123
FXUS63 KLOT 291826
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
126 PM CDT Tue Sep 29 2026

.KEY MESSAGES...

- Periods of rain are expected Tuesday night through Friday,
  with the greatest coverage on Thursday.

&&

.DISCUSSION...
Issued at 335 AM CDT Tue Sep 29 2026

While slower than previous model guidance suggested, shallow
fog is beginning to expand in coverage early this morning across
portions of the area outside of Chicago, some of which is
dense. The general expectation is for areas of fog to persist
through the first part of the morning commute, with some fog
potentially drifting into portions of Chicago toward daybreak
(though confidence in this remains on the lower side). Any fog
that does develop should erode quickly between 8-10 AM CDT.

GOES water vapor imagery depicts the high-amplitude upper level
pattern ongoing across the CONUS early this morning. Narrow
ridging located across the Mississippi Valley and western Great
Lakes is squeezed in between deep upper troughing and associated
closed low centered over southern CA/NV, and the remnants of
the slow moving Nor`easter currently drifting northeast across
New England.

In between the western trough and the upper ridge overhead
there is an expansive plume of deep moisture lifting across the
Great Plains toward the Upper Mississippi Valley, which is
expected to be further moistened with the arrival of the
tropical remnants of Polo which recently made landfall across
central Baja California. This will set the stage for a multi-
day stretch of rainfall across the region, beginning potentially
as early as tonight here locally into portions of northwest
Illinois as warm advective showers blossom atop a low-level
frontal zone. Shower coverage then gradually expands across much
of the area on Wednesday, including the Chicago metro.
Confidence in the timing of the various rounds of greater shower
coverage remain on the lower side, with some breaks possible at
times.

The combination of both Gulf and Pacific moisture will feed
anomalously high moisture content into the region. In fact,
modeled PWATs approaching or even exceeding 2" is toward the
upper echelon of climatology per DVN/ILX RAOBs for this time of
year. This would support the potential for heavy rainfall and
an associated localized flooding threat. Do think this may be
mainly tied to any isolated embedded thunderstorms that
potentially redevelop over a narrow more focused corridor,
though confidence in if and where that occurs remains low. The
aforementioned closed upper low across the SW CONUS will be slow
to merge back into the northern stream upper jet, which may
lead to showers persisting into the day on Friday as well before
the axis shifts fully south of the area Friday evening.

Temperatures through late week will be dependent upon shower
coverage each day with temperatures in the lower to mid 70s
favored in areas with greater shower coverage and upper 70s to
around 80 in areas south of the rainfall axis (likely south of
I-80).

The upper levels then transition toward a zonal to quasi-zonal
pattern for the weekend into early next week. While a few low-
amplitude waves likely move within the flow aloft, any
associated precipitation is currently favored to remain mainly
north of the area. North to northeasterly surface winds will
support cooler temperatures to start the first full week of
October with highs in the 60s and lows in the 40s to around 50.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 126 PM CDT Tue Sep 29 2026

Key messages for the 18Z TAF period:

* A system of rain moving in late tonight into early Wednesday
  and lasting through Wednesday. Associated MVFR likely, IFR
  possible.

A large and persistent storm system will bring widespread rain
to the area on Wednesday. Rain should begin falling at RFD
overnight and spread into Chicagoland during the early morning.
Rain is then expected through the daytime. Periods of MVFR vsbys
and MVFR to IFR cigs will be possible, especially pushing into
the later morning and afternoon. The potential for thunder is
low.

Meanwhile, SSW winds largely below 10 kt this afternoon will
remain S to SSW through the period. Gusts to near 20 kt are
anticipated during the day on Wednesday.

Doom

&&

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

&&

$$

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