828
FXUS63 KLOT 102028
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
228 PM CST Wed Dec 10 2025

.KEY MESSAGES...

- Light snow with likely some travel impacts tonight, mainly
  this evening, southwestern CWA

- Additional rounds of accumulating snow expected Thursday night
  and again Saturday, heaviest amounts likely south of I-80
  Thursday night.

- Bitterly cold temperatures and potentially dangerously cold
  wind chills are expected this weekend.

&&

.DISCUSSION...
Issued at 228 PM CST Wed Dec 10 2025

Through Thursday Night:

Active winter weather pattern will continue with a couple more
shots of snow just in the next 24-36 hours.

First up, sheared/channelized shortwave is noted on water vapor
imagery over western MN early this afternoon. Fairly widespread
snow is accompanying this feature over eastern MN, with 1-3SM
VSBY pretty common and a few less than 1SM snow obs. Most
guidance is really not handling this feature particularly well,
though RAP/NAMNest appear to be handling it the best. Have
increased pops to likely this evening over southwestern CWA
where RAP/NAMNest are favoring the best, albeit still quite
light QPF (just a hundredth or two).

Thicker stratus associated with this snow shows up quite nicely
on visible satellite imagery, with recent satellite trends
leaving me a little concerned that this light snow activity
could end up a bit farther east and deeper into our CWA than
guidance would suggest. For now stretched the chance pops
farther east, but oncoming evening shift will need to closely
monitor satellite and radar trends and may need to tweak
forecast accordingly. Thinking accumulation should end up an
inch or less, but with temps below freezing and lingering road
treatment probably mostly washed away, even a coating of snow on
roads could cause hazardous travel conditions.

Stratus deck blankets most of the western Great Lakes and
Midwest and is also not be handled particularly well by most
guidance. Forecast soundings maintain a frontal inversion
through the night and into tomorrow morning with this stratus
deck trapped beneath it. Lacking any particularly strong
subsidence, the stratus should hang on through the night, so
have bumped up cloud cover and temps accordingly in the grids
for tonight.

Shortwave on the nose of a 160kt 300mb jet have plowed onshore
into southern British Columbia this morning. This feature is
progged to zip across the southern Canadian Rockies tonight
before diving southeast around the western flanks of the deep
longwave trough over the Great Lakes and eastern Canada. There
remain split solutions in handling this feature, with most
highres/CAM guidance favoring a farther north track with our CWA
ground zero for the heaviest snowfall. Global models on the
other hand are farther south, with the axis of highest snowfall
totals across central IL. In fact, GFS and ECMWF would keep
accumulating snow south of the city of Chicago with a couple of
inches of accumulation across our far southern and southwestern
CWA.

Given this shortwave was still offshore at 12z this morning, I
tend to lean a bit more toward the global model`s handling of
this feature. Of the 3 big global models, the GFS is the
farther north and have largely followed that solution as a bit
of compromise. NBM pops have trended downward northern CWA and
this seems reasonable, so made no adjustments to NBM pops.
Which conveys the uncertainly with farther north CAM/highres
guidance. If global models don`t shift back north, then pops
over northern CWA will need to be lowered in subsequent
forecast updates.

- Izzi


Friday through Wednesday:

With good agreement in all accumulating snow likely having shifted
out of the area prior to daybreak Friday, PoPs nudged down to
slight chance in the morning will probably be able to be removed
with later forecasts. A short-wave associated with weak surface
low pressure over Lake Superior will push across the western
Lakes in the afternoon and early evening. This wave may bring a
glancing blow of flurries and perhaps a few true snow showers
(~20% PoPs) to approximately the northeastern 1/2 or 1/3 of the
CWA towards and after sunset. The cold front trailing from the
surface low will bring the first shot of modified Arctic air for
the bitterly cold weekend in store (more on that below).

Expansive 1040+ mb Arctic high pressure will gradually spread
southeastward Friday through the weekend, from the Canadian
Prairies Friday, to the mid-upper MO River Valley Saturday
night, and to central Illinois and Indiana Sunday night. The
initial cold front passage Friday night will bring lows in the
single digits and lower teens and wind chills of about 0 to 15
below. The final clipper-like system in the parade of these
systems over the past 7-10 days will likely bring a swath of
accumulating snow to portions of the region during the day on
Saturday, interacting with a tight lower to mid-level baroclinic
zone in place (implied frontogenesis for enhanced mesoscale
banding).

Similar to the Thursday night system, the southward shift of the
focus for a potentially plowable fluffy snowfall in recent model
cycles continued with the 12z global ensemble guidance. With this
in mind, the highest PoPs of nearly 70% are focused near/south of
the Illinois and Kankakee River Valleys. That said, felt
comfortable with likely threshold (55-60%) PoPs still up to the
I-88 corridor for a few key reasons. Low amplitude features like
Saturday`s are prone to larger shifts in relatively short lead
times in their meaningful QPF and accumulating snow footprints.
In addition, there`s still a respectable % of ensemble members
(rough estimate of about 30%) with the favored axis as far north
as the Chicago metro.

Another important item to keep in mind is that with surface temps
in the single digits and teens, much of the column will reside in
the DGZ, and thus it won`t take much for light snow to be wrung
out. Also, notably for this aspect, with air temps so cold, road
treatments are typically rendered less effective and often make
for worse travel impacts with only light amounts of snow vs. with
temps closer to freezing (such as today). Wherever the enhanced
mesoscale banding sets up, the deep DGZ and more favorably aligned
ascent through it could certainly present a chance for well above
climo 15-20:1 ratios and only modest QPF fluffing up to several
inches of snow. Experimental probabilities based on ensemble
membership currently would suggest the highest chances of this
being somewhere across central Illinois and Indiana.

Regardless of how the Saturday daytime snow plays out (or if some
areas are even missed altogether), the snow cover across the
region, approaching Arctic high, and 850 mb temps of -15 to -20C
will set the stage for the coldest night of the month thus far.
Sustained northwest winds of 10-15 mph and temps plunging below
zero for most of the area (possibly near -10F in spots west of
the Fox Valley) will yield wind chills of 15 to 30 below zero. A
Cold Weather Advisory will likely be needed for most of, if not
the entire CWA into Sunday morning.

The surface high will slide east overhead on Sunday, limiting
mixing heights but also bringing lighter winds as highs reach the
positive single digits and lower teens. The surface high position
over central IL Sunday evening will be close enough for temps to
tank everywhere except probably Chicago despite onset of warm air
advection aloft. Expect recovery into the 20s on Monday and then
lows in the teens Monday night. This will be merely a prelude to
notably milder conditions expected the rest of next week with a
change to a zonal jet stream pattern. Temperatures may approach
40F next Wednesday barring substantial effects from low clouds,
which is possible this time of year. Current signs point to the
overall milder pattern lingering towards Christmas. Precip
chances will remain low through the current day 7.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1139 AM CST Wed Dec 10 2025

Snow is coming to quick end from northwest to southeast across
the terminal airspace late this morning. In spite of this, MVFR
stratus remains rather expansive late this morning, and is
currently encompassing the Upper Midwest and western Great
Lakes region. There thus is no near term hope of scattering the
clouds out, so expect MVFR CIGs to persist, possibly even into
the day on Thursday. Otherwise, winds will remain gusty
(25-30kt) from the northwest into this afternoon, but will abate
with sunset late this afternoon.

Another quick shot of light snow and flurries is expected this
evening as an impulse over MN late this morning shifts into our
area. For the latest set of TAFs we have maintained just a
flurry mention with this activity. However, we will have to keep
an eye on the potential for some brief IFR VSBYs with any
heavier pockets of snow that occur this evening, particularly
at KRFD. This activity will end prior to midnight.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning until 3 PM CST this afternoon for Winthrop Harbor
     IL to Gary IN.

     Gale Warning until 6 PM CST this evening for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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