135
FXUS63 KLOT 012329
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
629 PM CDT Thu Oct 1 2026

.KEY MESSAGES...

- Locally heavy rainfall could result in some isolated flooding
  through mid-late evening.

- Chilliest night of the fall is expected Friday night with
  widespread lows in the 40s away from the Chicago urban heat
  island.

- Extended period of dry, pleasant fall weather expected Friday
  through potentially all of next week.

&&

.DISCUSSION...
Issued at 100 PM CDT Thu Oct 1 2026

Earlier discussion regarding this afternoon and evening`s heavy
rain threat (issued at 952 AM)...

Have opted to issue a flood watch for portions of northern IL
into Lake County Indiana for this afternoon and evening.

A cold front stretching from near Milwaukee to north central
Missouri will move slowly southeastward through tonight. An
unseasonably moist air mass is in place ahead of this front with
an axis of 70F+ dewpoints noted pooling just ahead of the
boundary across southeastern IA. The 12z ILX sounding had a PWAT
of 1.77" with SPC mesoanalysis depicting a corridor of 1.9-2.1"
PWATs pooling near the front from NW IL into SE IA and northern
MO. This is about as moist of an air mass as can occur this time
of year in this part of the country.

As the cold front moves slowly southeastward, a mid-level
shortwave trough over northwest MO is expected to shear east-
northeastward into northern IL, providing enhanced ascent
coincident with the timing of the cold front moving into
northern IL. The incredibly high dewpoints for early Oct should
contribute to some weak low level instability near and ahead of
the front despite the poor lapse rates. The weak low level
instability coupled with progged strong omega all below -10C,
should result in a very favorable warm rain process set-up,
which tends to result in very efficient heavy rain production.

Given this afternoon`s heavy rainfall looks to set up not far
from where the axis of heaviest rainfall (>1") fell yesterday,
there is increased concern for some flooding problems developing
this afternoon into this evening. The latest 3-hour flash flood
guidance is around 1.75", though naturally lower than that in
the highly urbanized areas of the Chicago metro area. There has
been a very consistent signal in every run of the HRRR since 6z
with an axis 3 hour rainfall totals of 2.5 to nearly 4" of rain
this afternoon ahead of this front, which is given the rather
low flash flood guidance, would be quite problematic.

Remainder of the discussion (late this evening onward)...

While the heavy rainfall threat is likely to diminish late this
evening, some light rain may linger through midnight north of
I-80 and only gradually end from the north south of I-80. It is
possible that light rain may not end south down to the Hwy 24
corridor until just after sunrise Friday.

Other than the chance of some lingering light rain near Hwy 24
early Friday morning, dry weather is expected through the
weekend and potentially most of next week as well. Skies will
gradually clear from north to south Friday. Despite increasingly
sunny conditions, northerly winds behind the cold front should
result in cooler temperatures and noticeably less humid
conditions Friday.

Clear skies and light winds should set the stage for the
chilliest night of the fall for most Friday night. Widespread
40s are expected outside of Chicago`s urban heat island.
Couldn`t even totally rule out an isolated 39 in the typically
chillier cold air drainage areas.

A moisture starved cold front will sweep across the area Sunday.
After a couple of days with highs in the 60s Friday and
Saturday, it could briefly warm up into the lower 70s Sunday
ahead of the front. Behind the front, highs in the 60s are
expected the beginning of next week, gradually moderating back
toward 70 by midweek. All in all, dry and very pleasant fall
weather is on tap after the rain ends tonight/very early Friday
morning.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 629 PM CDT Thu Oct 1 2026

Steady rain with IFR cigs and MVFR visby will continue at the
terminals for the next few hours. Visibility may drop as low as
1/2 to 1 mile particularly at GYY through 02Z as heavier
convective elements move overhead. Otherwise, a gradual
improvement trend is expected through the early portion of the
overnight period with cigs improving from IFR to MVFR and then
VFR.

Northeasterly winds may back north to even briefly northwesterly
overnight before a reinforcing surge off Lake Michigan turns
directions northeasterly after daybreak. Northeasterly winds
should then prevail through the remainder of the TAF period.

After a wet and soggy few days, tomorrow looks decidedly
pleasant with only a few high clouds. Will have to watch for fog
development tomorrow night given residually moist soils and
clear skies.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 10 PM CDT this evening for ILZ006-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 10 PM CDT this evening for INZ001.

LM...Small Craft Advisory from 4 AM to 10 PM CDT Friday for the IL
     and IN nearshore waters.

&&

$$

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