434
FXUS63 KLOT 190455
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1055 PM CST Tue Nov 18 2025

.KEY MESSAGES...

- Patchy drizzle through late this evening mainly south of I-80.

- Pesky low-level clouds may limit temperatures from changing
  much over the next few days.

- Chances for rain Thursday night into Friday continue to
  dwindle.

&&

.UPDATE...
Issued at 859 PM CST Tue Nov 18 2025

Had earlier added patchy drizzle mention across the area
through about 9pm CST. Based on obs of 2-6 mile visibility in
light mist/fog over roughly the southern half of the CWA as of
this writing, continued with patchy drizzle mention for another
couple hours south of I-80. There may be localized visibility
as low as 1-2 miles in drizzle and then visibility should
generally improve overnight. Socked in stratus will only allow
temps to drop 1-2F at most (if that) by daybreak.

Castro

&&

.DISCUSSION...
Issued at 151 PM CST Tue Nov 18 2025

Recent water vapor imagery depicts the center of an upper-
level vort max sliding across northern Illinois at press time.
Differential mid-level cyclonic vorticity advection and a
pocket of relatively steep mid-level lapse rates have allowed
for the development of another round of showers and drizzle with
the upper-level shortwave, which will continue to slide through
the area through this evening.

Tonight, mid-level heights will gradually rise as deep
troughing becomes established across the southwestern United
States. Subsequent mid-level subsidence will allow a surface
high pressure system to settle into and low-level inversion to
strengthen across the Great Lakes. The stage is hence set for
trapped low-level moisture to manifest as a lingering pesky
stratus deck through the overnight hours across much of the
area. For this reason, anticipate a muted temperature drop
overnight with lows generally in the upper 30s to lower 40s.

Tomorrow, low-level northeasterly steering flow on the southern
periphery of the surface high centered just to our north may
encourage dry air to chew away at the northeastern edge of the
stratus deck near Lake Michigan. However, since the surface high
will migrate eastward with time, steering flow will tend to
weaken and back southeasterly and even southerly through the
day. Accordingly, the low-level stratus deck may stagnate across
the area, leading to yet another cloudy day. So, will go ahead
and limit the impact of the diurnal curve on temperatures and
forecast highs only in the mid 40s. With subtle upper-level
ripples propagating within the ridge overhead, cannot rule out a
few instances of drizzle tomorrow. However, forecast soundings
indicate the thickness of stratus by tomorrow afternoon may not
be thick enough to support precipitation generation. For now,
will tuck in "silent" 10 PoPs across the area in favor of
watching trends. While southwesterly steering flow will increase
modestly tomorrow night, suspect stratus will continue to
linger and cause overnight lows to be very similar to tonight.

On Thursday, an upper-level trough is expected to propagate
along the US/Canadian border well north of a separate,
southern-stream upper-level trough lifting toward the middle
Mississippi River Valley. A modest uptick in southwesterly
steering flow will hopefully be enough to finally scour away any
lingering stratus, but can certainly envision a scenario where
the stratus ends up overstaying its welcome (especially if a
low-level pressure col develops between both upper-level
features). In scenarios/areas where stratus does clear,
temperatures Thursday should rebound nicely into the upper 50s
to around 60. However, if/where stratus lingers, temperatures
could very well verify in the mid 40s.

The signal remains for the southern-stream trough to get
effectively squashed Thursday night into Friday while attempting
to lift northeastward into confluent upper-level flow (in the
wake of the northern-stream trough). Accordingly, felt it was
time to finally drastically cut back on bullish NBM PoPs across
the area given the increasingly favored scenario where most
(all) rain remains to our south. Assuming we`re done with the
low-level stratus problem by then, temperatures Friday should
be in the low to mid 50s.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1055 PM CST Tue Nov 18 2025

The primary weather concerns will continue to center around CIG
trends through the period.

While the drizzle has come to an end in most locations, IFR CIGS
persist. Overall, through the remainder of the night, some small
improvement to low end MVFR remains possible, but given recent
trends, have opted to go more pessimistic and keeping IFR CIGs
through the night. Also cannot rule out some lower VSBYs
overnight in patchy fog, particularly outside the main Chicago
terminals. Expect MVFR CIGs to persist on Wednesday.

NE winds 5 to 10 knots this evening will gradually veer E for
Wednesday, then southerly Wednesday night.

KJB

&&

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

&&

$$

Visit us at weather.gov/chicago