768
FXUS63 KLOT 091240
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CST Tue Dec 9 2025

.KEY MESSAGES...

- A brief period of freezing rain may result in a glaze of ice
  on untreated surfaces this evening, impacting part of the PM
  commute (~5-8 PM CST), mainly northwest of a Dixon to McHenry,
  IL line (30% chance).

- Strong southwest winds are expected late this evening before
  becoming northwest on Wednesday (gusts to 35-40+ mph).

- The potential for hazardous travel conditions is increasing
  for the Wednesday AM commute due to robust snow showers or
  squalls.

- More fast-moving clipper systems could result in some
  additional periods of accumulating snow in or near our area
  late this week into the weekend.

- Temperatures will turn colder for the latter half of the week
  and especially this coming weekend, when dangerously cold wind
  chills may also be observed at times.

&&

.DISCUSSION...
Issued at 405 AM CST Tue Dec 9 2025

Through Wednesday night:

A complex forecast is in store through the next 24-48 hours
with multiple periods where there may be travel impacts.

The overnight snow showers and flurries are on their way out of
the area but likely resulted in a quick dusting in some spots.
In their wake, southerly warm and moist advection is already
well underway with winds steadily increasing out of the south
here locally. In fact, above freezing temperatures have already
lifted out of Missouri into southwest Illinois. The freezing
line will continue to lift northeast across Illinois and into
Indiana through the day today.

The potential for freezing drizzle continues to wane early this
morning with the lingering stratus deck expected to thin
through the morning. This should inhibit additional drizzle
development. However, there are weak echoes on radar across
parts of Wisconsin but so far have not seen any observations of
this reaching the ground. Still something to keep an eye on for
counties along the WI/IL stateline. As temperatures warm over
the snowpack, any lingering stratus is expected to continue
thinning which will limit any potential drizzle developing later
this morning, but overall, not expecting any hazardous weather
through the daylight hours.

The big forecast question after sunset early this evening is
whether or not there is a brief window for freezing rain across
northwest Illinois and along the WI/IL state line (generally
north of a Dixon to McHenry, IL line (including the Rockford
metro). A strengthening surface low is forecast to track across
southern Wisconsin overnight. Out ahead of it, light
precipitation will move into the area after sunset. Forecast
vertical profiles suggest that the primary precipitation type is
expected to be liquid/rain (perhaps mixed with some snow and/or
sleet initially). If ground temperatures are even just an hour
or two slower at warming above freezing than forecast, this
could lead to a quick glaze of ice on untreated surfaces
coinciding with part of the PM commute. Confidence in this
occurring here locally remains quite low, however, and have
noted a trend toward a slower onset of precipitation with some
of the latest guidance which further narrows this window (closer
to 5-8 PM CST). Have opted to message this with an SPS for
Ogle, Winnebago, Boone, and McHenry counties for now outlining
the threat of slick roadways. If confidence increases in slower
warming and a larger window for icing, a short-fused Winter
Weather Advisory may be needed given it would coincide with the
latter half of the PM commute. Stay tuned!

Once temperatures warm solidly above freezing across the area
late Tuesday, cold rain is expected to expand across the area
through the evening and continue overnight (total QPF around
0.1-0.2"). Winds will also become strong out of the southwest
through the evening, gusting up to 35 to 40+ mph.

There may be a bit of a break in the precipitation late tonight
into early Wednesday morning before a formidable cold front
races across the area. This will likely be paired with gusty
snow showers to potentially robust snow squalls. Winds along
the front will likely approach or reach wind advisory criteria
for a 1-3 hour window for any given location. Opted to hold off
on a wind advisory issuance with this update in case the day
shift determines that the snow squall potential itself warrants
a Winter Weather Advisory (as some offices have done upstream).
The trend toward higher Snow Squall Parameter values is also
telling (incorporates low-level instability, moisture, and
winds). This could result in a quick coating of snow up to an
inch within a short window of time paired with very low
visibility and strong winds. Accordingly, impacts to the
Wednesday morning commute are becoming increasingly likely
across the area between roughly 5-10 AM CST from northwest to
southeast.

A few show showers may linger into the afternoon with
temperatures in the upper 20s to around 30 degrees. Temperatures
then steadily fall into Wednesday night with lows back into the
teens (around 20 near the immediate lakeshore). There is a
signal for a robust lake effect band to drive down the lake in
the wake of the front Wednesday afternoon into the evening. This
could bring a quick 1-2" of snow to portions of northwest
Indiana before ending overnight.

Petr


Thursday through Monday:

The stubborn northwest flow pattern will persist over much the
CONUS through the latter half of the week and into the upcoming
weekend. With the energetic jet stream and associated baroclinic
zone lining up directly over Illinois and Indiana, the barrage
of clipper-type systems getting sent into the Midwest/Great
Lakes will continue through at least this weekend, and each of
these systems will likely produce additional snow accumulations
somewhere in or very near our forecast area.

While there are still some differences across the broader suite
of medium range deterministic and ensemble guidance that are
leading to persisting spatiotemporal uncertainties with our
precipitation forecast, there appear to be three primary
disturbances of note that will affect the region between
Thursday and Sunday. The first of these disturbances will be a
somewhat sheared-out shortwave trough that looks to arrive at
our longitude sometime late Thursday/Thursday night.
Strengthening mid-level frontogenesis along the attendant
baroclinic zone during this time should help induce an elongated
band of snow somewhere over or just southwest of our forecast
area as this wave passes by.

A second shortwave trough then looks to follow closely on the
heels of this first shortwave, arriving in the region on Friday.
Depending on when this wave arrives and how far south it dips,
it could help prolong the Thursday night snowfall into Friday,
or it could induce its own separate area of snow that may
largely remain to our north. Finally, a third shortwave trough
is then favored to zip into the region on Saturday, with an
overwhelming majority of 00Z EPS members and a smaller majority
of 00Z GEFS members outputting QPF in our forecast area as this
disturbance passes overhead. It`s still too early to have much
confidence in snow amounts with each of these snowfall
opportunities. However, it can be said that the cold and dry air
infiltrating the region should support generally lower QPF
amounts, but higher snow-to-liquid ratios that correlate to
fluffier snow characters (and possibly even dust-like snow,
particularly in the case of this weekend).

Of potentially greater note for this coming weekend will be the
bitterly cold temperatures. Successive shots of cold air
brought in by the aforementioned disturbances will likely
culminate in single digit and sub-zero lows and possibly even
single digit highs in many areas over the weekend. Confidence in
our overnight/morning wind magnitudes isn`t particularly high
at this point in time, but until a Canadian surface high settles
over the region sometime Sunday into Monday, suspect that
there`s a good chance that northwesterly winds will remain high
enough for minimum wind chills to reach (or at least get close
to) our -20F threshold for Cold Weather advisories in some
locations on at least one morning this coming weekend.

Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 640 AM CST Tue Dec 9 2025

Key Messages:

- Strong winds are expected tonight and into tomorrow, initially
  from a southwesterly to westerly direction before turning
  northwesterly behind a cold front tomorrow morning.

- A period of rain is expected this evening, possibly preceded
  by a brief wintry mix.

- Gusty snow showers/squalls are likely tomorrow morning with a
  brief period of sharply reduced visibility.

- MVFR/IFR ceilings are expected through most of the TAF period.


A busy TAF period is on hand as a deepening low pressure system
passes to the north of the area.

First, the southern edge of an MVFR stratocumulus deck is
bisecting the Chicago metro this morning. The expectation is
that this cloud deck will gradually ooze southward over the
course of the morning, but early morning satellite trends
suggest that there will be some VFR time at our Chicago metro
TAF sites as well until the ragged edge of the cloud deck has
made a more solid push southward. A lowering/strengthening
temperature inversion should then cause ceilings to lower later
today, possibly to IFR levels. The greatest probability of IFR
ceilings occurring is at RFD.

Early this evening, precipitation is expected to develop and/or
spread over the terminals. The precipitation is predominantly
expected to fall as rain, but at the onset of it, a brief
wintry mix could be seen. While the chances for that at the
Chicago metro terminals may have decreased slightly since the
06Z TAF issuance, elected to largely maintain the inherited
PROB30 groups for -PLSN for now.

It appears that there will likely be a relative lull or break
in the precipitation overnight before snow showers/squalls
closer to an incoming cold front move into the area. While these
wind-whipped snow showers will not last long, they will result
in sharp visibility reductions, briefly lowered ceilings, and
possibly some quick coatings of snow on the ground as well.
Prior to the onset of the snow showers, the lower clouds from
earlier may scatter out or lift to VFR levels, but a return to
MVFR ceilings is favored behind the cold front.

Lastly, winds will ramp up tonight as the low pressure system
gets closer by. 30+ kt gusts may begin as early as this evening
while winds are still from a southwesterly direction. Winds
will then gradually turn westerly tonight before shifting
northwesterly behind the aforementioned cold front, with 30+ kt
gusts remaining likely through this whole time. Some 40+ kt
gusts could be seen as well late tonight or tomorrow morning.
It`s also possible that the gustiness may be slow to get going
this evening. If that happens to be the case, then a 50+ kt low-
level jet could result in a period of low-level wind shear.
Didn`t have enough confidence in that happening to include a
formal LLWS mention in the TAFs, but that may need to be
reconsidered in forthcoming TAF packages.

Ogorek

&&

.MARINE...
Issued at 405 AM CST Tue Dec 9 2025

- Gale Warning in effect late Tuesday evening through Wednesday
afternoon.

The Gale Watch has been upgraded to a Gale Warning for strong
southwest winds quickly increasing through this evening ahead
of an approaching weather system to 35-40 kt gales. Wind
directions then turn northwest behind a cold front Wednesday
morning paired with gusty snow showers/squalls. A lake effect
snow band diving down the lake Wednesday afternoon may lead to
another brief period of higher gusts to 40 kt Wednesday
afternoon before winds gradually ease late afternoon into the
evening.

Petr

&&

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

     Gale Warning from 9 PM this evening to 3 PM CST Wednesday for
     Winthrop Harbor IL to Gary IN.

     Small Craft Advisory until 3 PM CST this afternoon for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

     Gale Warning from 9 PM this evening to 6 PM CST Wednesday for
     Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

Visit us at weather.gov/chicago