Alert archive / Illinois / Severe Thunderstorm Warnings in 2017

Severe Thunderstorm Warning 129, NWS Chicago, IL

July 21, 2017DeKalb County, LaSalle County, Lee County, Illinois

NWS Chicago, IL issued this severe thunderstorm warning at Jul 21, 2017, 12:32 AM CDT for DeKalb County, LaSalle County, Lee County, Illinois. It was in force for 43 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: Jul 21, 2017, 12:32 AM CDT (05:32 UTC)
Expired: Jul 21, 2017, 1:15 AM CDT
In force: 43 min
Issued by: NWS Chicago, IL
Area: Storm-based polygon
Hail: 0.75 in
Wind: 60.0 mph

Counties DeKalb County, IL, LaSalle County, IL, Lee County, IL

The radar

The Chicago radar (KLOT), 75 km from the middle of the area, began a volume at 05:32 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

Davenport, (KDVN), launched 00:00Z, 5.5 hours before the alert went out and 126 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: 2392 J/kg
Mixed-layer CIN: -69 J/kg
Mixed-layer LCL: 1284 m above ground
0-6 km bulk shear: 28 kt
0-1 km storm-relative helicity: 56 m²/s²
0-3 km storm-relative helicity: 151 m²/s²
Significant tornado parameter: 0.6
Supercell composite: 7.0

Open this sounding in Storm Lab

What forecasters said that day

The local office's own words before this severe thunderstorm 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 severe thunderstorm warning went out at Jul 20, 7:25 PM CDT, 5 hours 7 minutes earlier.

Area forecast discussion, Jul 20, 7:25 PM CDT, 5 hours 7 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, Jul 20, 4:59 PM CDT, 7 hours 33 minutes before

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
459 PM CDT Thu Jul 20 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019-
212200-
Winnebago-Boone-McHenry-Lake Illinois-Ogle-Lee-De Kalb-Kane-
DuPage-Cook-La Salle-Kendall-Grundy-Will-Kankakee-Livingston-
Iroquois-Ford-Lake Indiana-Porter-Newton-Jasper-Benton-
459 PM CDT Thu Jul 20 2017 /559 PM EDT Thu Jul 20 2017/

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL ILLINOIS...
NORTHEAST ILLINOIS AND NORTHWEST INDIANA.

.DAY ONE...Tonight.

WEATHER HAZARDS EXPECTED:
  Limited Thunderstorm Risk...with an associated:
      Limited Hail Risk...up to quarter size.
      Limited Thunderstorm Wind Damage Risk...up to 60 MPH.
  Limited Flooding Risk.
  Elevated Excessive Heat Risk through this evening.
  Elevated Fog Risk.

DISCUSSION:

Scattered thunderstorm development is possible along and south of a
Rochelle, IL to Rensselaer, IN line late this evening through the
overnight hours. The main threat from all storms will be
torrential rainfall. However, the strongest storms will be
capable of producing large hail and damaging winds.

Flooding will continue along portions of the Fox and Des Plaines
Rivers.

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

Friday...
  Significant Flooding Risk.
  Elevated Severe Thunderstorm Risk.
  Elevated Excessive Heat Risk.
  Limited Fog Risk.
Saturday...
  Limited Flooding Risk.
  Elevated Severe Thunderstorm Risk.
  Elevated Excessive Heat Risk.
Sunday...
  Limited Thunderstorm Risk.
Wednesday...
  Limited Thunderstorm Risk.

DISCUSSION:

An active and humid weather pattern will continue through
Saturday. Thunderstorms are likely at times Friday and more so
Friday evening into Saturday morning, especially north of
Interstate 80. The potential for heavy rain in the region is
significant which could cause worsened flooding. However, the
exact location of the heaviest rainfall is uncertain. In addition
there is a severe thunderstorm threat, mainly north of Interstate
80, on Friday and Friday night as well. Thunderstorm chances
continue on Saturday, shifting mainly along and south of
Interstate 80 by Saturday night.

Heat index readings will approach 105 degrees both Friday and
Saturday, mainly along and south of Interstate 80.

In addition, flooding will continue along portions of the Fox and
Des Plaines Rivers and may develop along other rivers of far
northern Illinois.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed late this evening.

GENERAL STORM MOTION OF THE DAY:

Moving toward the southeast at 35 to 40 MPH.

&&

LMZ740>745-212200-
Winthrop Harbor to Wilmette Harbor-
Wilmette Harbor to Northerly Island-
Northerly Island to Calumet Harbor-Calumet Harbor to Gary-
Gary to Burns Harbor-Burns Harbor to Michigan City-
459 PM CDT Thu Jul 20 2017

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE ILLINOIS NEARSHORE
WATERS OF LAKE MICHIGAN AND THE INDIANA NEARSHORE WATERS OF LAKE
MICHIGAN.

.DAY ONE...Tonight.

  Limited Thunderstorm Risk.

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

Friday...
  Significant Thunderstorm Risk.
Saturday...
  Elevated Thunderstorm Risk.
Sunday...
  Limited Thunderstorm Risk.
Wednesday...
  Limited Thunderstorm Risk.

&&

LMZ080-261-362-364-366-563-565-567-669-671-673-675-777-779-868-
870-872-874-876-878-212200-
Lake Michigan from Michigan City IN to St. Joseph MI 5 NM
offshore to Mid Lake-
Lake Michigan from Seul Choix Point to Rock Island Passage 5 NM
offshore to Mid Lake-
Lake Michigan South of a line from Seul Choix Point to the
Mackinac Bridge and north of a line from Charlevoix MI to South
Fox Island 5 NM offshore-
Lake Michigan from Charlevoix to Point Betsie MI 5 NM offshore to
Mid Lake-
Lake Michigan from Point Betsie to Manistee MI 5 NM offshore to
Mid Lake-
Lake Michigan from Rock Island Passage to Sturgeon Bay WI-
Lake Michigan from Sturgeon Bay to Two Rivers WI 5 NM offshore to
Mid Lake-
Lake Michigan from Two Rivers to Sheboygan WI 5 NM offshore to
Mid Lake-
Lake Michigan from Sheboygan to Port Washington WI 5 NM offshore
to Mid Lake-
Lake Michigan from Port Washington to North Point Light WI 5 NM
offshore to Mid Lake-
Lake Michigan from North Point Light to Wind Point WI 5 NM
offshore to Mid Lake-
Lake Michigan from Wind Point WI to Winthrop Harbor IL 5 NM
offshore to Mid Lake-
Lake Michigan from Winthrop Harbor to Wilmette Harbor IL 5 NM
offshore to Mid Lake-
Lake Michigan from Wilmette Harbor IL to Michigan City IN 5 NM
offshore to Mid Lake-
Lake Michigan from Pentwater to Manistee MI 5 NM offshore to Mid
Lake-
Lake Michigan from Whitehall to Pentwater MI 5 NM offshore to Mid
Lake-
Lake Michigan from Grand Haven to Whitehall MI 5 NM offshore to
Mid Lake-
Lake Michigan from Holland to Grand Haven MI 5 NM offshore to Mid
Lake-
Lake Michigan from South Haven to Holland MI 5 NM offshore to Mid
Lake-
Lake Michigan from St. Joseph to South Haven MI 5 NM offshore to
Mid Lake-
459 PM CDT Thu Jul 20 2017

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE OPEN WATERS OF LAKE
MICHIGAN.

.DAY ONE...Tonight.

There is a slight chance of thunderstorms over the far southern
end of the lake.

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

Periods of thunderstorms are expected through Saturday and
again possible late Wednesday.

&&

Rodriguez

Visit us at http://weather.gov/chicago

Follow us on Facebook, Twitter, and Youtube at:
www.facebook.com/nwschicago
www.twitter.com/nwschicago
www.youtube.com/nwschicago

In the hours before

Special weather statement, Jul 21, 12:30 AM CDT, 2 minutes before

Special Weather Statement
National Weather Service Chicago/Romeoville Il
1230 AM CDT FRI JUL 21 2017

ILZ010-011-019-210600-
De Kalb IL-La Salle IL-Lee IL-
1230 AM CDT FRI JUL 21 2017

...SIGNIFICANT WEATHER ADVISORY FOR NORTHWESTERN LA SALLE...
SOUTHEASTERN LEE AND SOUTHWESTERN DE KALB COUNTIES UNTIL 100 AM
CDT...

At 1230 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Amboy to near Mendota to Spring Valley.
Movement was east at 35 mph.

Winds in excess of 40 mph will be possible with these storms.

Locations impacted include...
Peru, Mendota, Oglesby, La Salle, Amboy, Utica, Earlville, North
Utica, Paw Paw, Tonica, Lostant, Sublette, Compton, Cedar Point, Troy
Grove and West Brooklyn.

Including the following interstates...
 I-39 between mile markers 41 and 87.
 I-80 between mile markers 73 and 83.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4177 8932 4175 8884 4110 8907 4110 8916
      4158 8917 4158 8938
TIME...MOT...LOC 0530Z 265DEG 30KT 4169 8925 4153 8907 4133 8921

BMD

The office that day

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

Issued	Product	Office	Counties	Lasted
Jul 21, 12:30 AM CDT	Special Weather Statement	LOT	Lee	30 min
Jul 21, 12:57 AM CDT	Severe Thunderstorm Warning	LOT	DeKalb, LaSalle	14 min
Jul 21, 8:39 AM CDT	River Flood Advisory	LOT	LaSalle	5 days
Jul 21, 9:55 AM CDT	River Flood Warning	LOT	Winnebago	10 days
Jul 21, 9:55 AM CDT	River Flood Warning	LOT	Winnebago	11 days
Jul 21, 9:55 AM CDT	River Flood Warning	LOT	Ogle	9 days
Jul 21, 2:39 PM CDT	Severe Thunderstorm Warning	LOT	Boone, Winnebago	36 min
Jul 21, 3:08 PM CDT	Severe Thunderstorm Warning	LOT	Boone, DeKalb, McHenry	37 min
Jul 21, 4:10 PM CDT	River Flood Advisory	LOT	Cook	51 hr
Jul 21, 4:14 PM CDT	Severe Thunderstorm Warning	LOT	Cook, DuPage, Kane	46 min
Jul 21, 4:41 PM CDT	River Flood Warning	LOT	Cook	3 days
Jul 21, 4:41 PM CDT	Severe Thunderstorm Warning	LOT	Cook, DuPage	49 min
Jul 21, 4:53 PM CDT	River Flood Warning	LOT	LaSalle	5 days
Jul 21, 5:00 PM CDT	River Flood Advisory	LOT	Will	51 hr
Jul 21, 5:00 PM CDT	River Flood Advisory	LOT	Will	66 hr
Jul 21, 5:00 PM CDT	River Flood Advisory	LOT	DuPage	40 hr
Jul 21, 5:06 PM CDT	Special Marine Warning	LOT		84 min
Jul 21, 5:09 PM CDT	River Flood Warning	LOT	Boone	4 days
Jul 21, 5:18 PM CDT	Areal Flood Advisory	LOT	Cook, DuPage, Kane	3 hr
Jul 21, 5:22 PM CDT	River Flood Warning	LOT	Lee	5 days
Jul 21, 6:30 PM CDT	Severe Thunderstorm Warning	LOT	Ogle	45 min
Jul 21, 6:39 PM CDT	Severe Thunderstorm Warning	LOT	Grundy, Kankakee, Livingston, Will	51 min
Jul 21, 2:34 AM CDT	Special Weather Statement	LOT	Benton, Jasper, Newton	41 min

Nearby

Severe Thunderstorm Warnings in Illinois, 2017 Every severe thunderstorm warning in 2017

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.