NWS Chicago Decision Support Briefing - Forecast

509
FXUS63 KLOT 042329
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
529 PM CST Wed Nov 4 2026

.KEY MESSAGES...

- Times of rain continue through Friday, greatest chances occur
later Thursday into Thursday night. Some drizzle could occur
tonight.

- Occasionally breezy and cool conditions are expected through
Friday.

- Freezing temperatures will be possible Friday and Saturday
nights and Sunday night could have a small portion in far
northwest IL.

&&

.DISCUSSION...
Issued at 1140 AM CST Wed Nov 4 2026

A cutoff low pressure system continues lumbering northeastawrd
and interacts with another vigorous southward-moving area of
vorticity from central Canada today through Friday. As a cold
front and associated dry slot swings through this afternoon, 850
mb temperatures drop from the low teens to the single digits
below zero Friday and Friday night indicating cooler air
entering the region. Temperatures fall back into the 30s
Thursday night with lower 40s in NW IN where extra lake effect
clouds will be possible. Then, lows fall back into the upper 20s
to mid 30s across NW IL Friday night and similar temperatures
will also be possible Saturday night. Additionally, gusts
between 20 and 30 mph decrease this evening, but a few sporadic
20 to 25 mph gusts cannot be ruled out overnight. Gusts between
20 and 25 mph could still occur on Thursday as a trough drops
down within the north and western side of the low. Some wrap
around spotty showers and drizzle are likely along and out in
front of this disturbance. This large area of resultant
vorticity now slides southeastawrd as a surface high slides
through overhead into the south central CONUS. Then, as the
disturbance and cold air move Lk MI, lake effect showers will be
possible over NW IN and over Lk MI. High pressure follows behind
and switches trajectories around decreasing instability, and
dry air comes in. This allows high temperatures to warm back up
through the weekend in the 50s to the 60s for next week. Lows
warm through the 30s around the middle of next week.

Northwest flow aloft takes over into next week as ridging ins
entrenched across the southwestern CONUS. This keepss the
forecast area dry through the rest of the forecast period as
energetic waves stay to the north near the border with Canada.

Roller

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 529 PM CST Wed Nov 4 2026

Forecast concerns include...

Mvfr cigs through the period, with ifr cigs possible Thursday.
Periods of drizzle and showers through the period.
Wind shift/cold front to northerly Thursday afternoon/evening.

Areas of drizzle and a few showers will move across the
terminals this evening with patchy drizzle likely to continue
overnight and into Thursday morning. Though there remains
uncertainty for how widespread the drizzle will be. Brief vis
reductions will be possible with any of the drizzle that occurs.
Scattered showers will be possible Thursday, especially
along/ahead of a cold front that will move across the area in
the late afternoon and early evening.

Prevailing mvfr cigs are expected through the period with areas
of low mvfr cigs this evening, mainly with the drizzle. Low
mvfr cigs are expected to become prevailing Thursday morning and
and its possible ifr cigs may become prevailing by Thursday
afternoon, especially across northwest IL and at RFD. The best
chance of ifr cigs for the entire area appears to be with the
cold front as it moves across the area. Cigs are expected to
slowly lift back through mvfr Thursday evening, behind the cold
front.

Southwest winds are expected through midday Thursday, gusting at
times into the 20kt range. Winds will steadily turn northwest
through mid afternoon Thursday, then shift northerly with the
cold front. Its possible directions may remain around 360
degrees for several hours Thursday evening, as gusts increase
in the 15-20kt range. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 PM CST Thursday for the IL and IN
nearshore waters.

&&

$$

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