574
FXUS63 KLOT 301749
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 PM 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 /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1249 PM CDT Wed Sep 30 2026

- Rain, at times rather heavy, is expected this afternoon for
  immediate Chicago terminals with ocnl IFR VSBY in the heavier
  rain.
- CIGS will vary from IFR to VFR through the TAF cycle.
- Another round of rain, rather heavy at times, is expected to
  move into RFD late tonight and into the remaining terminals
  Thursday morning.

A 75-100 mile wide band of rain, locally heavy, stretches from
southern lower Michigan/northern Indiana west across northern IL
and into southern Iowa. The southern flanks of this rain is
lifting gradually northward across northern MO and southern IA
with generally lesser coverage in this area in general.
Expectation is for northward shift in the rain axis and lower
coverage of rain to overspread the terminals this evening. The
exception will likely be RFD, where a resurgence of rain is
expected late this evening into the overnight hours. The axis of
rain, heavy at times, should them begin to shift back southeast
into the immediate Chicago area terminals by mid-late morning
Thursday, likely continuing into the afternoon. Prevailing MVFR
VSBY is expected in the rain with occasional IFR VSBY associated
with the more intense rainfall.

CIGS forecast is trickier with a sharp east-west oriented
gradient across the area delineating IFR CIGS to the north from
VFR CIGS just south of the boundary. This boundary has been
bisecting MDW and and ORD for the past couple of hours. In time,
guidance suggests that this boundary and lower CIGS will shift
north with CIGS improving to MVFR and likely VFR at ORD and DPA
later this afternoon. Precise timing of this improvement is
difficult to nail down. Also tricky is whether improved CIGS
will make it into RFD this evening. For now, maintained IFR
transitioning to MVFR, but a period of VFR CIGS as far north as
RFD is certainly plausible.

South to southwesterly winds are expected through much of the
TAF cycle, though a wind shift to northerly will occur Thursday
morning at RFD and early-mid afternoon for MDW/ORD.
Synoptically, winds should tend to favor north-northwest,
however lake influence could push wind direction at ORD and MDW
more toward north-northeasterly. For now, went with 360 given
the lower than average confidence.

- Izzi

&&

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

&&

$$

Visit us at weather.gov/chicago