351
FXUS63 KLOT 030012
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
612 PM CST Tue Dec 2 2025

.KEY MESSAGES...

- Isolated to scattered snow showers possible on Wednesday with
  a point-based 20-40% chance for any measurable snow

- Well below normal temperatures Wednesday night through the
  weekend, with the coldest period bringing minimum wind chills
  of about 10 to 20 below Thursday morning and Thursday evening.

- Additional chances for snow Saturday night-Sunday and again
  early next work week.

&&

.DISCUSSION...
Issued at 250 PM CST Tue Dec 2 2025

Tonight through Wednesday:

The stubborn low stratus deck kept temps for most locations
solidly below guidance (teens to around 20F), except near the
lake in Illinois and NW Indiana where a break in the overcast
allowed for highs to reach the lower to locally mid 20s.

As high pressure centers over the lower Ohio Valley tonight into
Wednesday morning while a cold front (trailing from strong low
pressure over Hudson Bay) approaches from the northwest,
strengthening low-level southwesterly flow should at least
temporarily erode the low clouds. Skies will still remain mostly
cloudy from mid and high level clouds though. Expect nearly
steady to slowly rising temps in response to southwest winds
gusting to 20-25 mph at times. Thinking is that the snowpack
should generally limit stronger winds than might have occurred
without snow on the ground.

The cold front will move across the area from the late morning-
midday across the northwest half or so of the CWA and through
the southeast half in the early to mid afternoon on Wednesday.
With the strongest cold advection lagging well behind the front
and only modest large scale forcing, this is not a typically
favorable setup for much, if any snow, near the cold frontal
trough. Largely discounted the more aggressive NAM and 15z RAP
depictions in favor of the conceptual model and global guidance
which is far more paltry with QPF.

In general, with the non-NAM forecast soundings lacking
saturation above -10 or -11C, even getting snow showers will
likely depend on a sufficiently deep saturated layer (and steep
enough lapse rates) reaching cold enough temps aloft. With the
stratus layer possibly eroding tonight, it`s unclear to what
extent this will be the case. With all of this being said,
lowered PoPs down to 20-40% for potential isolated to scattered
(likely non-accumulating) snow showers and perhaps brief reduced
visibility. There may be a short window for some lake
enhancement into northwest Indiana, though primarily east of
Porter County.

The other item of note on Wednesday is that it offers the best
chance to approach or locally exceed the freezing mark (30-34F
forecast range) until sometime next week.

Castro

Wednesday Night through Friday Night:

A lobe of the tropospheric polar vortex that`s been wobbling
around Hudson Bay area of Canada for a few days (and associated
with the deep surface low driving Wednesday`s cold fro-pa) is
expected to remain there for at least the next week. A strong
shortwave trough digging down the western flanks of the polar
vortex into Wednesday evening will result in an amplifying
upper trough, dislodging some pretty brutal early season Arctic
air south into the region Wednesday night through Thursday
evening.

The duration of the bitterly cold air looks short lived, but
could be rather potent with both high and low temps potentially
getting within a few degrees of the records for Chicago and
Rockford (see climate section below). One interesting forecast
element is that the progged 925 mb temps are not exceptionally
cold, so the unusually deep snow cover for early December will
be doing some work.

Temps Wednesday night should drop below zero across most of
interior northern IL assuming skies at least partially clear
out, which it looks like they should. Temps may struggle to get
much above 10F for the coldest interior areas of northern IL on
Thursday, with even "milder" areas close to the lake in NW IN
and in the Chicago urban corridor (and near the lake in NW IN)
only reaching the 15-20F range for highs. The surface high will
slowly shift east Thursday evening, likely supporting pretty
ideal radiational cooling conditions, allowing for temps to
rapidly drop to below zero (locally well below zero) outside of
Chicago. It wouldn`t be surprising if typical favored cold
spots briefly dip down to 10 below zero or even a bit colder by
the early to mid evening.

As the surface high moves farther east in the late evening and
overnight, southerly winds will develop and result in temps
leveling off and then rising overnight Thursday night.
Temperatures will then "warm" up to the lower-mid 20s on
Friday ahead of another weaker cold front approaching on Friday
night. We`ll need to watch for a short period of light snow or
snow showers (~20% PoPs) for now as a low-amplitude short-wave
traverses the region ahead of the front.

Castro/Izzi

Saturday through Tuesday:

The large scale pattern will remain active but northern
stream/Polar jet dominated next week. This could yield a few
additional opportunities for snow (~30-50% PoPs), but unlikely
to cause any higher end impacts. The first of these windows is
with a clipper/hybrid type system passing nearby to our
southwest Saturday night-Sunday, with another (possibly more
wound up) system Monday night into Tuesday. Temperatures will
remain below normal, but not as cold as our late Wednesday night
through Thursday stretch this week.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 612 PM CST Tue Dec 2 2025

Key Messages:

- IFR/MVFR ceilings are expected through most of the TAF period.

- There is about a 40% chance for snow showers from around
  midday tomorrow through the afternoon.

- A northwesterly wind shift is expected tomorrow behind a cold
  front.


The region remains socked beneath a largely IFR stratus deck
early this evening. The western terminus of this cloud deck was
inching eastward, but it`s still not entirely clear whether it
will reach the terminals and/or whether some additional
scattering of this cloud deck will occur to allow for a period
of VFR conditions tonight into tomorrow morning. Either way, IFR
or MVFR ceilings will likely be observed tomorrow both ahead
and behind a cold front dropping southeastward through the
region before the low ceilings likely clear out for good
tomorrow evening.

As the cold front passes through the area, it may be accompanied
by some snow showers. There is still some uncertainty over how
widespread the coverage of these snow showers will be in
northern Illinois and northwest Indiana, so the going PROB30
groups still appear to be appropriate for conveying this
possibility. If snow showers were to occur, they would likely
struggle to accumulate, but the snowflakes would likely be of
poor quality and be capable of knocking visibilities down even
lower than what is presently advertised in the going TAFs.

Southwesterly winds will turn a bit gusty tonight as the cold
front approaches the area, but the existing snowpack and low
inversion heights should play a role in tempering the magnitude
of gusts. Winds will then shift to a northwesterly direction
behind the front with post-frontal winds likely retaining
magnitudes similar to those of the pre-frontal winds.

Ogorek

&&

.CLIMATE...
Issued at 542 PM CST Tue Dec 2 2025

Updated at 250 PM CST Tue Dec 2 2025

Here are the current record low temperatures for Thursday into
Friday this week:

Chicago           Cold High       Low
Thursday 12/4     13 (1991)      0 (1893)
Friday 12/5                      4 (2005)

Rockford          Cold High       Low
Thursday 12/4      7 (1991)     -4 (1991)
Friday 12/5                     -5 (2005)

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 PM this evening to 10 AM CST
     Wednesday for Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 9 PM Wednesday to 9 AM CST Thursday
     for Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from midnight tonight to 3 PM CST
     Thursday for Gary to Burns Harbor IN-Burns Harbor to
     Michigan City IN.

&&

$$

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