633
FXUS63 KLOT 181742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1142 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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1141 AM CST Tue Nov 18 2025

Widespread low-end MVFR stratus at TAF issuance will lower into
IFR levels for much of the afternoon and early evening as an
upper-level low over eastern Iowa crosses northern Illinois.
Periods of drizzle and -SHRA are expected during this time,
particularly mid to late afternoon. As the low exits to the east
this evening, ceilings should rise slightly into low-end MVFR
levels. MVFR ceilings will then persist through Wednesday,
though should lift into higher MVFR levels by the afternoon.

ENE winds around 10 knots this afternoon will settle NE under
10 knots tonight, then gradually veer ESE for much of Wednesday.

Kluber

&&

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

&&

$$

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