Hail and Damaging Winds in Illinois, August 29, 2022

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1056 AM CDT Mon Aug 29 2022

.UPDATE...
Issued at 1056 AM CDT Mon Aug 29 2022

Scattered severe thunderstorms continue to be possible this
afternoon.

An upper-level trough continues to reside across the upper Midwest
with a surface cold front located along a line from Dodgeville,
WI to Cedar Rapids, IA. Ahead of this cold front a warm and very
moist airmass is in place characterized with temperatures already
in the upper 70s and lower 80s with dew points in the low to mid
70s. This warm moist airmass has continued to become unstable with
RAP mesoanalysis showing 2500 to 3000 J/kg of MLCAPE located
across northeastern Illinois and northwest Indiana which has
allowed the showers and thunderstorms located over eastern Iowa to
continue to develop and begin to approach severe limits. Wind
shear this morning continues to be the limiting factor with 0-6 km
bulk shear currently being analyzed around 15 to 20 kts, but
these values are expected to increase as the mid-level flow
increases this afternoon with values forecast to reach around 35
to 40 kts which is sufficient to support the development of
organized severe thunderstorms. These storms would be capable of
producing damaging winds upwards of 80 mph, possibly some large
hail, and possibly a tornado or two. Given this potential a Severe
Thunderstorm Watch has been issued for most of our northeastern
Illinois counties until 4 PM this afternoon, but an additional
watch may be need later this afternoon as storms move into
northwest Indiana.

While guidance has continued to struggle with the ongoing
thunderstorms moving across northern Illinois, the potent
environment across the area should allow additional storms to
develop this afternoon which guidance has continued to support.
These additional thunderstorms would also pose a severe threat
given the more favorable environment characterized by better
instability and wind shear as noted above. The severe threat is
expected to continue through at least mid evening as the cold
front moves through the area. Once the front clears the area this
evening the going forecast continues to look in good shape with
high pressure moving in which will keep the weather pleasant for
the rest of the upcoming week.

Yack

&&

.SHORT TERM...
Issued at 355 AM CDT Mon Aug 29 2022

Through Tuesday...

The main forecast focus this morning revolves around the
increasing threat for an episode of severe wind producing
thunderstorms this afternoon and evening, some potentially
significant. Very heavy rainfall is also of concern with these
storms, and as such at least a localized flash flood threat could
develop for some areas later this afternoon into the evening.

Aside from a few lingering storms across far eastern sections of
my northwestern IN counties, all is quiet across the area, at
least for the time being. Our main focus for convective
development today will be tied to a potent cold front expected to
shift southeastward across the area this afternoon through the
evening. An extremely moist and unstable environment
characterizes the prefrontal environment across the region, and
this was sampled well in yesterday evenings RAOBs upstream of the
area. For example, both the 00z TOP and OAX soundings indicated
MLCAPE values of nearly 4,000 J/KG thanks to the presence of
extremely steep mid-level lapse rates (8.5C + per KM) within an
Elevated Mixed Layer (EML). This is of concern for our area today
as our airmass overhead will be similar and will support explosive
thunderstorm development once the capping inversion erodes. This
looks very likely to occur over northern IL as soon as the early
afternoon timeframe (1-3 pm) when mid-level height falls/cooling
associated with the digging upper trough over the Upper Midwest
begin to overspread the surface frontal zone.

Prior to our main severe weather threat this afternoon, it
remains possible that a scattered band of elevated storms could
develop across WI southward into northern IL along the nose of
the better lower-level Theta-E advection around daybreak and shift
eastward across northeastern IL through the mid-morning hours.
Confidence on the extent of these storms remains low at this time,
but should these storms develop, there could be a hail threat
with them given the very steep mid-level lapse rates noted above.

Once surface based storm initiation takes place this afternoon,
potentially right overhead across far northern IL, it is expected
to rapidly intensify and organize thanks to the strong
instability and increasing mid-level flow. Initial storm
development may be in the form of supercell structures, with an
associated large hail and wind threat. However, rapid upscale
growth into a high wind producing MCS is likely by mid to late
this afternoon, with very strong damaging wind gusts (60-80 mph)
becoming the primary hazard. The severe and heavy rainfall threat
with the MCS should gradually sag southward late this afternoon
and evening, before the convection shifts out of my southern
counties sometime late this evening into the overnight. The
primary severe threat north of I-80 looks to be in the 2 to 6 pm
timeframe, with areas south of I-80 likely to be more in the 4 to
9 pm time frame.

Heavy rainfall and possible flash flooding will also be of concern
later today through this evening given the very rich moisture.
Precipitable water values near 2" will support extremely heavy
rain rates with these storms, so even short residence times could
result in a quick 1 to 2" rainfall amounts. In areas that become
favorable for west-to-east training, excessive rainfall amounts in
excess of 3" could easily occur. At this time, areas along and
south of I-80 look to have the highest potential for at least a
short period favorable for west-to-east training storms late this
afternoon into the evening.

Once the storms end with the frontal passage into tonight, we
will be transitioning to a cooler and much less humid airmass for
Tuesday. Expect pleasant conditions on Tuesday with highs in the
70s to around 80 amidst breezy northwesterly winds.

KJB

&&

.LONG TERM...
Issued at 355 AM CDT Mon Aug 29 2022

Tuesday night through Sunday...

Dry and pleasant weather is expected through the remainder of the
work week, with high temperatures forecast to be in the upper 70s
to around 80. Temperatures do look to moderate back into the 80s
next weekend as upper level ridging tries to build eastward into
the area, but at this time the weather continues to look quiet.

KJB

&&

.AVIATION...
For the 12Z TAFs...

Primary aviation forecast concerns:

 * Widely scattered showers/storms possible early this AM across
   far northern IL
 * Timing of afternoon thunderstorms, including severe risk

A few widely scattered showers and storms are possible across far
northern IL through the mid morning hours. One such storm is
currently moving across RFD. Areal coverage of these should remain
low, so we have opted to leave a formal mention for any showers or
storms at the Chicago area terminals this morning.

Attention then turns to today's thunderstorm potential this
afternoon. Confidence continues to increase that we will see
robust thunderstorms develop somewhere across far northern IL
after 19z. These storms are expected to become severe, with the
potential for some instances of very strong wind gusts in excess
of 60 kt and very heavy rainfall. The favored timing for these
storms continues to be the mid to late afternoon hours for the
Chicago area terminals, and the early to mid afternoon hours for
RFD. The only change from the 06z TAFs was to upgrade the mention
from a PROB30 to a tempo group. The focus for these storms will
sag south with the cold front into this evening, with generally
light westerly winds expected overnight. Winds then shift
northwesterly and become breezy Tuesday morning.

Petr/KJB

&&

.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...UPDATED
National Weather Service Chicago/Romeoville IL
1138 AM CDT Mon Aug 29 2022

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-301645-
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-
1138 AM CDT Mon Aug 29 2022 /1238 PM EDT Mon Aug 29 2022/

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

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
     Limited Tornado risk.
     Limited hail risk...up to quarter size.
     Elevated damaging wind risk...up to 80 mph.
  Limited Flooding Risk.
  Limited Excessive Heat Risk.

DISCUSSION...

While there is a chance for a few severe thunderstorms this
morning, more robust thunderstorms are expected to develop after
about 1 PM. Rapid intensification could support some isolated
instances of hail. However, strong potentially destructive winds
are expected to quickly become the primary severe threat through
the late afternoon and early evening hours. A tornado or two is
also possible.

Very heavy rainfall is also expected to accompany these storms,
and this could result in areas of flooding, especially in any
locales that experience prolonged periods of storms. The main
threat for isolated flash flooding may be realized south of
Interstate 80 this afternoon and evening.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

  No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed this afternoon and evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward east southeast at 40 to 45 mph.

LMZ740>745-301645-
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-
1138 AM CDT Mon Aug 29 2022

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...This Afternoon and Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Limited Waterspout Risk.
     Elevated thunderstorm wind gust risk...up to 65 knots.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

  No hazardous weather is expected at this time.

Yack

Visit us at www.weather.gov/chicago

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

Special Weather Statement
National Weather Service Chicago/Romeoville
1056 AM CDT Mon Aug 29 2022

ILZ003-004-008-010-011-291700-
De Kalb IL-Boone IL-Winnebago IL-Ogle IL-Lee IL-
1056 AM CDT Mon Aug 29 2022

...Gusty thunderstorms will impact portions of eastern Ogle, central
Lee, De Kalb, eastern Winnebago and Boone Counties through NOON
CDT...

At 1054 AM CDT, Doppler radar was tracking a line of thunderstorms
producing gusty winds, moving east at 40 mph. This line of storms
may intensify.

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...
Rockford, DeKalb, Belvidere, Loves Park, Machesney Park, Sycamore,
Dixon, Roscoe, Rochelle, South Beloit, Rockton, Poplar Grove, Genoa,
Amboy, Shabbona, Rockford Airport, Candlewick Lake, Cortland, Cherry
Valley and Davis Junction.

Including the following interstates...
 I-39 between mile markers 80 and 123.
 I-88 between mile markers 48 and 95.
 I-90 between mile markers 1 and 31.

This includes...  Northern Illinois University, Boone County
Fairgrounds, Lee County Fairgrounds, Rock Cut State Park, Rock Valley
College, Rockford Aviators Baseball, and Rockford Speedway.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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

A Severe Thunderstorm Watch remains in effect until 400 PM CDT for
north central and northeastern Illinois.

&&

LAT...LON 4249 8871 4176 8870 4163 8894 4179 8963
      4200 8926 4222 8918 4249 8917
TIME...MOT...LOC 1554Z 281DEG 36KT 4209 8925

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

Borchardt

Special Weather Statement
National Weather Service Chicago/Romeoville IL
1127 AM CDT Mon Aug 29 2022

ILZ005-006-012-013-019>021-023-032-103>108-291930-
McHenry-Lake IL-Kane-DuPage-La Salle-Kendall-Grundy-Kankakee-
Livingston-Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-
Including the cities of Crystal Lake, Algonquin, McHenry,
Woodstock, Waukegan, Buffalo Grove, Mundelein, Gurnee, Aurora,
Elgin, Naperville, Wheaton, Downers Grove, Lombard, Carol Stream,
Ottawa, Streator, La Salle, Mendota, Marseilles, Oswego,
Yorkville, Plano, Morris, Coal City, Minooka, Kankakee,
Bourbonnais, Bradley, Pontiac, Dwight, Fairbury, Evanston,
Des Plaines, Schaumburg, Palatine, Northbrook, Chicago, Cicero,
Oak Lawn, Oak Park, La Grange, Calumet City, Oak Forest, Lemont,
Orland Park, Park Forest, Joliet, Bolingbrook, Plainfield,
Mokena, Channahon, Manhattan, Wilmington, Crete, Peotone,
and Beecher
1127 AM CDT Mon Aug 29 2022

...Thunderstorms with gusty winds moving across northern
Illinois...

Thunderstorms with gusty winds will continue moving across
northern Illinois through early afternoon. The strongest storms
will move through Kane, Kendall, DuPage, and Will counties, where
winds may be strong enough to move lawn furniture and even down a
few tree limbs. Otherwise, all thunderstorms will produce frequent
lightning and downpours.

Additional thunderstorms may develop this evening with a damaging
wind threat. Stay up to date on the forecast.

Borchardt