888
FXUS63 KLOT 062036
AFDLOT

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

.KEY MESSAGES...

- Accumulating snow expected tonight into Sunday morning,
  particularly within the Winter Weather Advisory area along and
  north of I-80 where 2-5 inches is forecast.

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

&&

.DISCUSSION...
Issued at 236 PM CST Sat Dec 6 2025

Through Sunday:

The primary forecast concern continues to center around the
quick hit of accumulating snow across northern IL and far
northwestern IN tonight into Sunday morning (2-5" amounts).
While not a whole lot has changed with the forecast thinking, we
did opt to add Grundy, southern and eastern Will, Lake IN and
Porter IN to the going Winter Weather advisory tonight into
Sunday morning, since some amounts in these counties could end
up in the 2 to 3" range over a short period.

Snow (moderate to heavy, with rates likely up around 1" per
hour) is currently ongoing in association with an approaching
clipper system west of the area across western IA early this
afternoon. The higher rates of snow in this region are being
driven by robust mesoscale frontogenetic response within a
region of low mid-level static stability. Expectations continue
to support the eastward development of this area of snow into
northwestern IL after 7-8 pm this evening, then into
northeastern IL and northwestern IN later in the evening (after
10 PM). Periods of accumulating snow will then persist in the
advisory area overnight before tapering off early to mid morning
on Sunday (~9am), making for roughly an 8 to 10 hour period of
accumulating snow.

Interestingly, while a similar mesoscale frontogenetically forced
response (centered around 850 mb) is anticipated to occur
across northern IL overnight tonight, the strongest resulting
forced ascent may largely remain within a warmer less optimal
thermal environment (below the -12 to -18C DGZ) for the most
efficient dendritic growth. Accordingly, this may favor a bit
lower snow-to-liquid ratios (10-13:1) than would otherwise be
expected with such strong forced ascent (13 to 15:1). The net
result of this could be that many areas end up in the lower end
of the 2"-5" range. However, with the presence of steep mid-
level lapse rates (3-6 km near ~6.8C per KM) present through the
DGZ, I was not comfortable backing off on snow amounts at this
time. The period of highest rates (0.5-1" per hour) are
generally favored in the 2 am through 7 am period.

Snow totals will be much lighter with southward extend south of
I-80, owing to lighter rates and a bit warmer temperatures. In
fact, as temperatures hover around freezing in my south
overnight, there could be some light rain attempting to mix with
the light snow. This should thus result in only a dusting
across central IL and IN.

KJB

Sunday night through Friday:

Not much has changed with the forecast thinking during the
extended period. Here is the previous discussion for reference.

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-ILZ021-
     ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Winter Weather Advisory from midnight tonight to 9 AM CST
     Sunday for INZ001-INZ002.

LM...Small Craft Advisory from 8 AM Sunday to 9 AM CST Monday for
     the IL nearshore waters.

     Small Craft Advisory from 11 AM Sunday to 9 AM CST Monday for
     the IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago