438
FXUS63 KLOT 171736
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1136 AM CST Mon Nov 17 2025

.KEY MESSAGES...

- Rain is likely tonight into Tuesday morning.

- There`s another chance of rain late in the week.

&&

.DISCUSSION...
Issued at 303 AM CST Mon Nov 17 2025

Water vapor imagery early this morning shows a vigorous closed
mid-upper level low moving eastward across the central Rockies.
This upper low is progged to emerge out over the central High
Plains this morning, then weaken and devolve into an open wave
as it moves eastward into confluent mid-upper flow over the
Midwest later tonight into Tuesday. Despite the system progged
to be in a weakening state as it moves across the region, it
should still result in a period of rain late tonight into
Tuesday morning.

In advance of this system, look for mid-high level cloudiness
to be on the increase today, especially this afternoon as mid
level warm air advection strengthens in response to backing
flow in advance of this system. Low-mid level warm air advection
is progged to really ramp up in our area tonight with strong
pressure advection noted on the 285-300K isentropic surfaces.
Initially dry low levels will likely take some time to saturate
from top down, with more widespread rain likely holding off
until later this evening or more likely overnight. Guidance is
in good agreement on depicting thermal profiles that would favor
all rain in our area, however it is not uncommon in top-down
saturation/wet-bulb cooling situations like expected tonight, to
see a brief period of ice pellets at the onset of the precip.
Boundary layer temps are expected to remain above freezing and
any ice pellets would likely be too short of duration to result
in any travel impacts.

The stronger isentropic ascent should shift east of our CWA
Tuesday morning, likely resulting in an end in the more
widespread, organized rain. However, overnight rain is expected
to result in deeply saturated layer in the lower troposphere,
with forecast soundings depicting a 4-6kft deep saturated
layer. Modest shear progged within the resultant thick stratus
deck could result in sufficient collision and coalescence
leading to drizzle Tuesday morning possibly continuing into the
afternoon. In addition, the thick stratus deck should result in
temps nearly flatlining Tuesday, holding in the upper 30s to
lower 40s most areas.

Seems likely that easterly low level flow will result in a
sharpening low level frontal inversion which will probably mean
this stratus deck will have some staying power. With no strong
subsidence or dry air advection expected, seems like a good bet
that stratus will linger through Wednesday and probably into
Wednesday night as well. Should this occur, the diurnal range in
temps will likely be smaller than forecast Tuesday night
through at least Wednesday night. For now, only adjustment to
NBM was to lower Tuesday`s high temps, but wouldn`t be
surprising to see low temps a bit warmer and high temps a bit
cooler than temps offered up by the NBM Tuesday night through at
least Wed night.

Confidence in the evolution of the strong shortwave that medium
range guidance has been ejecting from the western U.S. southern
strong long wave trough has been decreasing amidst increasing
model spread. Given the growing spread in guidance, including a
growing contingent of GFS and EMCWF ensemble members completely
missing us, didn`t feel comfortable making any adjustments to
NBM pops late week. If the trend toward more guidance keeping us
dry continues, then subsequent forecasts will likely have lower
pops. Given the progged complex split flow regime with a closed
southern stream mid-upper low near the west coast, it is quite
possible that there will be further, potentially large, swings
in guidance solutions across the Lower 48 later this week and
beyond into the weekend as well.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1136 AM CST Mon Nov 17 2025

The primary aviation concerns are as follows:

* Periods of showers tonight and tomorrow morning, turning into
  drizzle

* Gradually lowering cigs from VFR to MVFR and eventually IFR

* Potential for long-duration low stratus event continuing well
  beyond the end of the TAF period

Discussion:

Light and variable winds will continue through the early
afternoon hours thanks to the passage of a surface ridge axis.
Directions should trend easterly by early-evening owing to lake
influence, before increasing in magnitude toward 10kt through
the overnight hours. An easterly wind direction will then
prevail through the remainder of the TAF period.

Showers are beginning to develop from far southeastern South
Dakota southeastward into Missouri within a strengthening
region of broad low-level warm air advection induced by an
upper-level wave entering western Nebraska. Shower coverage will
increase markedly this evening and spread eastward, reaching
the terminals in the 06-08Z timeframe. Cigs should remain VFR
through the overnight hours owing to top-down saturation
processes eroding a low-level dry layer. Visbys should remain
more or less between 4 and 6 miles in showers overnight.

After daybreak, showers should gradually transition to drizzle
as a low-level inversion strengthens across the region. Cigs
should lower into MVFR and IFR as the transition to drizzle
occurs. Visibility may drop as low as 2-3 miles where drizzle
cells are most prevalent, perhaps between 18-00Z (last 6 hours
of the 30-hour TAF window at ORD/MDW) tied to when the upper-
level wave slides directly overhead. For now, will advertise 6SM
RADZ in favor of downward adjustments in visibility as model
and observational trends allow.

Finally, the potential exists for IFR to MVFR stratus to linger
well beyond the end of the TAF period as low-level moisture gets
trapped beneath the strong inversion.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until noon CST today for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

Visit us at weather.gov/chicago