Special Weather Statement, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1155 PM CDT Wed Aug 11 2021

.SHORT TERM...
330 PM CDT

Through Thursday night...

Primary concern is the intensity, timing and coverage of the line
of severe storms this evening. While there was some reduction in
the insolation today due to lingering cloud cover, we are still
seeing some recovery late this afternoon. KILX 19z sounding
indicated MUCAPEs had increased to 1300 J/kg and will continue
climbing as afternoon heating continues. We also are seeing a bit
of a cap around 850 mb, but additional heating should work to
overcome any warm layer aloft. So assuming sufficient instability
will be present, low level shear profiles are becoming more
favorable for rotating updrafts. SFC-3km SRH is increasing as
well, already around 250 m2/s2. SPC mesoanalysis page is
projecting that to climb over 300 by the time the line of storms
arrives this evening. Timing looks to be slightly delayed from
the timing we had in the morning forecast, with arrival in our NW
counties now closer to 530-6pm. Severe threat will include
widespread damaging winds possibly up to 70 mph, with a few
tornadoes possible. Hail threat will be present, but no
significant hail sizes are anticipated.

Looking ahead, the next main concern will be the severe potential
late Thursday and Thursday evening. Yet another wave of strong to
severe storms is projected to develop across northern Illinois
late in the afternoon ahead of an advancing cold front. Dynamics
and kinematics point toward damaging winds as the primary hazard,
but a few tornadoes will also be possible. Hail threat will be
marginal with Thursday evenings storms.

Heat on Thursday looks to remain below heat advisory levels, and
even today had reduced heat levels from persistent cloud cover.
Will let the current heat advisory ride through expiration today.

Shimon

&&

.LONG TERM...
330 PM CDT

Friday through Wednesday...

The slowly departing cold front will keep chances of showers and
storms in our SE counties on Friday, but they should remain south
of the Chicago Metro area. The storms should remain below severe
levels on Friday, with those chances primarily in southern and
SE Illinois. Precip should depart to the SE by Friday evening,
setting the stage for a pleasant weekend with low heat and
humidity, by August standards.

A shift in the longwave pattern will allow Canadian high pressure
to slowly advance along the northern CONUS, providing northerly
breezes and lowering dewpoints. High temps through the weekend
into Monday look to remain in the upper 70s to lower 80s. A
warming trend will develop on Tuesday and Wednesday as southerly
winds develop on the west side of departing high pressure. A few
showers/storms could bubble up on Tuesday afternoon and again on
Wednesday, but coverage will be isolated for the most part.

Shimon

&&

.AVIATION...
For the 06Z TAFs...

Primary forecast concern remains thunderstorm chances early this
morning and again Thursday afternoon/evening.

There remains a chance for a few thunderstorms early this morning
with new activity developing ahead of a frontal boundary. Low
confidence for coverage with this activity but there is enough
development to include tempo thunder mention for a few hours early
this morning.

Southwest winds will remain gusty into the lower 20 kt range
ahead of these storms and then winds are expected to shift to the
west/northwest as the frontal boundary moves across the area.
Winds will turn back southwesterly by mid Thursday morning with
gusts returning to 20kt through the afternoon. Another frontal
boundary will move across the area Thursday evening shifting winds
to the northwest with winds turning more northerly early Friday
morning. Ahead of this front, there is another chance for
thunderstorms later Thursday afternoon into Thursday evening. Low
confidence for where thunderstorms will develop, which could be
south of the terminals but will continue prob mention for now. cms

&&

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

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
Issued by National Weather Service Lincoln IL
341 PM CDT Wed Aug 11 2021

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-122045-
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-
341 PM CDT Wed Aug 11 2021 /441 PM EDT Wed Aug 11 2021/

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:
     Limited Tornado risk.
     Limited hail risk...up to quarter size.
     Elevated damaging wind risk...up to 70 mph.
  Limited Flooding Risk.

DISCUSSION...

Severe thunderstorms will again be possible into this evening, with
organized thunderstorms expect to move southeast from Wisconsin
into the region. The primary threat is expected to be damaging
winds, though isolated large hail and brief tornadoes are also
possible. Locally heavy rainfall will occur with some of these
storms.

Afternoon heat index values may temporarily reach values of 100
to 105 degrees depending on the amount of sunshine in any one area.

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

Thursday...
  Limited Severe Thunderstorm Risk.
  Limited Flooding Risk.
  Elevated Excessive Heat Risk.
Friday...
  Limited Thunderstorm Risk.
Tuesday...
  Limited Thunderstorm Risk.

DISCUSSION:

Scattered thunderstorms are possible Thursday afternoon and
night. A few of these could be severe. Scattered storms will
also be possible south of I-80 on Friday.

.SPOTTER INFORMATION STATEMENT...

 Spotters may be needed this evening, especially north of I-80.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east southeast at 30 to 45 mph.

LMZ740>745-122045-
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-
341 PM CDT Wed Aug 11 2021

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...

  Hazardous conditions for small craft.
  Elevated Thunderstorm Risk...with an associated:
     Limited Waterspout Risk.
     Limited Hail Risk...up to quarter size.
     Elevated thunderstorm wind gust risk...up to 50 knots.

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

Thursday...
  Limited Severe Thunderstorm Risk.
Friday...
  Limited Thunderstorm Risk.

Shimon

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
739 PM CDT WED AUG 11 2021

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

1008 AM     TSTM WND DMG     2 SSW ROCKFORD          42.25N 89.11W
08/11/2021                   WINNEBAGO          IL   BROADCAST MEDIA

            *** 1 FATAL ***
            THREE PEOPLE WERE RESCUED FROM A MOBILE HOME
            THAT WAS CRUSHED BY A FALLEN TREE. ONE
            FATALITY WAS REPORTED. TIME CONFIRMED BY 911
            CALL CENTER.

&&

BB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
933 PM CDT WED AUG 11 2021

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

0932 PM     TSTM WND GST     DUPAGE AIRPORT          41.92N 88.25W
08/11/2021  M56 MPH          DUPAGE             IL   ASOS

            ASOS STATION KDPA DUPAGE AIRPORT REPORTS 56
            MPH GUST.

&&

WSL

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