133
FXUS63 KLOT 282344
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
644 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 /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 644 PM CDT Mon Sep 28 2026

A high pressure ridge crossing the area tonight will result in
winds of around or less than 5 knots gradually veering from ENE
to SSE, with calm winds likely for much of the overnight period.
Winds will then settle SSW up to 10 knots mid morning Tuesday
into the afternoon before shifting SSE in the evening.

The combination of light winds, SKC skies aloft, and higher
low-level moisture advecting off Lake Michigan provides a
decent radiational fog setup overnight. However, the influence
of the nocturnal urban heat island may delay, or even prevent,
substantially lower visibility at ORD and especially MDW.
Conditional on fog blossoming across northeast Illinois
overnight, visibility at ORD and DPA would be expected to lower
into LIFR/VLIFR levels for several hours from the pre-dawn hours
through an hour or two after sunrise (roughly 09 to 13/14Z).

Any fog that develops should be shallow enough to quickly erode
mid to late morning, leading to VFR conditions with increasing
cirrus through the remainder of the day and into the night.

Kluber

&&

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

&&

$$

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