756
FXUS63 KLOT 070211
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
811 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 2-6 inches is forecast. The highest totals
  may set up in the vicinity of a Rochelle to Midway Airport
  line.

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

&&

.UPDATE...
Issued at 811 PM CST Sat Dec 6 2025

Snowfall timing remains right on track, spreading across I-39
shortly (through 930 PM) then into Chicagoland 10-11 PM. No
significant changes needed to PoP timing as a result.

The main development this evening is a slight increase in the
modeled 850 mb wind response overnight as a southwesterly low-
level jet pushes east of the Mississippi River and briefly
intensifies, with the latest iterations of the RAP and HRRR now
consistently suggesting wind speeds pushing towards 35-40 kts.
This mass response is a bit more significant than what has
previously been modeled by global guidance, and suspect there
may be some very modest upper jet/PV interaction at play based
on latest model analyses. It`s still a bit unclear if this will
fully be realized, particularly given the gradually rising
surface pressures within the parent surface low across Kansas
(slowly weakening), but the trends have been fairly persistent.

The main result of this has been for a general sharpening and
increase of the 925-800 mb frontogenesis, maximizing more-or-
less right along the I-88 corridor overnight tonight into early
Sunday morning. A commensurate increase in peak UVVs nearing 25
ubar/second within this area are evident, and while these
maximum values do not perfectly overlap the depth of a somewhat
narrow dendritic growth zone, they are strong enough that mean
UVVs through the DGZ are advertised near 15+ ubar/second for a
3-5 hour window tonight which may help boost SLRs a bit.

Given all of this, have slightly increased QPF and snowfall
amounts in the vicinity of a Rochelle to Midway Airport line
where peak hourly snowfall rates around or a hair above an inch
per hour will be possible during a brief window late tonight.
We`ll call it a general 3-6 inches in this area, with the
highest amounts likely falling in a very narrow corridor, and a
few totals beyond 6 inches can`t be ruled out. Finally, suspect
there will be a serious snowfall gradient near and south of
I-80, and likely greater than we can reasonably depict in the
gridded forecast due to mesoscale subsidence on the south side
of the main f-gen circulation.

Don`t see a need for changes to the going Winter Weather
Advisory configuration. There continues to be a potential for a
little wintry mix/freezing rain at times south of the Kankakee
River tonight, but temperatures will gradually warm to near or
even locally just above freezing, likely mitigating impacts.

Depending on how quickly winds come up Sunday morning, some
blowing and low drifting of snow may occur, mainly in open/rural
areas, so this is something we`ll be keeping an eye on. Finally,
lake effect snow may focus across parts of Lake/Porter counties
in NW Indiana Sunday afternoon before pivoting west towards NE
Illinois/Chicago Sunday evening and overnight. Thermodynamic
parameter space isn`t particularly robust or concerning, but
some additional localized accumulations up to an inch or two may
occur through this timeframe.

Carlaw

&&

.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 /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 603 PM CST Sat Dec 6 2025

A compact but potent storm system currently over Iowa will
affect all TAF sites late this evening through around daybreak
Sunday, with a potential period of moderate to briefly heavy
snow overnight.

Ongoing top-down saturation across northern Illinois will take
at least a few hours to bring SN down to the surface. SN is
expected to reach RFD by around 03Z and in the 04-05Z window for
the Chicago terminals, resulting in a period of IFR visibility
for an hour or two after onset. A narrow band of SN with rates
over 1 inch per hour is then expected to shift somewhere across
the Chicago metro and possibly RFD for a 3 to 4 hour window
overnight. Latest trends indicate that this band will affect the
Chicago terminals, particularly MDW and GYY. If the core of the
band were to directly affect a given terminal, VLIFR visibility
will be possible for a couple hours.

Synoptic-scale snow will quickly end by around daybreak (13Z),
but lingering lake-enhanced SHSN with IFR/MVFR ceilings should
persist through mid-morning. Winds will also increase and favor
just west of north with gusts around 20 knots for much of the
day. Some near-surface DRSN is also possible in the morning for
E/W oriented runways.

Sub-VFR ceilings will likely scatter for the afternoon before
redeveloping with scattered lake-effect SHSN and a NE wind shift
during the evening. The SHSN may struggle to reach more than a
few miles inland across northeast Illinois, but both MDW and GYY
could experience a 1-2 hour window of IFR/LIFR visibility mid
to late evening Sunday.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory until 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.

&&

$$

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