982
FXUS63 KLOT 011726
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1226 PM CDT Thu Oct 1 2026

.KEY MESSAGES...

- Periods of rain, at times heavy, will continue through this
  evening. Additional rainfall up to 1-2" (and possibly narrow
  corridors in excess of 2") may lead to localized flooding and
  river rises.

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

&&

.UPDATE...
Issued at 952 AM CDT Thu Oct 1 2026

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.

- Izzi

&&

.DISCUSSION...
Issued at 348 AM CDT Thu Oct 1 2026

Regional radar imagery shows an expansive area of showers
ongoing across the region early this morning, with the earlier
focused axis of torrential rainfall in eastern Nebraska having
since scattered out for the most part. A broader area of steady
rainfall, with embedded higher rainfall rates, is beginning to
set up across northeast Iowa, far northwest Illinois, into
southwest Wisconsin initially this morning. This favors mainly
lighter rain and somewhat intermittent coverage for much of the
area through the morning before this activity ultimately drifts
back south into northern Illinois toward mid-late morning.

Meanwhile a separate area of renewed heavier shower development
will likely occur in response to the phasing of a disturbance
lifting northeast from the southern KS/MO border with a separate
disturbance moving into southern IA, eventually reaching the
area toward the afternoon. In contrast to yesterday, there is a
brief window when a narrow corridor of weak instability (up to
250-500 J/kg MLCAPE) may develop in response to this feature as
it moves through the area into early evening. The presence of
weak instability and near-record moisture content (2"+ PWATs)
would support higher rainfall rates, potentially locally in
excess of 1"/hr. While guidance has been fairly consistent in
the cold front progressing steadily across the area today, any
training of heavier showers and isolated embedded thunderstorms
may still yield localized corridor(s) of rainfall amounts in
excess of 2". The threat of flooding would be mainly tied to
these narrow corridors of higher amounts. However, confidence in
if and where these ultimately set up remains low and
accordingly, plan to hold off on any additional flood-related
headlines. Widespread rainfall amounts of 1-2" will still lead
to some rises on area rivers and streams.

In the wake of this evening`s cold front, persistent northerly
winds and cold air advection will lead to a seasonably cool
end to the work week on Friday with high temperatures in the
60s, lowest along the lakeshore.

The upper level pattern transitions toward a more zonal WNW to
ESE orientation this weekend into next week. Temperatures
remain near seasonable to seasonably cool with lower and more
comfortable humidity. Several low-amplitude waves will move
within the upper jet to our north and swing mainly dry cold
fronts across the area at times.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1226 PM CDT Thu Oct 1 2026

Key Aviation Messages for the current TAF period:

- Steady rain through this evening that will result in lower
  cigs/vis down to IFR conditions with a slight chance for LIFR
  conditions.

- Southwest winds are expected to turn to the north behind a
  front with the heavier rain this afternoon

Rainy conditions are expected to prevail through the day today.
Winds remain out of the southwest and just under 10 knots. As a
front over northern Illinois (currently over RFD) moves
southeast, rain rates are expected to increase and winds will
flip to the north. There is a little uncertainty on the exact
wind direction as they may flip between NNW and NNE at times.
However, there is higher confidence that the periods of
steadier, heavier rain will be accompanied by lower cigs and vis
down to at least IFR conditions. There is a chance for LIFR
conditions, but too low for formal mention in the TAF. Lastly,
there is a slight chance (less than 20 percent) for an isolated
lightning strike this afternoon, but confidence remains too low
for mention in the TAFs.

Rain will gradually taper off from northwest to southeast
through the evening. Drier, VFR conditions are expected after
midnight with northerly winds between 10 to 15 knots prevailing
through the day tomorrow.

DK

&&

.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.

&&

$$

Visit us at weather.gov/chicago