613
FXUS63 KLOT 291941
AFDLOT

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

.KEY MESSAGES...

- Periods of rain, heavy at times, expected late tonight through
  early Friday morning. Significant rises on area rivers and
  streams may result.

&&

.DISCUSSION...
Issued at 241 PM CDT Tue Sep 29 2026

Early this afternoon, GOES water vapor imagery shows an
expansive plume of deep Gulf moisture (PWATs ~1.5"+) streaming
northward from the Plains across the Upper Midwest and into
Ontario. This broad latitudinal transport is driven by a strong
south- southwesterly low-level mass response east of a northern-
stream trough traversing the Canadian Rockies and a southern-
stream system advancing into the Desert Southwest. As the
northern- stream trough moves rapidly east across southern
Canada through midweek, it will push a surface cold front west-
southwestward into the Midwest and western Great Lakes.
Meanwhile, as the Desert Southwest trough moves toward the Great
Plains through Thursday, it is forecast to eject several
smaller-scale impulses east-northeast over the Corn Belt into
the lower Great Lakes. These waves, combined with a persistent
northward feed of Gulf moisture into the frontal boundary, will
set the stage for multiple rounds of heavy rain Wednesday
through Thursday night.

Initial rain showers driven by warm air advection may move into
portions of northwestern Illinois as early as later this
evening. However, the first primary wave of rain is expected to
shift into northern Illinois and parts of far northwestern
Indiana late tonight (likely after 4:00 AM) and persist through
the morning. The threat of lightning with this activity appears
low (but non-zero), but locally heavier downpours may accompany
it. Scattered shower activity will likely continue into
Wednesday afternoon and evening. The primary area of focus may
begin to shift northward toward the Wisconsin state line, though
the exact extent of this northward shift remains uncertain.
Substantial cloud cover and periods of rain will keep
temperatures in the 60s to around 70 across most of northern
Illinois on Wednesday.

The axis of rainfall, potentially accompanied by some
thunderstorms, is expected to shift back south into the area
Thursday into Thursday night as the surface frontal boundary
drops southward. The presence of near-record precipitable water
values near 2 inches (rare for early October per sounding
climatology at ILX and DVN) suggests periods of moderate to
heavy rainfall will accompany this system. Rainfall amounts of 2
to 3 inches (with locally higher totals) are possible across
the area. This threat will be highlighted in a Hydrologic
Outlook (ESF).

The surface cold front will shift south of the area on Friday,
bringing an end to the wet pattern as high pressure builds in
for the weekend. Conditions for the weekend look dry and
coolermore typical for early Octoberwith temperatures topping
out in the 60s.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 126 PM CDT Tue Sep 29 2026

Key messages for the 18Z TAF period:

* A system of rain moving in late tonight into early Wednesday
  and lasting through Wednesday. Associated MVFR likely, IFR
  possible.

A large and persistent storm system will bring widespread rain
to the area on Wednesday. Rain should begin falling at RFD
overnight and spread into Chicagoland during the early morning.
Rain is then expected through the daytime. Periods of MVFR vsbys
and MVFR to IFR cigs will be possible, especially pushing into
the later morning and afternoon. The potential for thunder is
low.

Meanwhile, SSW winds largely below 10 kt this afternoon will
remain S to SSW through the period. Gusts to near 20 kt are
anticipated during the day on Wednesday.

Doom

&&

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

&&

$$

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