946
FXUS63 KLOT 061804
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1204 PM CST Sat Dec 6 2025

.KEY MESSAGES...

- A clipper system will deliver a swath of snow to the area
  tonight into Sunday morning. A Winter Weather Advisory has
  been issued for a portion of northern Illinois where snow
  totals in the 3-5" range are expected. A wintry mix also
  looks increasingly likely to be seen in locations south of
  I-80.

- Light lake effect snow may develop into portions of northwest
  Indiana and northeast Illinois late Sunday through Monday
  morning.

&&

.DISCUSSION...
Issued at 419 AM CST Sat Dec 6 2025

Through Sunday:

The main concern in the short term portion of the forecast is
the likelihood of accumulating snow from a clipper system
tonight into tomorrow. This system`s surface low (analyzed over
northeastern Wyoming at press time) will track east-
southeastward today across the central Plains and into the
Midwest in tandem with an upper-level shortwave, likely
weakening over the course of the day as it loses better upper-
level jet support. The earlier chasm between the hi-res model
guidance depicting a stronger and more northerly tracking system
and global guidance favoring a weaker and more southerly
tracking system has shrunk considerably, with the broader suite
of model guidance honing in on a happy medium solution with a
more northerly track to the system, but a less-amped up or ill-
defined surface low. The lingering model spread regarding the
strength of this incoming system is still, however, leading to
persisting finer-scale uncertainties in our snowfall and
precipitation type forecasts less than 24 hours away from the
onset of precipitation tonight.

The larger-scale picture with this incoming system remains
fairly clear, with isentropic ascent inducing a swath of
precipitation that will spread over the area late this evening
and into the overnight hours before a Canadian surface high
filters in behind the clipper and puts and end to most of the
precipitation Sunday morning (lake effect snow may persist
beyond this time near Lake Michigan). Thermal profiles are, for
the most part, expected to remain below 0C through most or all
of the event in most locations, resulting in snow being the
dominant precipitation type. However, with the more northerly
track to the system now, parts of our southern CWA appear
increasingly likely to be an exception to this as warming low-
level and surface temperatures look to push the lowest portion
of the thermal profiles there above the freezing mark for at
least a brief period of time. This would result in a wintry mix
or even outright rain occurring there for a good chunk of this
precipitation event, especially in a scenario featuring a
surface low that`s still somewhat well-defined as it tracks
across Illinois.

The presence/strength of the surface low will also modulate the
magnitude of low- to mid-level frontogenesis occurring
coincident with the snow swath. Some of the more aggressive
latest model solutions, such as the 00Z HRRR and RAP, actually
depict fairly intense ascent within the heart of their modeled
frontogenetical banding, with omega peaking within or just below
the dendritic growth zone (which would feature steep lapse
rates that would further enhance snowfall rates). If such an
outcome were to be realized, then there could be a narrow
corridor somewhere that overperforms accumulation-wise. However,
these models have a tendency to run a little hot with wintry
systems 24+ hours out, so there are reasons to be skeptical of
their snowfall output.

Regardless, the latest multi-model consensus suggests that
there is a fairly good likelihood of locations north of I-80
picking up between 2 and 5 inches of snow accumulation (highest
wherever the f-gen banding sets up) tonight through Sunday
morning. Temperatures are expected to remain in the 20s here
while snow is falling, which will allow the snow to accumulate
on roadways and make for a messy drive for anyone who is
planning to head out early Sunday morning. Have issued a Winter
Weather Advisory for our Illinois counties north of I-80 to
highlight this.

Snow totals farther to the south and eastward into northwest
Indiana are less certain due to the expectation that the system
will be weakening and the potential for warming temperatures to
cut into snow totals and reduce the overall magnitude of the
impacts that the snow may cause there. That said, it`s possible
that a southward expansion of the advisory may eventually be
warranted if the system wiggles a little farther south from its
presently favored track or if the likelihood of freezing rain
icing up roadways north of the I-74 corridor increases.

Ogorek


Sunday night through Friday:

A period of lake effect snow may develop in the wake of the
weekend winter system late Sunday into early Monday morning as
surface high pressure moves into the region. This will lead to a
period of north northeast winds which would be favorable for
the development of a lake effect snow band oriented into the
Chicago metro counties of northeast Illinois and northwest
Indiana. The combination of cooler lake temperatures, overall
lower inversion heights (barely touching the DGZ around 5-6kft)
and a shallower cloud layer, suggests that snow ratios will lean
lower and limit accumulations overall. Have opted to maintain a
roughly 20-40% chance of lake effect snow showers from Sunday
evening through Monday morning but have kept QPF under 0.1"
which results in additional snowfall accumulations of a dusting
to locally up to around 1".

The advertised active clipper pattern continues with the next
disturbance moving across Wisconsin Monday night into Tuesday
morning, which may graze far northern Illinois with some light
snow. Current indications are that accumulations may be mainly
limited to a dusting toward the WI/IL state line. A stronger
system (sub 1000mb low) then arrives right on its heels Tuesday
night into Wednesday. The warm advective regime that sets up
ahead of it will support temperatures warming into the 30s and
likely above freezing here locally if the current forecast
track holds. Before this occurs a wintry mix of precipitation
would be possible before switching over to mainly rain for most
if not the entire area. Temperatures then quickly drop back
below freezing Wednesday morning in the wake of the Tuesday
night system with a continued signal for gusty snow showers
possible mid morning into early afternoon Wednesday. Something
to keep an eye on over the coming days.

Periodic snow chances then continue through the end of the week
(20-30%) as the active clipper wave train continues.
Temperatures trend cooler for the latter half of the week with
forecast highs in the teens and 20s and lows in the single
digits to lower teens.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1204 AM CST Sat Dec 6 2025

Forecast concerns for the terminals...

- Low chance (20-30%) for period of freezing drizzle this
  afternoon mainly at the Chicago terminals

- Accumulating snowfall to arrive late this evening and persist
  through daybreak Sunday resulting in IFR ceilings and LIFR
  visibilities at times

- Breezy northeast winds on Sunday


The broad surface high that was overhead last night will
continue to move east of the area this afternoon as the
developing storm system in the central Plains begins to traverse
towards the area. As a result, expect winds to become more
variable this afternoon with speeds generally remaining light
(<5 kts). While BKN to OVC MVFR ceilings are expected to prevail
with otherwise dry conditions, there is a low chance (20-30%)
for a period of freezing drizzle to develop this afternoon as
the system`s warm front lifts into the terminals. Since
confidence is still low on whether or not sufficient moisture
will materialize, have opted to handle this potential with a
PROB30 at the Chicago terminals for now. That said, if FZDZ does
develop expect it to occur between 20z and 00z this afternoon
and possibly result in a light glaze on untreated surfaces.

Heading into this evening, the aforementioned storm system will
begin to move into northern IL around 03z this evening and
persist into Sunday morning. This system will bring a widespread
bought of accumulating snow in addition to IFR ceilings and IFR
to LIFR visibilities. In total accumulations generally in the
2-5 inch range can be expected with the higher totals forecast
across northwest IL. As snow tapers Sunday morning, there may be
a period of light flurries and/or freezing drizzle that could
linger through 18z Sunday. Since surfaces should already be
treated by time this round of freezing drizzle/light snow
occurs suspect that impacts should be limited.

Otherwise, expect skies to gradually scatter out by Sunday
afternoon with breezy north-northeast winds gusting around 20
kts. However, some lake effect cloud cover may continue to
result in BKN 2500-3500 ft ceilings at ORD, MDW, and GYY through
the end of the TAF period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory from 9 PM this evening to 6 AM CST
     Sunday for ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-ILZ019.

     Winter Weather Advisory from midnight tonight to 9 AM CST
     Sunday for ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ103-
     ILZ104-ILZ105-ILZ106.

IN...None.
LM...None.

&&

$$

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