743
FXUS63 KLOT 011452
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
952 AM 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 /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 711 AM CDT Thu Oct 1 2026

Key Messages:

- Periods of rain this morning will transition to a steady rain
  this afternoon (mid-late morning at RFD) before rain tapers
  off this evening.

- MVFR and IFR ceilings and visibilities are expected at times
  through tonight. A brief period of LIFR could not be ruled
  out either.

- A northerly wind shift will occur behind a cold front this
  afternoon.


Initially, waves of rain showers will continue to pass through
the C90 airspace through this morning. Primarily VFR conditions
will be favored at the Chicago metro terminals during this time,
though brief MVFR visibility reductions may occur at times
beneath the steadier showers.

A more solid band of rain will set up over RFD by the mid-late
morning before shifting over the Chicago metro during the
early-mid afternoon. Prevailing MVFR to IFR conditions are
expected to occur for several hours as this band of steady rain
sits over the terminals. The rain will then taper off from
northwest to southeast during the evening, with VFR conditions
returning at some point thereafter.

A brief period of LIFR also could not be ruled, particularly
this afternoon at the Chicago metro terminals. The HRRR forecast
model, in particular, has had several recent runs with a narrow
band of deeper convective showers/possible storms with notable
rainfall rates setting up somewhere over the Chicago metro this
afternoon. If such a scenario were to verify, then temporary
visibility reductions to 1/2 mile or less and lightning would
become more likely at the Chicago metro terminals. However, most
other models currently do not favor such a scenario and
generally keep the heaviest rainfall and greater instability
south of the terminals, so held off on depicting this realistic
worst case scenario in the 12Z TAFs. If more model support for
this scenario crops up this morning or if observational trends
start to point towards such a scenario occurring, then these
TAFs will need to be trended in a more pessimistic direction.

Lastly, during today`s rainfall, a cold front will drop across
the area from northwest to southeast. This will result in a wind
shift from southwesterly to northerly winds between midday and
the mid-late afternoon. Confidence in the exact timing of this
wind shift still remains somewhat low, but the wind shift
should come within 1-2 hours of the forecasted wind shift times
in the TAFs.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 4 AM to 10 PM CDT Friday for the IL
     and IN nearshore waters.

&&

$$

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