702
FXUS63 KLOT 302332
AFDLOT

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

.KEY MESSAGES...

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

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

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

- Dry and seasonably cool conditions will arrive this weekend
  and continue 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 /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 631 PM CDT Wed Sep 30 2026

Ceilings are expected to continue improving through MVFR and
then VFR through the next few hours as a warm frontal boundary
extending eastward from the Plains lifts northward through the
Great Lakes region. Shower coverage may temporarily decrease for
a time this evening before increasing during the overnight hours
as isentropic ascent intensifies and a series of upper-level
vorticity maxima slide overhead. With winds becoming nearly
light and variable overnight, will have to watch for areas of
drizzle between showers with associated drops in ceilings and
visibility. For now, will keep any formal mention out of the
TAF.

After daybreak, winds should gradually adopt a southwesterly
direction in advance of an approaching cold front. Shower
coverage will increase markedly by late morning and become
steady rain by early afternoon. Ceilings will lower from MVFR to
IFR as rain becomes steady. A wind shift from southwest to
northerly is expected as the front moves overhead, with a
continuation of steady rain and IFR ceilings. Have noted a
slight slowing trend in guidance in the arrival of the front,
which has been reflected in the outgoing TAFs.

Borchardt

&&

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

&&

$$

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