793
FXUS63 KLOT 281940
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
240 PM CDT Mon Sep 28 2026

.KEY MESSAGES...

- More fog possible late tonight into early Tuesday morning,
  some may become locally dense.

- Widespread rain is likely across the region in the late
  Wednesday through Thursday night time frame.

&&

.DISCUSSION...
Issued at 240 PM CDT Mon Sep 28 2026

Surface high pressure remains dominant across the Great Lakes
region tonight into Tuesday. Light winds and mainly clear skies
tonight will facilitate additional fog development (locally
dense in spots) late tonight into early Tuesday morning as air
temperatures drop below crossover values. We have maintained the
inherited mention of fog through early Tuesday morning and
expanded it to include most of the forecast area east of I-39.
Fog will dissipate by mid-morning, giving way to a mild and dry
autumn day, albeit with increasing mid- and high-level
cloudiness in the afternoon. Temperatures will again peak in the
70s to near 80, except slightly cooler along the Illinois Lake
Michigan shoreline.

While quiet weather persists through Tuesday, a wetter pattern
takes over mid- to late week. Following tonight`s landfall of
Hurricane Polo in Baja California, its remnants will track
rapidly northeastward into New Mexico before merging with a
longwave trough advancing across the Desert Southwest into
Wednesday. At the same time, strengthening southerly low-level
flow will stream deep Gulf moisture (precipitable water values
exceeding 1.9 inches) northward toward an approaching cold
front. The front will gradually slide southward into the Midwest
and western Great Lakes Wednesday into Thursday, setting the
stage for periods of heavy rainfall from the Desert Southwest
into the lower Great Lakes.

Rainfall chances will ramp up across northwestern Illinois and
southern Wisconsin on Wednesday as a lead wave ejects eastward
across the Corn Belt. Uncertainty remains regarding how far
southeast this axis of rainfall will extend during the day, as
the surface boundary lags to the northwest. Most areas south of
I-80 could remain dry through the day, leading to a sharp
temperature gradient across the region: highs in the 60s to low
70s in rainy areas, and near 80 in drier southern areas.

Eventually, the frontal zone and main axis of rainfall will
shift east-southeastward across the entire forecast area on
Thursday as combined Pacific and Gulf moisture streams into the
Great Lakes. Total rainfall amounts and the exit speed of the
rain shield remain uncertain. However, near-record precipitable
water values near 2 inches (rare for early October per sounding
climatology at ILX and DVN) suggest rainfall could exceed 2 to 3
inches in localized areas through Thursday night.

The surface cold front should move south of the area by Friday,
ending the wet pattern as high pressure builds in for the
weekend. While the forecast maintains low-end rain chances
Friday afternoon into the weekend, conditions are expected to
remain largely dry. Cooler temperatures with highs in the 60s
will return Friday through the weekend.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1240 PM CDT Mon Sep 28 2026

Key aviation forecast message:

- Fog, possibly dense, is likely across the Chicago metro areas
  late tonight into early Tuesday morning.

A north-south oriented surface high pressure ridge was centered
across the upper Midwest early this afternoon. This ridge will
shift east across the western Great Lakes into this evening,
with light/calm winds and generally clear skies setting up good
radiational cooling and conditions favorable for fog development
across the area overnight. Latest model guidance is in general
agreement in favoring northeast IL and the Chicago metro
terminals for widespread fog, some of which may possibly become
dense (LIFR/VLIFR). Have maintained inherited timing from
previous TAFs (development as early as 06Z, lowest vis in the
08-13Z range), though did nudge visibilities down a bit more
solidly into IFR at ORD/MDW/DPA and GYY. Always tricky how low
visibilities will get at the warmer, more-urban ORD/MDW
terminals, but the potential for a period LIFR/VLIFR toward
daybreak does exist. If later model runs maintain these trends,
then confidence may increase to further lower forecast vis at
these locations this evening. Model forecasts continue to
indicate mainly low-end VFR/MVFR vis for RFD.

Otherwise, outside of early fog Tuesday, VFR conditions are
expected. Light/calm winds overnight should become south to
south-southwest by mid-morning Tuesday. Current guidance
suggests a potential weak south-southeast lake push toward the
end of the ORD/MDW 30 hour forecasts.

Ratzer

&&

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

&&

$$

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