550
FXUS63 KLOT 011735
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 PM CDT Mon Sep 1 2025

.KEY MESSAGES...

- A pleasant pattern will continue through the remainder of the
  holiday weekend.

- 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 245 AM CDT Mon Sep 1 2025

A tranquil weather pattern continues through Tuesday as high
pressure remains firmly in place across the region. Accordingly,
conditions both today and Tuesday will be similar to Sunday`s
weather. The main exception being that afternoon temperatures
will be a couple of degrees of warmer, with inland readings
generally in the upper 70s to near 80 today and around 80 on
Tuesday. Weak gradient flow will also again foster lake breezes
that make steady inland progress both afternoons, thus
supporting slightly cooler afternoon temperatures (mid 70s)
along the Lake Michigan shore. Patchy shallow ground fog will
also be possible again early Tuesday morning in spots away from
Chicago as temperatures cool towards their crossover values.

Beyond Tuesday, attention continues to focus on the significant
weather pattern shift expected across North America 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.

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. The signal for this feature has began to show some
run-to-run consistency over the past 24 hours in the ensemble
guidance as a compact and fast-moving wave slingshots around the
base of the departing upper low. However, forecast confidence
remains on the lower side during this period, owing largely to
differences in the strength of this secondary impulse.
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

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1235 PM CDT Mon Sep 1 2025

Main forecast concern is fog overnight/early Tuesday morning.

Easterly winds may increase to 10kts this afternoon and then
winds will become light and variable this evening and likely
calm overnight. Light and variable winds will return after
sunrise Tuesday morning with a lake breeze expected to move
inland for the Chicago terminals, shifting winds easterly
Tuesday afternoon. Light west/southwest winds are possible for
northwest IL by mid/late Tuesday morning.

Patchy fog is expected for most locations outside of ORD/MDW
overnight into early Tuesday morning but there remains quite a
bit of uncertainty for how widespread the fog may become and how
low visibilities may drop. Trends in guidance would suggest the
lowest visibilities may be across northwest IL and central IL
and did include tempo 1sm at RFD. However, trends will need to
be monitored later this evening into the overnight hours. cms

&&

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

&&

$$

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