464
FXUS63 KLOT 021948
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 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.

- Windy conditions possible Thursday night into early Friday
  morning with gusts possibly as high as 35 mph.

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

&&

.DISCUSSION...
Issued at 248 PM CDT Tue Sep 2 2025

Through Wednesday Night:

The prolonged quiet and comfortable weather pattern will come to
an abrupt end on Wednesday. Tonight into early Wednesday will
be dry for most of the area, though not as cool as recent mornings
due to increasing cloud cover. A minor exception to the dry
conditions tonight may be in the far northwestern 1/4 to 1/3 or
so of the CWA (ie. primarily near and north of a Dixon to
Waukegan line). Remnant isolated to widely scattered showers
related to the current showers and storms from far northeast
Iowa into far western Wisconsin may sneak in during the pre-dawn
hours (20-30% PoPs in the gridded forecast). A very sharp drop-
off in instability into our area precludes any embedded
thunderstorm mention with this possible shower activity.

Wednesday-Wednesday night will feature a transition from late
summer to decidedly fall-like conditions as a strong cold front
pushes across the area. The front will press steadily
southeast after daybreak Wednesday, reaching roughly the US-24
corridor by the early afternoon. As this occurs, increasing
large scale ascent paired with good deep layer moisture will
cause shower coverage to ramp up with and behind the front,
earliest (mid morning through midday) for far northern IL, then
spread across the remainder of the Chicago metro and points
south and east through the afternoon. It does appear that the
rain should cutoff fairly quickly by sunset for areas near and
northwest of I-55, and most of the CWA dry (except perhaps far
southeast sections) by around or shortly after midnight.

Ahead of the front, breezy west-southwest winds will cause dew
points to mix out into the 50s, particularly with southward
extent, as temps warm into the upper 70s to lower 80s. Amidst
already modest mid-level lapse rates with equilibrium levels
only briefly topping 400 mb, limited instability within fairly
deep inverted-V profiles plus veering winds in advance of the
front should keep most of the precip confined to near and behind
the cold front, as alluded to earlier. Despite the overall poor
thermodynamic environment, there may be tall enough updrafts at
times for charge separation and isolated to widely scattered
thunderstorms embedded within the larger area of showers.
Confidence is high in periods of rain at times, though it
doesn`t appear to be a total washout during the most extensive
shower coverage, thus officially capped highest precip chances
at 70% (periods of showers).

The most favored area for occasional heavy downpours and a bit
more prolonged period of rain looks to be south of the Kankakee
River Valley in eastern Illinois and northwest Indiana in the
early to mid evening hours. In this area, rainfall amounts may
top out near or a bit above 3/4", primarily during the evening.
For the rest of the area, aside from locally higher amounts in
any thunderstorms, rainfall totals should generally be in the
1/4 to 1/2 inch range.

Robust cold air advection in the wake of the cold front,
especially after dark, will drive a notably chilly airmass into
the region by early Thursday. Low temperatures are currently
forecast to fall into the lower to mid 40s across interior
northern Illinois, with mid 40s to around 50F elsewhere (low-mid
50s downtown Chicago and Indiana shore). In addition, brisk
northwesterly winds on the lake will result in hazardous
conditions (large waves and strong currents) developing and
lingering into Thursday for the Indiana shore in particular.

Castro


Thursday through Tuesday:

Primary forecast concern are the winds Thursday night into
Friday morning.

A fast moving wave is expected to dive southeast on the
southwest side of the first departing upper low with a deepening
surface low developing across the western Great Lakes Thursday
night and then quickly lifting northeast Friday morning. There
is still quite a bit of uncertainty for how all of this will
evolve, including the strengthen and speed of the surface low,
but given current trends, have increased winds into the 15-25
mph range with gusts into the 30-35 mph. And higher gusts are
certainly possible during this time period.

Rain chances will be short-lived in any one location Thursday
night ahead of this system, but blended pops into the likely
range across far northern IL look on track with precip chances
quickly diminishing further south into the local area. Dry
weather is then expected until the middle of next week.

Temperatures will continue to be below normal through the period
with high temps generally in the 60s Thursday, Friday and
Saturday and then low/mid 70s Sunday-Tuesday. The coldest
mornings will be Saturday morning and Sunday morning with much
of the area seeing low temps in the 40s. With high pressure
right over the area Sunday morning, the usual cool spots may dip
into the upper 30s. cms

&&

.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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