464
FXUS63 KLOT 011755
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1155 AM CST Mon Dec 1 2025

.KEY MESSAGES...

- Winter Weather Advisory for the entire area this afternoon
  into tonight for 1-4 inches of snow, with the highest amounts
  generally near and south of the Kankakee River.

- Well below normal temperatures will persist through this
  weekend with wind chills -5 to -15 possible Thursday and
  Friday mornings.

&&

.UPDATE...
Issued at 1130 AM CST Mon Dec 1 2025

No changes were made to the Winter Weather Advisory in effect.
The main tweaks were to accumulating snow onset time, and snow
ratios and amounts.

The short-wave bringing the snow later today 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.

It will take some time to complete top-down saturation for
steady, accumulating snow to start, and as such refined the
categorical PoPs a bit later than the previous forecast, but
not a big change to the message. There probably will be a short
~1-2 hour window of flurries and then visibility quickly dropping
to near or under 1 mile with untreated roadways becoming snow
covered and slippery. While it may take until closer to, or
even a bit after, 4pm for most of the Chicago metro and points
east to see accumulating snow, we opted to graphically message
the expected onset time vs. any changes to the advisory start
time.

Despite a deep DGZ today and tonight, 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. With this thinking in mind, updated snowfall
amounts came down a bit particularly for the northern 1/2 of the
CWA. Most locations should end up in the 1-3" range by the time
the snow ends tonight, with best chance for amounts in the
~3-4" range south of the Kankakee River Valley.

Castro

&&

.DISCUSSION...
Issued at 239 AM CST Mon Dec 1 2025

Patchy freezing fog will be possible early this morning, mainly
west and south of the Chicago metro area. Fog here at LOT has
slowly improved over the past hour and with a rather shallow
moisture layer, expect the fog to remain patchy and variable and
plan to continue SPS mention and will monitor trends through
daybreak.

Winds have shifted southeast across the southeast portion of
Lake Michigan and this has allowed a narrow band of lake effect
snow to develop over the lake and as winds turn southerly
through the morning, this lake effect snow is expected to remain
over the lake and then shift further northeast today and thus,
no mention of light snow or flurries this morning.

Snow is expected to spread and develop across the entire area
this afternoon impacting the afternoon/evening commute. Some
minor changes this morning included increasing snow/water ratios
into the 17:1 range to account for a bit deeper DGZ zone passing
across the area for a 2-4 hour period, which may result in a
period of moderate to perhaps briefly heavy snow. Still some
uncertainty for exactly where the best banding may develop,
which could end up being along/south of I-80. Qpf amounts still
look reasonable in the 0.2 to 0.25 inch range, though perhaps
higher in a narrow band. This would then yield 2 to 4 inches of
snow with locally higher amounts possible, mainly south of I-80
as noted above. A shift further south would then lower amounts
across northern IL and its possible across far northern IL,
amounts may only be in the 1-3 inch range. While amounts remain
on the low end of advisory criteria, at least for the northern
half or so of the cwa, had to make a decision for an advisory or
SPS and given the potential for moderate snow falling during the
rush hour, opted to go with an advisory for the entire area, two
segmented to account for an earlier start in IL and a later end
across northwest IN. Falling snow is expected to end prior to
the Tuesday morning rush hour, but any untreated surfaces may
be slippery or snow covered.

Tuesday is expected to be dry but cold. After morning lows in
the teens for most areas, highs may struggle to reach the mid to
upper 20s. Temps may increase back to around or just above
freezing on Wednesday ahead of a strong cold front. Precip
chances remain uncertain along and ahead of the front and its
possible, maybe likely that precip will develop right over the
cwa during the afternoon. Previous discussion noted the
potential for some freezing drizzle and that is possible along
with a mix. For now, have not made any changes to the blended
guidance which is high chance pops along with all snow for
precip type. Precip would be short in duration and quickly move
east of the area Wednesday evening.

Northwest winds will increase behind the front with gusts into
the mid 20 mph range during the evening, slowly diminishing into
the 10 mph range by Thursday morning. Low temps by this time
will likely be in the single digits for most of the area, with
subzero single digits across the northwest cwa, with wind chills
into the -5 to -15 range. High pressure will move across the cwa
on Thursday as high temps struggle into the lower/mid teens. The
high moves east Thursday evening with southerly winds by Friday
morning but low temps still in the subzero single digit lows for
most areas away from Chicago and wind chills in the -5 to -15
range again.

Quite a bit of uncertainty for the end of the week into the
weekend. There seems to be some consensus for a system moving
across the area, but large differences in timing, strength,
temps, precip types, etc. Blended guidance is now just slight
chance pops for the weekend and made no changes. cms

&&

.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

&&

.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 from 4 PM CST /5 PM EST/ this
     afternoon to 3 AM CST /4 AM EST/ Tuesday for INZ001-
     INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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