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524
FXUS63 KLOT 130147
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
847 PM CDT Sat Jul 12 2025

.KEY MESSAGES...

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- A few thunderstorms are possible across central IL and IN
  Sunday afternoon, dry weather is expected through the rest of
  the weekend.

- After a couple day break Sunday and Monday, humidity returns
  mid-week along with chances of showers and thunderstorms.
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&&

.UPDATE...
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Issued at 847 PM CDT Sat Jul 12 2025

Isolated showers across far northern IL should dissipate this
evening.

Weak secondary surface cold front was analyzed from just west
of Rockford to near Moline as of 8 pm, moving slowly east across
the region. SPC/RAP mesoanalysis depicts ~1000 J/kg of MLCAPE
ahead of this front prior to sunset, though veered westerly
winds ahead of the boundary (minimizing low-level convergence)
and poor mid-level lapse rates and weak subsidence in the wake
of a departing short wave trough have resulted in minimal
shower/thunderstorm threat across the area. A few weak/shallow
updrafts have bubbled at times since late afternoon mainly north
of I-90 across far northern IL, though the lack of sustained
low-level forcing has allowed only a few isolated showers. The
few lingering shallow updrafts should gradually dissipate over
the next hour or so with the decrease in diurnal instability
with sunset.

Otherwise, quiet weather is expected for the remainder of the
night. Have noted a return of wildfire smoke to the upper
Midwest behind the front per visible satellite imagery. Even a
few surface obs along the Mississippi River had reported some
5-6SM visibilities in haze this afternoon, though most of the
smoke appears to be aloft at least in HRRR model smoke
depictions. Hazy skies aloft may linger into Sunday morning per
these forecasts.

Ratzer
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&&

.DISCUSSION...
Issued at 304 PM CDT Sat Jul 12 2025

Through next Saturday:

A couple of weather features we are monitoring may provide parts
of the area with a few isolated to widely scattered late day
showers. The first is a mid-level impulse currently noted in the
water vapor imagery tracking east-northeastward across central
parts of IL. This feature will be the driver of some widely
scattered showers and possibly a few storms for the next few
hours across my far southeastern counties. The second feature is
a more robust northern stream impulse currently shifting out of
the Upper Midwest into the western Great Lakes. Increased
forcing for ascent in advance of this wave has been driving
scattered showers and storms to our north in WI just in advance
of an eastward shifting cold frontal boundary. The primary focus
for these showers is expected to largely remain to our north
through the late afternoon hours. However, there remains about a
20% chance for a few isolated late day showers across northern
IL as this cold front shifts into the area. Otherwise, a
majority of the area is expected to remain dry the remainder of
the day.

A less humid airmass will filter in across northern IL in the
wake of this front tonight into Sunday, with dewpoints falling
into the 60s. This less humid airmass will remain in place
through Monday. Accordingly, primarily dry weather is
anticipated for much of the area for Sunday and Monday. However,
we are keeping an eye on a small (~20%) chance for showers and
thunderstorms Sunday afternoon, particularly across central
parts of IL and IN (areas well south of the Kankakee and
Illinois River Valley). This is a change in the forecast as this
area now looks to remain close enough to the deeper moisture to
support another threat for some isolated to widely scattered
afternoon convection. Model guidance is also now in better
agreement in tracking what appears to be a convectively enhanced
impulse east-northeastward out across MO Sunday morning, then
into central parts of IL later in the day. Given this trend we
have opted to include a 20% mention for afternoon showers and
storms across far southern sections of the area. The main threat
from any storms in this area would be locally heavy downpours.

High pressure will scoot off to our east late Monday, with
southerly flow transporting the humid, Gulf air mass back
northward into the region Tuesday through Thursday. The main belt
of westerlies should remain to our northwest Tuesday and
Wednesday, though medium range guidance does hint at perhaps a
weak southern stream disturbance meandering northeastward into the
region supporting at least chances of primarily afternoon and
evening scattered showers and thunderstorms Tuesday and especially
Wednesday. By Wednesday night into Thursday, it looks like a
northern stream trough will push eastward across the Great Lakes
dragging a cold front across the area with the threat for more
showers and thunderstorms. Another day or two break from rain
chances and humidity looks possible Friday and perhaps into
Saturday.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 622 PM CDT Sat Jul 12 2025

No significant aviation concerns this evening aside from a few
sprinkles or showers moving eastward across the terminals prior
to 02z. Otherwise, west-southwest winds will turn more west or
west-northwest but diminish in speed early this evening.
Remaining cumulus will diminish as well. West to southwest
winds under 10 kt are expected Sunday but a lake breeze will
develop along the shoreline. Expect winds to shift northeasterly
at GYY but do not have a wind shift at ORD or MDW, but it may
reach MDW late in the afternoon so will keep monitoring.

MDB

&&

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

&&

$$

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