Special Weather Statement, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
350 AM CDT Thu Aug 12 2021

.SHORT TERM...
347 AM CDT

Through Friday...

Forecast concerns again focus on thunderstorm trends, and one more
day of very warm and humid weather conditions. Afternoon heat
indices expected be just below advisory criteria today. Cold front
moving through the area later tonight and Friday will bring
seasonably cooler and less humid air into the area, with thunderstorm
focus shifting south of the cwa.

Strong/severe thunderstorms finally developed along/south of the I-
80 corridor early this morning as low level jet strengthened, and
will continue to present a sporadic damaging wind and locally heavy
rainfall threat for the next few hours. Had collaborated with SPC
earlier to locally extend and expand severe thunderstorm watch until
4 am. Expectation is that this activity will gradually weaken toward
sunrise while gradually shifting east/southeast. Cold front is
expected to stall roughly south of the I-88 corridor across northern
IL this morning, and eventually develop back northward during the
day as low level winds turn south-southwest in response to large
scale height falls associated with a strong mid-level short wave
trough across the upper Midwest/northern Lakes. While some
convection may fester this morning/midday across the far south cwa,
though forecast soundings depict weak capping farther north across
area. However, the digging upper Midwest short wave will push
another cold front toward the region later this afternoon and a
combination of peak diurnal heating/instability and increasing low
level convergence ahead of the approaching front should result in
scattered thunderstorm development across far north/northwest IL by
late afternoon/early evening. Steep low-level lapse rates and a
pronounced mid-level dry layer in the 800-700 mb layer should support
some threat of damaging winds with stronger cores/linear clusters
despite relatively modest deep-layer shear. Focus for storms should
shift south of the I-80 corridor for the most part after midnight as
the stronger cold front pushes southeast.

Guidance varies with the frontal location Friday morning, through a
loose consensus is that it will be across the central/southern cwa
to start the day. Have limited chance pops generally to areas south
of I-80 on Friday, along and south of the southward moving cold
front. It's possible the better shower/thunderstorm threat shifts
south of the forecast area during the day.

Heat wise, low level temps are progged slightly cooler than the past
few days. Outflow/stabilization from early storms across the south
will also likely help to keep temps from over-achieving. Lower dew
points have settled in behind the weak cold front in the north,
though we should see low-mid 70s surface dew points across much of
the area as the front mixes back north today. Current forecast heat
indices are thus a little lower than the past couple of days,
generally upper 90s/low 100s. Cooler and less humid weather moves in
Friday, with temps in the lower 80s, and upper 70s along Lake
Michigan with modest lake breezes.

Ratzer

&&

.LONG TERM...
348 AM CDT

Friday night through Wednesday...

No significant update to extended forecast due to ongoing
convective weather overnight. Previous extended discussion remains
valid:

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/Ratzer

&&

.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.
&&

Visit us at weather.gov/chicago

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

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
442 AM CDT Thu Aug 12 2021

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-130945-
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-
442 AM CDT Thu Aug 12 2021 /542 AM EDT Thu Aug 12 2021/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Today and Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Limited hail risk...up to quarter size here.
     Limited damaging wind risk...up to 65 mph.
  Limited Flooding Risk.
  Elevated Excessive Heat Risk.

DISCUSSION...

Scattered thunderstorms may produce isolated damaging wind gusts
south of the Interstate 80 corridor early this morning, along with
locally heavy rainfall and possibly some flooding. Additional
scattered thunderstorms are possible late this afternoon into
tonight across the entire area. Some of these storms may be
severe, with damaging wind gusts the main threat, with the severe
threat decreasing late evening into the overnight.

Warm and humid conditions will persist today, though peak
afternoon heat indices should top out just under 105 degrees.

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

Friday...
  Limited Thunderstorm Risk.
Tuesday and Wednesday...
  Limited Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed late this afternoon and this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward east at 35 mph early this morning, and to the
  southeast at 35 mph later today and tonight.

LMZ740>745-130945-
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-
442 AM CDT Thu Aug 12 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...Today and Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Limited Waterspout Risk.
     Limited hail risk...up to quarter size.
     Limited thunderstorm wind gust risk...up to 55 knots.

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

Friday...
  Limited Thunderstorm Risk.
Wednesday...
  Limited Thunderstorm Risk.

Ratzer

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
345 AM CDT THU AUG 12 2021

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

0335 AM     TSTM WND GST     5 SE LISBON             41.43N 88.42W
08/12/2021  M54 MPH          GRUNDY             IL   AWOS

            AWOS STATION KC09 MORRIS AIRPORT.

0318 AM     TSTM WND GST     4 ESE VALPARAISO        41.45N 87.00W
08/12/2021  M51 MPH          PORTER             IN   ASOS

            ASOS STATION KVPZ VALPARAISO AIRPORT.

0155 AM     TSTM WND GST     3 NNE PONTIAC           40.92N 88.62W
08/12/2021  M54 MPH          LIVINGSTON         IL   AWOS

            AWOS STATION KPNT PONTIAC AIRPORT.

&&

Special Weather Statement
National Weather Service Chicago/Romeoville
351 AM CDT Thu Aug 12 2021

ILZ032-033-039-120930-
Livingston IL-Iroquois IL-Ford IL-
351 AM CDT Thu Aug 12 2021

...Strong thunderstorms will impact portions of southeastern
Livingston, Ford and Iroquois Counties through 430 AM CDT...

At 350 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Watseka to near Gibson City. Movement was
east at 30 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...
Watseka, Paxton, Fairbury, Gibson City, Gilman, Clifton, Milford,
Chatsworth, Onarga, Forrest, Sheldon, Chebanse, Cissna Park, Piper
City, Ashkum, Buckley, Cullom, Martinton, Stockland and Crescent
City.

Including the following interstate...
 I-57 between mile markers 258 and 301.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Severe Thunderstorm Watch remains in effect until 400 AM CDT for
central and east central Illinois.

&&

LAT...LON 4040 8846 4062 8846 4062 8857 4076 8858
      4099 8825 4101 8753 4049 8753 4049 8794
      4040 8793
TIME...MOT...LOC 0850Z 270DEG 25KT 4083 8781 4052 8838

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

Petr

Special Weather Statement
National Weather Service Chicago/Romeoville
420 AM CDT Thu Aug 12 2021

ILZ033-039-INZ010-011-019-121000-
Iroquois IL-Ford IL-Newton IN-Jasper IN-Benton IN-
420 AM CDT Thu Aug 12 2021

...Strong thunderstorms will impact portions of southern Ford,
southeastern Iroquois, Benton, Newton and southern Jasper Counties
through 500 AM CDT/600 AM EDT/...

At 420 AM CDT/520 AM EDT/, Doppler radar was tracking strong
thunderstorms along a line extending from near Brook to near Milford
to near Gibson City. Movement was east at 40 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...
Rensselaer, Paxton, Fowler, Kentland, Milford, Otterbein, Remington,
Morocco, Oxford, Sheldon, Goodland, Brook, Boswell, Earl Park,
Stockland, Hanging Grove, Loda, Collegeville, Donovan and Elliott.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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

&&

LAT...LON 4042 8846 4058 8780 4102 8753 4101 8693
      4091 8693 4091 8699 4084 8699 4084 8710
      4048 8709 4049 8794 4040 8793 4040 8846
TIME...MOT...LOC 0920Z 284DEG 34KT 4093 8737 4057 8773 4041 8837

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

Kluber

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
427 AM CDT THU AUG 12 2021

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

0410 AM     TSTM WND GST     4 SSW MARTINTON         40.86N 87.74W
08/12/2021  M52 MPH          IROQUOIS           IL   MESONET

            PEAK GUST FROM A PERSONAL WEATHER STATION.

0318 AM     MARINE TSTM WIND 1 NNW EAST CHICAGO      41.65N 87.47W
08/12/2021  M41 MPH          LAKE               IN   MESONET

            MESONET STATION XCAV CLINE AVENUE BRIDGE.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
450 AM CDT THU AUG 12 2021

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

0445 AM     TSTM WND GST     2 WNW RENSSELAER        40.95N 87.18W
08/12/2021  M56 MPH          JASPER             IN   AWOS

            AWOS STATION KRZL RENSSELAER AIRPORT.

&&

Special Weather Statement
National Weather Service Chicago/Romeoville
459 AM CDT Thu Aug 12 2021

ILZ032-121030-
Livingston IL-
459 AM CDT Thu Aug 12 2021

...A strong thunderstorm will impact portions of southeastern
Livingston County through 530 AM CDT...

At 459 AM CDT, Doppler radar was tracking a strong thunderstorm near
Chenoa, or 7 miles west of Fairbury, moving east at 45 mph.

HAZARD...Wind gusts up to 50 mph and nickel size hail.

SOURCE...Radar indicated.

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

Locations impacted include...
Fairbury, Chatsworth, Forrest, Cullom and Strawn.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4062 8857 4067 8858 4076 8858 4076 8875
      4084 8876 4089 8824 4062 8823
TIME...MOT...LOC 0959Z 271DEG 41KT 4075 8865

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...50 MPH

Kluber