430
FXUS63 KLOT 291201
AFDLOT

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

.KEY MESSAGES...

- Areas of fog, some locally dense, may impact the start of the
  commute this morning.

- 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 /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 701 AM CDT Tue Sep 29 2026

Key Messages:

- Fog and low stratus will persist in the area this morning
  through around 14-15Z.

- Rain is expected tomorrow, but showers may begin as early as
  late tonight.


Fog and associated very low stratus is ongoing this morning.
Conditions are varied across the area, with the worst
conditions (VLIFR) occurring at DPA at press time. LIFR
conditions will also remain possible at ORD through around
13-14Z there as the fog and very low clouds in central DuPage
County slowly ooze towards the ORD airfield. All TAF sites
should return to prevailing VFR conditions by 15Z.

Rain is expected to move into the region tomorrow morning, but
scattered showers out ahead of the main wave of rainfall will
likely reach RFD late tonight and may also affect some of the
Chicago metro terminals overnight as well. Initially dry low-
level air should favor VFR conditions prevailing until the end
of the TAF period, but lower flight categories may need to be
introduced into the TAFs later on.

Otherwise, outside of any rain/shower influence, sustained
winds from mid-morning through pretty much the remainder of the
TAF period should generally fall in the 5-10 kt range and
largely come from a south-southwesterly to south-southeasterly
direction.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 10 AM CDT this morning for ILZ005-
     ILZ006-ILZ012-ILZ013-ILZ103.

IN...None.
LM...None.

&&

$$

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