734
FXUS63 KLOT 110138
AFDLOT

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

.KEY MESSAGES...

- Additional clipper systems will move across the broad region
  tomorrow and Saturday. At this point, shoveable-snow appears
  most favored along and south of Interstate 80 with both.

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

- There is a growing signal for a pattern change toward milder
  (and less snowy) conditions next week.

&&

.UPDATE...
Issued at 738 PM CST Wed Dec 10 2025

Recent radar imagery across the region depicts a compact region
of flurries and snow showers extending from near Davenport,
Iowa to Bloomington, Illinois. The area of snow appears to be
tied to a weakening mid-level wave, which is poised to more or
less shear out while passing into central Illinois over the next
3 to 6 hours. With surface observations indicating largely 10
mile visibility beneath all radar returns, earlier concerns
about a quick hit of snow and slippery travel conditions are
thankfully going away. Nevertheless, will maintain somewhat high
PoPs (>60%) for snow showers, focused southwest of a line from
Mendota to Cissna Park, IL.

Outside the compact region of snow showers, nighttime
microphysics satellite imagery captures expansive stratus across
the region quite well at this hour, which has thus far limited
the ability for temperatures to fall much from where the were
before sunset. Model guidance is pretty awful at handling cool
season stratus, so do share the same dose of skepticism as the
day shift that clouds will clear allowing for temperatures to
tank overnight as depicted in most model guidance. With that
said, have been watching the very slow progress of the clearing
line across northeastern Wisconsin, which does appear poised to
eventually work southward eroding stratus across far northern or
northeastern IL by daybreak Thursday. At this point, feel
pretty comfortable maintaining the inherited overnight
temperature forecast showing teens north of I-88 and low to mid
20s south of I-80, keeping in mind either location could end up
lower (north) or higher (south) depending on eventual cloud
trends.

With regard to the clipper system tomorrow... Am noting a
surprising spread in the structure of the system advertised by
the latest model guidance given the short lead time. It seems
like the structural differences are at least partially tied to
the speed at which the upper-level low currently over the
northern Great Lakes shifts northeastward toward Nova Scotia
tonight into tomorrow. A slower evolution of the departing
upper-level low keeps the clipper system somewhat suppressed and
to the southwest, with the surface low passing more or less
from Kansas City, Missouri to Paducah, Kentucky. The band of
snow with the clipper would be tied to a plume of steep mid-
level lapse rates just north of the surface low extending from
central Iowa toward eastern Kentucky, with a sharp edge more or
less slicing through northern Illinois (terminating somewhere
along a line from Mendota, Illinois to Fowler, Indiana?). This
scenario is advertised by the GEFS/EPS/NAM suite and is
currently reflected in our gridded database.

A slightly faster evolution of the departing upper-level low,
as depicted by RAP/HRRR/RRFS guidance, opens the door for a
stronger clipper/surface low tracking from Omaha, Nebraska to
Bloomington, Indiana. The RAP/HRRR/RRFS suite has been locked
onto such an evolution, including notably stronger/backed
850-700mb low-level flow directed right into northern Illinois.
Such stronger/backed 850-700mb flow would tighten the low-level
baroclinic zone and elucidate a stronger frontogenetical
response, altogether supporting a much broader shield of snow
encompassing much or our area well to the north of the lapse-
rate driven snow across central Illinois. While the QPF with a
stronger system would not be overly impressive (generally 0.15
near the Wisconsin state line to 0.35" near/south of US-24),
respectable lift (around 15 ubar/sec) focused right below the
DGZ (the so-called "cross-hairs" signature) would support
slightly fluffier snow ratios of 15:1, altogether leading to a
broad swath of 2 to locally 5 inches of snow right across
northern Illinois and northwestern Indiana. This scenario would
obviously be a bit more problematic in terms of impacts across
our area. (Though it should be noted that shovable-snow does
fall across our southern forecast area in either scenario).

At this point, it`s hard to say exactly which outcome is more
likely. Incoming 00Z HRRR guidance seems pretty similar to prior
runs, providing little insight into whether one camp is caving
toward toward the other. And, of course, something in between is
very much a reasonable outcome as well. Given differences
between the GEFS/EPS/NAM and RAP/HRRR/RRFS suites start to
become discernible by about 06 to 09Z tonight, the midnight
shift should be in a better position to evaluate the forthcoming
evolution of the clipper system and determine the need for a
Winter Weather Advisory across part (or all?) or our area. So,
will leave the gridded database unchanged for now.

Updated text products (really cosmetic updates) will be sent
soon.

Borchardt

&&

.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
hi-res/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/hires
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 /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 545 PM CST Wed Dec 10 2025

Forecast concerns for the terminals...

- Period of light snow showers/flurries this evening.

- MVFR ceilings to prevail through tonight before lifting
  Thursday afternoon.

- Another period of accumulating snow Thursday evening and
  night.


Winds will continue to subside this evening as the atmosphere
decouples with generally 8-10 kt winds overnight from the
northwest. However, a subtle disturbance in eastern IA will
pivot over northern IL and portions of northwest IN later this
evening resulting in a period of light snow showers and/or
flurries. While the better moisture aloft is forecast to reside
closer to RFD and western IL, it seems sufficient moisture will
be present in the snow growth region to support a 40-50% chance
for flurries as far east as the Chicago area terminals. Given
the lower coverage of any snow showers/flurries have opted to
introduce TEMPOs into the TAFs. Regardless, any accumulations
from said snow is expected to be minimal with up to a dusting at
worst mainly at RFD.

Snow will taper shortly after midnight tonight with otherwise
dry conditions expected for the rest of tonight and much of the
day on Thursday. While it does look as if ceilings will lift
towards the VFR range by Thursday afternoon, some low-level
moisture is forecast to hang around which may allow some MVFR
clouds to linger through this timeframe as well. As for winds,
expect directions to become more west-southwest on Thursday
before settling into an easterly direction Thursday evening.
Though speeds will remain light (around 5-6 kts) through the
period.

Finally, another disturbance will move into northern IL and
northwest IN Thursday evening bringing with it another period of
accumulating snow. Latest forecast guidance continues to favor
areas south and west of a Rochelle, IL to Rensselaer, IN line as
getting the most snowfall (2-4+ inch amounts) but there is a
growing signal for some better forcing to get as far east as the
Chicago terminals. With this signal being somewhat persistent
among hi-res guidance have opted to trend 30-hour TAFs towards
this potential by introducing MVFR visibilities after 03z
Thursday evening. That said, if confidence grows may need to
consider lower visibilities. As for amounts, it appears the TAF
sites will likely see snow totals in the 1-2 inch range (1-3
inch closer to RFD) from this event though locally higher
amounts could be seen if the aforementioned forcing pans out.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 AM CST Thursday for Winthrop
     Harbor IL to Gary IN.

     Small Craft Advisory until noon CST Thursday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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