309
FXUS63 KLOT 021725
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1225 PM CDT Tue Sep 2 2025

.KEY MESSAGES...

- Period of rain showers expected with a strong cold front
  Wednesday afternoon and evening. A few embedded non-severe
  thunderstorms are also possible.

- Below to well below normal temperatures are expected from
  Thursday through this weekend.

&&

.DISCUSSION...
Issued at 306 AM CDT Tue Sep 2 2025

Through Wednesday Night:

Following some patchy early morning fog, one more quiet weather
day is anticipated across the area. Expect afternoon
temperatures today to be a couple of degrees warmer than those
observed on Monday, which should generally result in inland
readings topping out around 80 under partly cloudy afternoon
skies. However, similar to the past couple of days, weak
gradient flow will favor an inland shifting lake breeze that
will foster slightly cooler conditions (mid 70s) along the Lake
Michigan shore.

Wednesday is still shaping up to be our day of transition as
the well advertised large scale weather pattern shift across
North America sends a strong surface cold front across the area
through the day. Rain chances will ramp up with and behind this
front by mid to late morning across northwestern parts of IL,
then spread across the Chicago metro area and points south and
east through the afternoon and evening hours of Wednesday before
ending.

Ahead of the front, dewpoints across southern sections of the
area (mainly south of I-80) are forecast to mix out into the
lower 50s amidst air temperature rising into the lower 80s. The
net result will be limited instability within fairly deep
inverted-V profiles in advance of the front. While the overall
poor thermodynamic environment with and ahead of the front is
expected to curtail the threat of severe weather, lapse rates
look sufficient to support some instances of charge separation
in the clouds and hence the presence of isolated to widely
scattered thunderstorms embedded within the larger area of
showers. The coverage of these showers will be rather widespread
for a short period Wednesday afternoon into early Wednesday
evening, though the fast pace of the front is expected to limit
total precipitation amounts (generally an inch or less in most
locations).

Robust cold advection in the wake of the front will shove a
notably cooler airmass into the region late Wednesday night into
Thursday. Temperatures by early Thursday morning are currently
forecast to fall into the low 40s across parts of northwest
Illinois, with mid to upper 40s favored elsewhere.

KJB


Thursday through Monday:

In the wake of Wednesday`s cold frontal passage, an
unseasonably cool air mass will settle into the region. Early
morning temperatures in the 40s and low 50s on Thursday look to
warm only into the 60s during the afternoon, which for most
locations, would make Thursday the first day to feature high
temperatures cooler than 70F since late May.

Another pronounced upper-level trough presently located over
Alaska will largely retrace the footsteps of the mid-week polar
trough and dive southeastward into Great Lakes late Thursday
into Friday. The strong dynamics associated with this
disturbance are likely to induce another round of precipitation
across the Midwest as it moves through. For our CWA, Thursday
night into early Friday appears to be the favored time frame for
this precipitation to occur, though depending on the strength
and track of an associated surface low, it is possible that some
locations (particularly in our southern/southeastern counties)
could miss out on this round of precipitation. A period of gusty
westerly winds can also be expected behind a reinforcing cold
front on Friday. The magnitude of these winds will likewise be
modulated by the precise evolution of the surface low, as will
Friday`s high temperatures, which could be a repeat of
Thursday`s temperatures or could end up being a little warmer
and closer to (but still below) early September normals, per
the latest ensemble and available MOS guidance.

Behind this reinforcing disturbance, surface high pressure
looks to settle back into the region over the weekend, favoring
another period of dry conditions with continued below normal
(though gradually warming) temperatures. Ensemble consistency
regarding the overall synoptic pattern evolution then breaks
down going into next week, resulting in lower-than-average
confidence in the forecast beyond Sunday.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1225 PM CDT Tue Sep 2 2025

Little to no pressure gradient across the Lower Great Lakes will
continue to afford light and nearly variable winds. The
exception will be along lakeshore TAF sites where a lake breeze
will keep winds decidedly easterly (northeasterly at GYY) from
early afternoon onward.

Will have to keep an eye on festering showers along a remnant
outflow boundary moving southeastward from Minnesota this
evening and overnight. With instability decreasing into northern
Illinois (especially after sunset), would have to think showers
will dissipate before reaching the terminals. Should they
manage to hold on, they would approach RFD sometime in the
03-05Z range. Additional showers and storms may develop in
Minnesota and northern Iowa overnight along a southeastward-
moving cold front and approach the terminals toward daybreak
tomorrow. However, a large remnant cold pool across Minnesota
may take time to mix away, thereby limiting instability
available for renewed development upstream. In addition, any
shower or storm may race ahead of the front, limiting forcing in
an otherwise fairly hostile regime for convection. Will
withhold any mention of precipitation at the TAF sites through
daybreak for now and let later shifts watch for emerging trends
upstream.

After daybreak, southwest winds will become gusty at the
terminals ahead of the approaching cold front. In spite of
meager low-level moisture return prior to the front, at least
filtered sunshine should allow for enough instability to develop
by mid-morning to support the development of showers and storms.
development should happen near RFD first and then spread
eastward toward DPA/ORD/MDW/GYY in the afternoon. Confidence in
thunder occurring at any given TAF site is between 30 and 50%.
So, will introduce -SHRA with PROB30 groups for thunder at
RFD/ORD/MDW (any precip should reach DPA/GYY after the 24-hour
TAF window). Largely VFR cigs should prevail ahead of and behind
the front.

Finally, a northwesterly wind shift is expected with the front
at ORD/MDW toward the very end of the 30-hour TAF window.

Borchardt

&&

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

&&

$$

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