444
FXUS63 KLOT 171144
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
544 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 /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 544 AM CST Mon Nov 17 2025

Key messages for the 12Z TAF period:

* A system of showers late this evening through the night likely
  transitioning to drizzle Tuesday morning

* MVFR cigs expected and MVFR vsbys possible associated with the
  rain/drizzle

Rain showers are expected to fill in across the area late this
evening. Light rain or drizzle may begin as early as 03 or 04Z,
but the steadier rainfall looks to move in after 05Z at RFD and
after 06Z in Chicagoland. A period or two of moderate rain will
be possible late tonight into early Tuesday morning. MVFR
cigs/vsbys may accompany any heavier showers that move over the
terminals. After sunrise, rain is expected to lighten to more of
a drizzle for the rest of the morning and possibly lingering
into the afternoon. MVFR cigs look to settle in by mid-morning
and persist through the afternoon.

Meanwhile, NE winds below 10 kt this morning will turn easterly
late this afternoon while remaining light. Speeds will get up
to around 10 kt this evening into Tuesday with occasional gusts
to near 20 kt as the storm system moves through.

Doom

&&

.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.

&&

$$

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