274
FXUS63 KLOT 012025
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
225 PM CST Mon Dec 1 2025

.KEY MESSAGES...

- Winter Weather Advisory for the entire area into tonight for
  1-3 inches of snow, with isolated 4 inch totals possible,
  especially south of the Kankakee River.

- Quick shot of Arctic air with near record cold temperatures
  possible late Wed night through Thursday night.

&&

.DISCUSSION...
Issued at 225 PM CST Mon Dec 1 2025

Through Tonight:

Once again, no changes were made to the Winter Weather Advisory
in effect through this evening/early overnight. Will let the
evening shift assess the need for any changes to the planned end
times.

The short-wave bringing the accumulating snow through this evening
is positively tilted, albeit packing fairly robust forcing for
large scale ascent. This will be augmented on the mesoscale by
lower to mid-level frontogenesis that will lead to transient
enhanced banding with associated higher snowfall rates, primarily
south of I-80 per extrapolation of most recent regional radar and
latest guidance trends.

Despite a deep DGZ for this event, forecast soundings depict some
limiting factors for widespread, more efficient fluffy dendrite
type snowflakes. Not seeing complete saturation through the DGZ
(lacking supersaturation with respect to ice) and the strongest
ascent is primarily focused above the DGZ. In these scenarios,
it`s common for relatively small snowflakes that are still
effective at reducing visibility but accumulate less efficiently
(ratios as low as 10:1). The wildcard here is wherever f-gen
associated banding sets up, which again should be into the
southern half or third of the CWA. These areas will be the most
likely to have a more sustained period of 15-20:1 type ratios and
temporary snowfall rates between 1/2" to 1"/hr.

As alluded to above, once the accumulating snow starts, the
smaller flake size (for most of the CWA) will actually be quite
effective in knocking down visibility. Upstream sites over the
past couple hours have featured plenty of 1/2 to 3/4 mile
observations. With temps in the 20s during the snow, untreated
roadways will become snow covered and slippery, making for
hazardous travel conditions during with the evening commute.

Updated event total snowfall amounts again came down a bit
particularly for the northern 1/2 of the CWA. Most locations north
should end up in the 1-3" range by the time the snow ends tonight,
lowest near the Wisconsin border, with the best chance for
amounts in the ~3-4" range south of the Kankakee River Valley.
We`ll continue to message a general north to south range of 1-4".
Temperatures will drop into the teens to lower 20s after the snow
ends, though with west-northwest winds only 5-10 mph by then,
wind chills won`t be much lower than the air temps.

Castro


Tuesday through Monday:

Low confidence sky cover forecast for Tuesday with big spread in
guidance. The typically more reliable ECMWF and HRRR with
handling post-frotal CAA stratus both would suggest we stay
BKN-OVC during the day Tuesday. Would normally fairly
confidently follow the lead of these models this time of year,
but interestingly, satellite imagery doesn`t show all that much
stratus. Have trended slightly higher for sky cover tomorrow,
but it wouldn`t be surprising if it were completely sunny nor
would it be shocking if we stayed OVC all day. If skies do clear
out, then high temps could end up a couple of degrees warmer
than our current forecast which leans more toward a cloudier
solution.

If skies clear out during the day Tuesday, it would be short
lived as mid-high level OVC should quickly arrive Tuesday
evening in advance of the next system. The expected cloud cover
and strengthening southerly flow should result in minimal drop
in temps Tuesday night, with temps possibly even rising a couple
few degrees late.

Tropospheric lobe of the Polar Vortex has been wobbling around
Hudson Bay area of Canada for a few days and is expected to
remain there for at least the next week. A strong shortwave
trough digging down the western flanks of the Polar Vortex
should Wednesday should result in an amplifying upper trough,
which will dislodge some pretty brutal early season Arctic air
south into the Midwest and Great Lakes 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).

Forcing associated with that digging trough should bring some
snow showers to the area Wednesday. While cold fronts aren`t
typically overly efficient snow producers, strong low-mid level
frontogenesis associated deep vertical circulation paired with
coupled upper level jet streaks should result in strong forcing.
It doesn`t look like a big snow, but if coupling of jets pans
out as progged, then some areas could end up with a coating to
an inch of snow and some travel impacts.

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,
with even "milder" areas close to the lake in NW IN and in the
Chicago urban corridor only looking to get just barely into the
teens for highs on Thursday. Depending on how quickly the
surface high moves east Thursday evening, we could have pretty
ideal radiational cooling conditions allowing for a rapid drop
in temps to below zero readings Thursday evening. As the sfc
high moves farther east, southerly winds should develop and
probably result in temps leveling off or even rising overnight
Thursday night.

A moderation in temps is expected Friday into the weekend when
another system could bring snow to somewhere in the region.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1155 AM CST Mon Dec 1 2025

* A period of snow and IFR conditions expected late this
  afternoon through mid-late evening

* Lingering IFR CIGS are possible overnight into Tuesday morning

A large area of snow over Missouri and southern Iowa, currently
approaching the Mississippi River, will overspread the terminals
late this afternoon into this evening. Recent aircraft
observations from MDW depict a deep layer of dry air, roughly
6000-7000 ft deep, that will need to saturate before snow can
begin. The trend in guidance has been to slow the onset timing
of the snow and have made a light adjustment in this direction
for the 18z TAFs. Upstream observations have shown VSBY rapidly
dropping from 10SM to IFR/LIFR as the snow overcomes the dry
layer and begins reaching the ground. Snow should taper off and
end late this evening or just after midnight. Total
accumulations of 1-3" are expected at the terminals.

Mixed signals in guidance regarding the potential for lingering
CIGS overnight into Tuesday, which is leading to lower than
average confidence forecast. Have opted to maintain IFR CIGS
overnight into Tuesday morning with a trend upward to MVFR late
Tuesday morning into Tuesday afternoon. Quite plausible that low
cloudiness scatters out and we lose CIGS as early as late
tonight, so this portion of the forecast may need to significant
adjustments made in later TAF updates.

- Izzi

&&

.CLIMATE...
Issued at 225 PM CST Mon Dec 1 2025

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...Winter Weather Advisory until midnight CST tonight for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-
     ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Winter Weather Advisory until 3 AM CST /4 AM EST/ Tuesday for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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