380
FXUS63 KLOT 300827
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
327 AM CDT Wed Sep 30 2026

.KEY MESSAGES...

- Periods of rain, heavy at times, expected through early
  Friday morning.

- Large footprint of rainfall across the region may lead to
  significant rises on area rivers and streams.

- Dry and seasonably cool this weekend into early next week.

&&

.DISCUSSION...
Issued at 327 AM CDT Wed Sep 30 2026

Steady rain associated with warm advection and isentropic
ascent is already spreading across the area early this
morning ahead of a weak wave moving across Iowa. The overall
expectation remains for periods of rain to then continue through
early Friday morning thanks to a steady feed of anomalously
high Gulf and Pacific moisture paired with multiple smaller
impulses lifting into the region ahead of high amplitude upper
troughing across the western CONUS.

Modeled PWATs locally approaching or exceeding 2 inches would
be near-record for this time of year, which seems attainable
given 0Z KTOP RAOB PWATs already at 2.05". This would support at
least periods of moderate to locally heavy rainfall and
widespread rainfall amounts of 1 to 3 inches across the area
(and potentially locally higher). Determining the timing of any
potential breaks in the higher shower coverage remains lower
confidence. Nevertheless, there has been a signal in higher-res
guidance that a relative minimum in rainfall coverage may occur
this evening before renewed shower development occurs overnight
through Thursday. The threat for thunderstorms with this system
remains fairly low but still can`t rule out isolated embedded
lightning strikes with any of the heavier showers (20% chance).

The position of the axis of highest totals and the rainfall
amounts have varied slightly between models and from run to run.
However, highest amounts continue to be favored in counties
along and north of I-80, including in the Chicago and Rockford
metro areas. Given the more drawn out nature of the rainfall
paired with some breaks in the higher rates, think that the
threat of flash flooding will remain fairly localized and tied
to any isolated heavier showers and embedded thunderstorms on
Thursday. Of greater concern is the areal footprint of rainfall
which is forecast to be fairly expansive across the region. This
will likely lead to more gradual flooding of poor drainage low
lying areas and rises on area rivers and streams, some
potentially significant. Those living along or near rivers and
streams should monitor forecasts closely over the next several
days.

Temperatures through late week will be dependent upon shower
coverage each day, with temperatures in the mid 60s to lower 70s
favored in areas with greater shower coverage and mid 70s to
around 80 in areas south of the rainfall axis (likely well 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 will 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 /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 125 AM CDT Wed Sep 30 2026

Key Messages:

- Periods of rain are expected through Thursday.

- MVFR/IFR ceilings and visibilities will be observed at times
  during the TAF period.


A swath of rain will spread over the terminals prior to daybreak.
Dry low-level air should permit VFR conditions to persist for a
couple of hours after the first raindrops before steadier rainfall
arrives and ceilings and visibilities correspondingly fall to
MVFR levels. Prevailing IFR ceilings will then likely develop
over RFD later this morning, and most forecast guidance favors at
least temporary IFR ceilings occurring at ORD, DPA, and possibly
MDW as well later today. During the periods of steadiest rainfall,
IFR visibilities may also be observed. Ceilings will likely
improve at the Chicago metro TAF sites sometime late this
afternoon or evening as a surface boundary lifts northward, though
confidence remains low in the timing of when this will occur.
This boundary likely won`t lift as far north as RFD, however.

It appears that there will be a relative lull in precipitation
during the late afternoon and evening, but another wave of rain
will arrive tonight and persist into Thursday. Flight conditions
will thus deteriorate once again, though forecast guidance
currently suggests that the worst conditions won`t settle in until
after the end time of the current TAF period. There is also about
a 20% chance for isolated thunderstorms to occur near the
terminals with this second wave of precipitation, but the
probability of this occurring near any particular TAF site at this
time is too low to warrant a formal TSRA or VCTS mention in the
TAFs.

Ogorek

&&

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

&&

$$

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