363
FXUS63 KLOT 072023
AFDLOT

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

.KEY MESSAGES...

- Accumulating snow with he next clipper system is expected to
  focus north of our area Tuesday night into Wednesday. However,
  a period of blustery winds is expected across our area.

- Temperatures will warm above freezing Tuesday through early
  Wednesday, before falling again by the end of the week.

- Another surge of Arctic air may push through the Great Lakes
  next weekend with wind chills dropping below zero.

&&

.DISCUSSION...
Issued at 223 PM CST Sun Dec 7 2025

Tonight through Monday:

The main forecast items of note are this evening and early
overnight: namely brief very cold temps for some locations and
lake effect snow showers into northeast Illinois and perhaps
far northwest Indiana.

High pressure of 1030-1035 mb will transit the region tonight
through Monday. With the deepest snow cover across interior far
northern Illinois (including 4-7" of new snow into this AM),
there appears to be a window of favorability for temps to tank
to as low as a couple degrees below zero west of the Fox Valley
as winds diminish to 5 mph or less.

Low level flow will turn northeasterly and then easterly this
evening, temporarily enhancing convergence on the southwest
portion of the lake. Marginal lake induced thermodynamics and
dry air above roughly 800 mb will serve to limit the intensity
of lake effect snow showers and coverage of any localized
measurable accumulation (ie. a coating to perhaps a couple
tenths) to generally 40-50% or less. Opted for isolated to
scattered lake effect snow shower mention in the gridded
forecast. The band(s) of lake effect snow should pivot westward
and weaken with time into the early overnight and then dissipate
altogether.

Cloud cover will slosh back westward through the overnight,
thickest near the lake, which will result in temps becoming
nearly steady if not slowly rising overnight. The temp rise
would be most pronounced for interior northern IL locations
favored to briefly tank this evening (possibly near to below
zero this evening up to near/around +10F Monday morning).

On the back side of the surface high Monday afternoon, winds
will become southerly at 10 mph. Temps will recover to the lower
to locally mid 20s if lingering cloud cover (plus extensive snow
cover) doesn`t temper modest warming too much. This is still
about 10-15 degrees below normal for December 8th.

Castro

Monday night through next Saturday:

The well-advertised clipper pattern will remain in place over
the broader region through the week with the next system
expected to move across Wisconsin Monday night into Tuesday
morning. While the system should only graze far northeast
Illinois, there is a continued signal in longer-range hi-res
guidance and forecast soundings for a precipitation type locally
of freezing drizzle. Will continue to hold 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 is then expected to race across the Great
Lakes late Tuesday into Wednesday. Robust low-level warm
advection ahead of the deepening low will transport warmer
temperatures into the local area back above freezing. Ensemble
model guidance continues to highlight a corridor from Minnesota
through Wisconsin and into Lower Michigan as favored for
accumulating snow, with a wintry mix to just rain across
northern Illinois and northwestern Indiana. Perhaps most
noticeable will be strong winds along the southern flank of the
(around 985mb) low pressure system. Ensemble mean gusts Tuesday
night into Wednesday morning fall somewhere in the neighborhood
of 30 to 40 mph, though some of the higher-end scenarios
advertised by the GFS suite would support gusts exceeding 45
mph. The warm temperatures should tend to limit the ability for
the snowpack to blow around across the region, but this will
also be something to monitor in the coming days.

Temperatures will quickly cool back below freezing Wednesday
morning behind an associated cold front. Steep low-level lapse
rates, cloud tops touching the DGZ, and a continued tight
pressure gradient will support 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 (20 to 40%) chances for snow will continue
through the end of the week as the active clipper wave train
continues. Pinpointing where the next few clippers track beyond
a few days out is a futile task. With that said, there is a
notable signal within ensemble guidance for another period of
bitterly cold temperatures next weekend with highs falling back
toward the single digits and overnight lows toward or below
zero. Depending on the eventual orientation of a building
surface high in the wake of the arctic cold front, blustery
northwest winds may lead to wind chills threatening Cold Weather
Advisory criteria of -20F at some point next weekend.

Petr/Borchardt

&&

.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 4 AM CST Monday for the IL and IN
     nearshore waters.

&&

$$

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