528
FXUS63 KLOT 311834
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
134 PM CDT Sun Aug 31 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 next week accompanied by a
  shot of well below average temperatures.

&&

.DISCUSSION...
Issued at 134 PM CDT Sun Aug 31 2025

A tranquil weather pattern will continue to start the week as
high pressure remains firmly established across the region through
Tuesday evening. Conditions on Monday and Tuesday will be similar
to today, just with a few degrees of warming each day. Weak
gradient flow will allow daily lake breezes to march steadily
inland each afternoon, holding temperatures in the low/mid 70s
lakeside. While widespread dense fog doesn`t look like it will
be an issue, patches of shallow fog appear possible both Monday
and Tuesday mornings in spots away from Chicago. Finally, some
cooling of a persistent 850-700 mb warm layer towards Tuesday
may result in a sliver of near-surface instability development
during the afternoon, suggesting perhaps a small chance for a
sprinkle or two out of a more developed Cu deck, but with such
low chances, no weather mention is needed in the grids at this
point.

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. There does continue
to be some ensemble spread in regards to the overall strength
and southward extend of the Upper Low into the Upper Great Lakes
late in the week. This spread becomes particularly noteworthy
in regards to the potential for a secondary impulse and
reinforcing shot of cold air to dig into our region along the
western periphery of the upper low on Friday. Nevertheless, the
main message 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 remains
unchanged.

The main precipitation chances with Wednesday`s incoming cold
front look like they should primarily focus during the
afternoon, evening, and early overnight, mainly 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 fairly fast-moving, so precipitation amounts don`t look
particularly significant, but coverage of showers does look
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 into
the lower 40s 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 60s (low 60s north, mid-upper 60s south).

Focus then turns to a potential follow-up system Thursday night
into Friday. The signal for this feature hasn`t necessarily been
consistent from run-to-run, but remains a notable signal within
the ensemble guidance suite as a compact but very fast-moving wave
slingshots around the base of the departing upper low. While
forecast confidence remains low, this will be a system to watch
given the rather significant low-level mass response advertised
should the pieces come together, resulting in a period of
strong/gusty winds and another round of showers.

Carlaw/KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1233 PM CDT Sun Aug 31 2025

No widespread significant aviation weather concerns are
anticipated through the period.

High pressure will remain in control through the period with
light and variable winds becoming somewhat steadier from the
east into mid afternoon. Stratocumulus clouds in the Chicago
metro will erode from the east as a more concentrated push of
lake air spreads inland. Winds will diminish to calm/light and
variable after sunset then increase modestly from the east early
Monday afternoon. Added a mention of patchy fog at DPA with
cooling and light winds supporting some potential development.
The patchy fog signal looks less favorable at KRFD but may need
to add a mention with later issuances.

MDB

&&

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

&&

$$

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