Flash Flood Warning 8, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
305 PM CDT Wed Jul 5 2023

.SHORT TERM...
Issued at 304 PM CDT Wed Jul 5 2023

Through Thursday night...

Convection has continued to blossom across sections of the CWA,
with focus remaining on three main areas:

1) NE portion of a well-defined MCV entering far west-central IL
2) Cold front across northwest Illinois
3) Lake breeze along the IL shore

More robust cells continue to favor the front and MCV with
stronger forcing, but the lake breeze and additional local
convergence areas in the Chicago metro have been also forced
several strong cells an an otherwise uncapped environment. While
deep layer shear of around 20 knots remains a limiting factor for
a more organized severe weather threat, the thermodynamic profile
with steep low-level lapse rates and low 70F dew points is
compensating to the marginal shear. Convective strength has shown
near severe to marginally severe levels into the Chicago metro,
and with loosely organized convective clusters becoming more
likely with the front MCV, this has the making of being a drawn-
out afternoon and early evening of rather messy radar
presentation. Overall activity will gradually congeal with
eastward extent across the CWA through mid-evening. While some
more discrete cells could produce marginally severe hail, damaging
wind gusts and locally heavy downpours/flooding will be the
primary concerns. Coverage of severe convection will be dependent
on the development of more cohesive and long-lasting clusters and
could foster bowing segments from localized RIJs in the dry mid-
levels. Regarding the active Cook/DuPage Flood Watch, potential
rainfall rates of just 1-2" in an hour or less may result in
renewed flooding from Sunday's substantial rainfall.

Behind the convection, lingering showers may persist across the
east half of the CWA through the overnight hours within the right
entrance of an exiting upper jet streak. A NNW/NW push behind the
cold front overnight through Thursday will then result in
dangerous swimming conditions along the Indiana shore, but
especially Porter County, Thursday afternoon into the evening.

Kluber

&&

.LONG TERM...
Issued at 205 PM CDT Wed Jul 5 2023

Friday through Tuesday...

Friday is shaping up to be a fairly nice day with highs around the
80 degree mark and lowering humidity levels.

A gradual warming trend is forecast to begin on Saturday through
the early part of next week with highs returning to the low to mid
80s. Meanwhile, as an upper level trough over Hudson Bay will
slowly move down over Southern Ontario, it will influence the
upper level steering flow to a more northwest pattern over
Northern Illinois. At the surface, dew points will likely start to
creep upwards, back in the low to mid 60s on Saturday. Between
Saturday and Monday, models are suggesting weak ripples of
vorticity passing over the area that will keep chances for rain
and even a rumble of thunder through the weekend and into the
early part of next week, but confidence will be low on exact
placement and timing. Being under the right entrance region of a
week 300 mb jet provides for a little more encouragement for
thunder for areas south of I-80 on Saturday, but as that jet
drifts northward over the weekend the confidence weakens as well
except for when another vorticity node passes overhead.

DK/Borchardt

&&

.AVIATION...
For the 18Z TAFs...

Convection continues to gradually mature in two general areas --
the first near Illinois and Wisconsin borders and the second
along the northern fringe of an approaching MCV over west-central
Illinois. Over the next few hours, convection is expected to
increase in coverage ("fill-in") from the Wisconsin border to
central Illinois and organized into a cluster or two by early
afternoon, which will then move east across northern Illinois
this evening. Scattered convection may develop ahead of any
cluster, as well. With this in mind, the inherited TEMPO groups
at the terminals remain appropriate with 2-3 hour windows for
thunderstorms at some points this afternoon and evening. Any storm
may bring gusty winds generally from the westerly direction with
peak gusts at or above 30kt. In fact, convective gusts as high as
50 kt cannot be ruled out, particularly near DPA/ORD/MDW/GYY this
evening. Brief reductions in visibility are expected with
convection, too.

Behind convection this evening, light and variable winds will
become northwesterly and remain light through daybreak. Patchy
MVFR cigs may develop near ORD/MDW/GYY, as well. After daybreak, a
lake breeze with a north/northeasterly wind shift is expected to
move inland particularly near ORD/MDW/GYY during the morning
hours.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch...ILZ013-ILZ103-ILZ104-ILZ105 until 9 PM Wednesday.

IN...Beach Hazards Statement...INZ002...1 PM Thursday to 10 PM
     Thursday.

LM...None.
&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
244 PM CDT Wed Jul 5 2023

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-061945-
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-
244 PM CDT Wed Jul 5 2023 /344 PM EDT Wed Jul 5 2023/

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 Hail Risk...up to quarter size.
     Limited Damaging Wind Risk...up to 65 mph.
  Limited Flooding Risk.
  Limited Excessive Heat Risk.

DISCUSSION...

Scattered storms will continue across the Chicago area this
afternoon. Meanwhile, a separate area of thunderstorms near the
Mississippi River will move eastward this evening. All storms may
produce damaging hail and winds, as well as torrential downpours
and frequent lightning.

Outside of thunderstorms, warm and humid conditions will continue
to lead to heat index values between 95 and 100 degrees.

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

Thursday and Friday...
  Limited Thunderstorm Risk.
Saturday...
  Limited Flooding Risk.
  Limited Thunderstorm Risk.
Sunday through Tuesday...
  Limited Thunderstorm Risk.

DISCUSSION...

.SPOTTER INFORMATION STATEMENT...

  Spotter reports would be greatly appreciated.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east northeast around 20 mph this afternoon, then
  storm motion increasing to east at 30 to 40 mph this evening.

LMZ740>745-061945-
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-
244 PM CDT Wed Jul 5 2023

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:
     Limited Hail Risk...up to quarter size
     Limited Thunderstorm Wind Gust Risk...up to 55 knots.

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

Thursday through Tuesday...
  Limited Thunderstorm Risk.

Borchardt

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