004
FXUS63 KLOT 302006
AFDLOT

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

.KEY MESSAGES...

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

- Large footprint of 1" to 3" rainfall amounts across the
  region may lead to significant rises on area rivers and
  streams.

- Potential for localized corridors of rainfall amounts in
  excess of 3" Thursday afternoon and evening.

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

&&

.DISCUSSION...
Issued at 306 PM CDT Wed Sep 30 2026

Several small-scale mid-level impulses, emanating from a
broader trough shifting across the southern Rockies, continue
to eject east-northeast from the Great Plains into the lower
Great Lakes this afternoon. The initial impulses have supported
a persistent, steady soaking rainfall across much of the area,
particularly along and south of the I-88 corridor. The airmass
remains unseasonably moist due to a continuous deep northward
feed of Gulf moisture. This was confirmed by a derived
precipitable water value near 1.7 inchesa near-record high for
late Septemberfrom a recent Valparaiso University sounding in
northwestern Indiana. Fortunately, a lack of appreciable
instability has kept rain rates manageable, generally below
0.25 inches per hour with only localized corridors reaching 0.5
inches per hour. Consequently, no significant hydrological
concerns are anticipated for the remainder of the day.

The focus for this persistent rain is expected to shift
gradually northward into southern Wisconsin and far northern
Illinois this evening. Shower coverage should temporarily ease
overnight before the next wave arrives with a cold front on
Thursday. A higher concern for torrential rainfall and
associated hydrological impacts exists Thursday afternoon and
evening as low-level lapse rates steepen ahead of the front.
Prefrontal instability will remain focused primarily below the
-10C level, which is unfavorable for lightning production but
highly conducive to efficient warm-rain processes and low
centroid cells given warm-cloud depths reaching 12,000 feet.

We will have to closely monitor the potential for
northeastward- training showers near the frontal boundary
capable of rain rates exceeding 1 inch per hour. Where repeated
heavy rain occurs, localized flash flooding could develop with
totals possibly exceeding 3 inches. While footprint amounts over
3 inches will be localized, broader rainfall totals of 1 to 3
inches are expected across the area through Thursday night,
likely causing aerial flooding and significant river rises
through the end of the week. These threats remain highlighted in
our Hydrologic Outlook.

The surface cold front will push south of the area on Friday,
bringing an end to the wet pattern as high pressure builds in
for the weekend. Dry and cooler conditionstypical for early
Octoberwill prevail for the weekend and early next week, with
high temperatures in the 60s.

KJB

&&

.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.

&&

$$

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