Beach Hazards Statement	LOT	Cook, Lake	57 hr
Aug 19, 8:55 AM CDT	River Flood Warning	LOT	Will	24 hr
Aug 19, 10:52 AM CDT	Areal Flood Advisory	LOT	DeKalb, LaSalle, Lee	6 hr
Aug 19, 10:53 AM CDT	Areal Flood Advisory	LOT	Cook, DuPage, Will	6 hr
Aug 19, 3:06 PM CDT	River Flood Warning	LOT	DuPage, Will	17 hr
Aug 18, 8:21 PM CDT	Flash Flood Warning	LOT	Lake	3 hr
Aug 18, 9:12 PM CDT	Flash Flood Warning	LOT	Porter	2 hr
Aug 18, 11:33 PM CDT	Areal Flood Advisory	LOT	Lake, Porter	13 hr
Aug 19, 12:38 AM CDT	Flash Flood Warning	LOT	Lake	112 min

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; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.