535
FXUS63 KLOT 180906
AFDLOT

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

.KEY MESSAGES...

- Look for at least some scattered showers today, especially
  this morning.

- Late week rain chances continue to dwindle.

&&

.DISCUSSION...
Issued at 306 AM CST Tue Nov 18 2025

Water vapor imagery early this morning shows the upper low
we`ve been tracking beginning to shear out, becoming
increasingly east-west elongated. Guidance continues to suggest
this upper low will devolve into an open wave and gradually
weaken as it moves across northern IL this afternoon. Despite
the expected weakening, a pocket of seasonably cold air aloft
(-21C at 500mb) will still move across the area, steepening up
mid level lapse rates.

The first arc of warm air advection driven showers will be
moving out of the area in the next hour or two, but a second
band of showers and thunderstorms from southeast IA into west
central IL will move across the area through the early morning
hours. The coverage of thunderstorms has been decreasing as it
moves farther from the cold core aloft and into more stable air
mass over IL. This trend of decreasing thunderstorm coverage
should continue and just expecting showers as it moves into and
across our CWA through the early-mid morning hours.

Some additional showers, accompanying the heart of the cold
core aloft/vort max, will probably move across the area in at
least a scattered fashion this afternoon. Sufficient elevated
instability looks to be present to support a chance of some
isolated thunderstorms this afternoon, mainly south of the
Kankakee River and closer to the stronger elevated instability.

There is still a strong signal in guidance for the development
of an extensive stratus deck over the area later this morning.
Assuming the stratus materializes as expected, it will likely
stick around for quite some time as low level easterly flow and
low cloudiness helps maintain a inversion that the stratus looks
to become trapped beneath. Forecast soundings continue to
suggest the stratus deck could be thick enough to support some
drizzle late this morning into this afternoon assuming cloud
bases get sufficiently low.

Some guidance does hint at the possibility of some drier low
level air advecting in from the east/northeast late tonight into
early Wednesday, potentially chipping away a bit at the stratus
over the lake and into adjacent areas of NE IL/NW IN. Certainly
can`t rule that out, but for now continue to place most of our
eggs in the cloudier basket through Wednesday night.
Accordingly, have adjusted high temps downward a bit Wednesday
and lows upward a bit Wednesday night, but assuming stratus
holds, then the diurnal ranges could be even smaller. No changes
made to NBM for Thursday high temps, but same story, if status
is still around, then those forecast temps will need to be
lowered.

No changes made to pop/wx late in the week into the weekend.
Medium range guidance has continued to trend stronger and
farther south polar front jet. The farther south northern stream
jet results in a more suppressed and farther south track of any
energy ejecting out from the cut off southern stream low over
the southwestern U.S. The NBM pops continue to trend downward
during this time frame and given the time lagged nature of the
NBM, are probably still too high. No adjustments were made to
NBM pops during this time frame, but pops will likely come in
lower with the afternoon forecast package. In fact, other than
perhaps a brief shower accompanying a northern stream shortwave
and associated cold front, it is looking increasingly likely
that most of the area will remain dry late this week through the
weekend.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1100 PM CST Mon Nov 17 2025

Main Concerns:

- Periods of rain tonight/early Tuesday and periodic showers
  with occasional drizzle until early Tuesday evening may knock
  visibility down at times.

- IFR to MVFR CIGs likely late tonight through Tuesday evening.

- A non-zero chance for an isolated thunderstorm on Tuesday.

A weather system will impact the area overnight through Tuesday
and bring an end to the quiet/VFR flying conditions. Widespread
light to occasionally moderate rain/showers will develop
overnight, with MVFR VSBY possible at times into the morning.
The steady rain should then give way to a showery and drizzly
regime sometime after daybreak Tuesday morning, and there`s a
chance for the drizzle to linger into the early evening (later
than in current TAF). Potential is there for VSBY temporarily as
low as IFR in persistent drizzle during the late morning, for
which PROB30 mention may need to be considered in the TAFs with
the next amendment. In addition, forecast soundings suggest
enough instability aloft to possibly generate a couple TS in the
area, which will need to be assessed for later updates.

Regarding CIGs on Tuesday-Tuesday evening, the pattern and time
of year favor them deteriorating to IFR, supported by NAM and
RAP/HRRR forecast soundings. Some guidance, particularly the
MAV/LAMP MOS and probabilistic LAMP, still aren`t excited about
prevailing IFR, which continues to lower confidence a bit.
Assuming our thinking is on the right track with prevailing IFR
developing, convective activity (SHRA and any isolated TS) may
occasionally scatter the IFR CIGs. CIGs may then improve some
later Tuesday night. Winds will be easterly (5-15 kt range)
through late in the day Tuesday, shifting northeast Tuesday
evening. Sporadic gusts in the 15-20 kt range are probable.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 6 AM this morning to 3 AM CST
     Wednesday for the IL nearshore waters.

&&

$$

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