004
FXUS63 KLOT 071715
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1115 AM CST Sun Dec 7 2025

.KEY MESSAGES...

- Lake effect snow is expected to develop late this morning and
  continue through tonight, affecting parts of northwest Indiana
  and northeast Illinois. Additional snow accumulations from
  this are largely expected to remain under one inch.

- Bitterly cold conditions return late week with forecast
  minimum wind chills of -10 to -20F Friday night into early
  Saturday AM.

&&

.UPDATE...

The Winter Weather Advisory for eastern Will, southern Cook,
Lake (IN), and Porter counties has expired.

The only other change to the forecast late this morning was to
introduce patchy blowing snow where more than 2 inches of snow
fell earlier this morning due to northerly wind gusts of 20 to
locally 30 mph. Otherwise, the going forecast is in good shape.

Castro

&&

.DISCUSSION...
Issued at 350 AM CST Sun Dec 7 2025

Through Monday:

Snow from a weakening clipper system is ongoing across the area
early this morning, with the steadiest snowfall rates being
observed north of I-80. A wintry mix including freezing
rain/drizzle remains possible across our southernmost counties
through a little past sunrise, but thus far, haven`t seen many
persistent observations of the like. The lone change of note
that was made for the forecast through the remainder of this
morning was to shift the sharp snowfall gradient farther north
and cut down on snow amounts a bit more in the southern half of
our CWA with radar trends suggesting that locations along and
south of I-80 likely won`t pick up a whole lot more in terms of
snowfall amounts. Otherwise, everything else appears to be
playing out as expected.

The synoptic system snow will come to an end by the mid-late
morning as surface high pressure begins to build into the
region from the northwest. By around this time, the inbound
surface high will have also helped reorient the low-level flow
over Lake Michigan to a north-to-south direction. Amidst
lake-850 mb temperature differentials averaging out around 15C,
the longer, northerly fetch will allow for a relatively thin
single band of lake effect snow to develop over Lake Michigan
and then penetrate inland into our eastern counties (primarily
northwest Indiana, but our Illinois counties that border Indiana
could also get grazed late this morning or this afternoon).
Then, this evening, winds over the lake will turn easterly,
steering the lake effect snow band into northeast Illinois.

Overall, while lake-induced thermodynamic parameters will be
favorable for lake effect snow, they won`t be particularly
stellar on the whole. Snowflake quality will likely also be
compromised by 4000-6000 ft inversion heights/lake-induced
equilibrium levels preventing the lake-induced stratocumulus
from extending well into the dendritic growth zone. As a result,
the lake effect snow is not expected to accumulate efficiently,
and most locations that see it will likely only pick up a
dusting or light coating of additional snow accumulation through
tonight. That said, when all is said and done, would not be
surprised to see a localized inch or two of snow accumulation
wherever the lake effect snow band is most persistent. A few
flurries also couldn`t be ruled out farther inland away from the
lake effect snow late tonight into Monday morning as the next
shortwave in our active northwesterly flow pattern drops into
the region. Didn`t add a mention of this to the forecast grids
just yet though as dry air and/or shallow low cloud depths could
very well end up being too prohibitive of factors.

Otherwise, expect blustery northerly winds today that will
prevent temperatures from climbing much, if at all, during the
daytime. Low temperatures tonight will be highly dependent on
cloud cover. Where skies manage to clear out tonight, light
winds will allow for temperatures to crater into the single
digits, while the prevalence of clouds should otherwise keep low
temperatures suspended in the teens. Monday then looks to be a
chilly day for the area with highs only expected to be in the
20s as the aforementioned surface high tracks across the Great
Lakes.

Ogorek


Monday Night through Saturday:

The advertised clipper pattern remains in place over the
broader region this week with the next disturbance expected to
move across Wisconsin Monday night into Tuesday morning. This
will mainly graze far northeast Illinois with light wintry
precipitation (closer to the WI/IL state line). While toward the
very end of the longer-range hi-res guidance, there is a signal
for patchy freezing drizzle on the southern end of the
precipitation. Have held off on a formal mention in the gridded
forecast for now but this will be something to monitor over the
next couple of days.

A stronger system then arrives late Tuesday into Wednesday.
Robust warm advection ahead of the deepening low will transport
warmer temperatures into the local area back above freezing.
This continues to suggest that this will be mainly a rain event
here locally, perhaps beginning as a wintry mix at onset across
far northern Illinois Tuesday evening. While precipitation is
expected, QPF looks rather light overall with forecast totals
only around 0.05-0.20".

Temperatures quickly cool back below freezing Wednesday morning
behind an associated cold front. Steep low-level lapse rates
and cloud tops touching the DGZ supports the potential for gusty
snow shower development mid morning into early afternoon on
Wednesday which may lead to additional streaky coatings of snow
over parts of the area in addition to localized low visibility.

Periodic (mainly low) snow chances then continue through the
end of the week (20-30%) as the active clipper wave train
continues, though it is possible much of the local area misses
out on meaningful snow accumulations with these depending on
when and where they track. This will likely be paired with
bitterly cold conditions to end the week with forecast lows
dropping below zero Friday and Saturday night. In fact, minimum
wind chills Friday night into early Saturday morning could drop
into the -10 to -20 degree range.

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1116 AM CST Sun Dec 7 2025

MVFR stratus lagging behind the recent clipper system will erode
from north to south through the afternoon. Winds will remain out
of the north with occasional gusts of 20-25kt through the
remainder of daylight hours.

Tonight, a land breeze originating off Lower Michigan is
expected to force the development of shallow lake effect snow
bands along the western shore of Lake Michgian. Marginal
thermodynamics will limit snow intensity and render snowflakes
small, leading to meager accumulations (up to a few tenths). The
more impactful element will be periods of reduced visibility,
perhaps as low as 3/4 to 1 1/2SM. In collaboration with CWSU
ZAU, opted to convert the inherited PROB30 groups to TEMPO
groups focusing on the early evening/overnight period at
GYY/MDW/ORD. The lake effect snow will also be marked by an
easterly wind shift and a return of MVFR stratus.

Lake effect snow should taper after midnight/06Z as shoreline
convergence erodes. MVFR stratus may linger as a surface ridge
slides overhead and causes steering flow to weaken. Nearly calm
surface winds toward daybreak will transition toward
southeasterly, and eventually southwesterly Monday morning.

Finally, cannot rule out a stray snow flurry or two tomorrow
during daylight hours as various cloud decks reside in/near the
DGZ. With forecast soundings depicting a dry near-surface layer,
will withhold any mention of P6SM -SN in the outgoing TAF.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 AM CST Monday for the IL and IN
     nearshore waters.

&&

$$

Visit us at weather.gov/chicago