621
FXUS63 KLOT 311941
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
241 PM CDT Thu Jul 31 2025

.KEY MESSAGES...

- Wildfire smoke will remain in the area through the weekend.

- High waves and rip currents will lead to a high swim risks at
  all Lake Michigan beaches at least into Friday.

- Pleasant conditions will prevail through the weekend with
  highs in the mid 70s to around 80, and lows in the 60s in
  Chicago and 50s elsehwere.

&&

.DISCUSSION...
Issued at 227 PM CDT Thu Jul 31 2025

Through Friday:

CONUS-scale GOES-19 water vapor imagery depicts broad upper-
level troughing across eastern Canada and the northern Great
Lakes atop an elongated surface high pressure system centered
over the Upper Mississippi River Valley. The resulting pattern
has allowed for pleasant conditions characterized by relatively
low humidity values and seasonably cool temperatures to prevail
across the region. The only fly in the ointment on an otherwise
comfortable day is hazy skies due to wildfire smoke rounding the
southeastern corner of the surface high. Indeed, visibilities
across the region have uniformly been between 3 and 5 miles,
with several locations dropping to as low as 2 miles from time
to time.

Tonight, the surface high pressure system in the Upper
Mississippi River Valley will shimmy eastward toward Lake
Superior. At the same time, a subtle inverted surface trough
(currently over central Lower Michigan) will evolve
southwestward over Lake Michigan and into northeastern Illinois.
The net effect should be a shift in wind direction from
northeasterly to easterly. As this occurs, the back edge of the
thickest near-surface smoke should shift southwestward away from
our area. Meanwhile, increasing convergence along the shoreline
of Lake Michigan in tandem with respectable lake instability
(marine airmass/850mb temperature differentials pushing 12-14C)
should allow for a stratocumulus field to develop and expand
westward into northeastern Illinois. In spite of marginal
moisture in the convective layer, the degree of shoreline
convergence and lake instability suggests scattered sprinkles if
not showers may develop overnight, favoring areas north of
I-80. So, opted to introduce a 20% chance of showers. Elsewhere,
the relatively low humidity levels should lead to another
relatively cool night with lows in the upper 50s to around 60.

The surface high pressure system will continue to serve as the
primary influence to our local weather tomorrow (Friday) as it
settles over the central Great Lakes. Forecast soundings depict
a pocket of dry air advecting into the region by mid-morning,
which should encourage any lingering lake-induced stratocumulus
clouds to erode. While near-surface smoke should be southwest of
our area tomorrow, upper-level smoke will still stream
deregulate overhead leading to a murky appearance to the sky.
Highs will hence be similar to today and in the low to mid 70s.

Borchardt


Friday Night through Thursday:

Expansive high pressure will remain centered across Wisconsin
and Lower Michigan and bring dry and mostly clear conditions
through the weekend. Expect high temps to range from the mid 70s
near the lake to around 80 inland. Min temps will be seasonably
cool with low/mid 60s in the Chicago urban core and 50s
elsewhere. Cannot rule out upper 40s for the typically colder
spots of interior northern Illinois Friday night.

The surface high will drift eastward to the East Coast early
next week as persistent clusters of convection track westward
over the southeast and Gulf Coast. Meanwhile, a mid-level trough
currently west of the California coast is progged to dislodge a
weaker wave across the Rockies and into the central CONUS on
Monday before crossing our area on Tuesday. Some of the
Atlantic-based moisture will be drawn NNW toward the lower Ohio
and mid-Mississippi River Valleys by Monday and especially
Tuesday. This would ultimately support isolated to widely
scattered diurnally-driven thunderstorms as close as central
Illinois Monday afternoon and over the entire forecast area on
Tuesday. A low-amplitude ridge nosing northeastward into the
southern Great Lakes may then bring drier conditions Wednesday
and Thursday next week.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1232 PM CDT Thu Jul 31 2025

Wildfire smoke reaching the surface will continue limiting
visibility to the 2 to 4 mile range through the afternoon and
evening. Per experimental HRRR smoke model output and upstream
observations across Michigan, do expect gradual improvement in
visibilities after 00Z. Outside of smoke, breezy northeast
winds will continue through the evening.

Tonight, an inverted surface trough will swing over Lake
Michigan causing winds to turn easterly. Lake instability
(water/850mb temperature differentials pushing 12-14C) will
encourage the development of a stratocumulus deck overnight,
favoring the 03 to 12Z window or so at ORD/MDW/DPA/GYY. In
spite of marginal moisture in the lake convective layer,
shoreline convergence should be enough to support at least
isolated showers during the overnight hours. (Will note a few
showers near Saginaw Bay in Michigan in the upstream airmass as
a sanity check). Chances for showers impacting any terminal
seems lower than 30%, so withhold formal mention for now. With
that said, can easily envision a need for at least VCSH to be
introduced at ORD/DPA/MDW at some point for the overnight hours.

Forecast soundings depict an erosion of moisture in the lake
convective layer after daybreak, suggesting that lake
stratocumulus clouds will similarly erode by mid-morning. For
now, will maintain SCT025 in favor of watching trends overnight
(as water/850mb temperature differentials will remain favorable
for at least a few clouds). Winds tomorrow will turn
northeasterly and remain so through the remainder of the TAF.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT Friday night for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-
     ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

     Beach Hazards Statement through Friday afternoon for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Friday afternoon for INZ001-
     INZ002.

     Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...Small Craft Advisory until 9 AM CDT Friday for the IL and IN
     nearshore waters.

&&

$$

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