811
FXUS63 KLOT 012329
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
629 PM CDT Mon Sep 1 2025

.KEY MESSAGES...

- Patchy fog possible overnight into early Tuesday morning.

- An early fall storm system will move through the Great Lakes
  region toward the middle to end of the week accompanied by a
  shot of well below average temperatures.

&&

.DISCUSSION...
Issued at 126 PM CDT Mon Sep 1 2025

Through Tuesday Night:

Primary forecast concern is fog potential overnight into early
Tuesday morning. Guidance is in fair agreement for at least
patchy or shallow ground fog to develop outside the immediate
Chicago metro area overnight into Tuesday morning. However,
there is still quite a bit of uncertainty for how widespread any
fog may become as well as how low visibilities may drop. It
appears the best chance for fog is along and north of I-55 and
then along west of the Fox Valley. Given the uncertainty, opted
to include patchy fog everywhere but Chicago and areas of fog
as well as patchy dense fog mention may be needed with later
updates for portions of northwest IL.

Easterly winds this afternoon will diminish to light and
variable or calm tonight and then return to light and variable
Tuesday morning with a lake breeze expected to move inland in
the afternoon for the metro area.

High temps will likely reach the upper 70s for most areas away
from the lake this afternoon. Lows tonight generally in the 50s,
with lower 60s in Chicago. Even with the uncertainty of fog
coverage Tuesday morning, it should dissipate quickly and with
mostly sunny skies, high temps may reach 80 for most areas away
from the lake Tuesday afternoon. cms

Wednesday through Monday:

Attention continues to focus on the significant weather pattern
shift expected across North America by the middle of the week
in response to the anomalously strong mid/upper-level ridging
building across northwestern Canada. The associated buckling of
the Polar jet over this region will send impulses originating
from the Beaufort Sea south-southeastward across south central
Canada, and ultimately carving out a large upper trough/low over
the Upper Great Lakes region later in the week. Ensemble spread
continues to shrink with the evolution of the initial system
and surface cold frontal passage through early Thursday.
Accordingly, there have been no major changes with the ongoing
thinking of a period of precipitation with a strong cold frontal
passage late Wednesday into Wednesday evening, followed by a
turn towards much cooler weather by Thursday.

The main precipitation chances with Wednesday`s incoming cold
front look like they should primarily focus during the afternoon
and evening, along and behind the front itself. Ahead of the
surface boundary, dewpoints are forecast to mix out into the
upper 40s/lower 50s amidst air temperature rising into the lower
to possibly mid 80s, which will yield limited instability
within fairly deep inverted-V profiles. This front and the
attendant vort lobe will be fast-moving, so precipitation
amounts don`t look particularly significant, but coverage of
showers does look fairly widespread for a period during the late
afternoon and evening. Instability is forecast to remain pretty
meager, but sufficient perhaps for a few embedded storms given
the magnitude of incoming forcing. While not the most likely
scenario, there is a play for a stronger storm or two if the
approaching upper-level jet and associated shear reaches the the
meager instability axis. At this point, the threat for severe
weather Wednesday appears to be 5% or lower at any given point.

Robust cold advection in the wake of the front will shove a
notably cooler airmass into the region overnight and into
Thursday. Temperatures by early Thursday morning may push
towards 40 degrees across parts of northwest Illinois. Latest
indications are that northwesterly winds around 10-15 mph will
continue post FROPA, limiting the frost potential, even if
temperatures end up a bit cooler in the upper 30s. Thursday will
feature persistent westerly breezes, and temperatures only
warming into the low to mid 60s.

Focus then turns to a potential follow-up system Thursday night
into Friday. Forecast confidence remains on the lower side
during this period, owing largely to differences in the strength
and track of the secondary impulse within ensemble guidance.
Nevertheless, this is a system to watch given the the potential
for a rather significant low-level mass response advertised by
some guidance. Should all the pieces come together, a period of
strong/gusty winds and another round of showers would be likely
sometime Thursday night into Friday.

KJB/Carlaw/Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 629 PM CDT Mon Sep 1 2025

Other than patchy shallow fog late tonight through sunrise
Tuesday at DPA/GYY/RFD, VFR conditions are expected through the
period. NE winds under 10 knots at TAF issuance will either
settle VRB under 5 knots or become calm overnight into Tuesday
morning as a surface ridge remains over northeast Illinois. A
weak lake breeze is likely to shift winds ENE around 10 knots at
ORD/MDW Tuesday afternoon, but has the potential to stall east
of the terminals and prevail W/WSW winds around 5 knots through
the afternoon. Winds will ultimately shift S late Tuesday
evening ahead of an approaching low pressure system and cold
front on Wednesday.

Kluber

&&

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

&&

$$

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