841
FXUS63 KLOT 091738
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1138 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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1138 AM CST Tue Dec 9 2025

Key Messages:

- Strong and increasingly gusty winds expected tonight into
  Wednesday, initially from a southwesterly to westerly
  direction, then turning northwesterly early Wednesday
  morning.

- A period of rain expected this evening.

- About a 1 to 3 hour period of wind-whipped snow showers and
  associated low VSBY`s expected early Wednesday morning.

A potent clipper system will race east-southeastward across
southern WI tonight into early Wednesday morning. This
approaching system will foster increasingly gusty winds
(25-30kt) from the west-southwest tonight, followed by a period
of 30-35 kt west-northwesterly gusts with a cold frontal
passage Wednesday morning. Northwesterly winds are then
expected to remain gusty into the afternoon, but magnitudes will
ease through the day.

A majority of the precipitation with this system is anticipated
to remain north of the the area. With that being said, a period
of rain is still expected for a few hours this evening before
waning overnight. Still cannot rule out a very brief (<1 hour)
wintry mix with the rain onset this early this evening, but the
chances and duration of any wintry mix are expected to remain
short enough to exclude the mention from the main terminals.

Finally, a second period of precipitation is expected with this
system early Wednesday morning (roughly 11 to 15Z timeframe) as
the cold front and colder airmass surges back southward into
the area. Precipitation type with this batch of precipitation is
expected to be snow, with some minor accumulations possible. In
combination with the strong gusty northwest winds, wind-
whipped snow showers may support a short (1-2 hour) period of
sub 1 mile VSBYs early Wednesday morning. Conditions should then
gradually improve mid to late morning as the snow shifts out of
the area, though some MVFR CIGs look to persist into the
afternoon.

KJB

&&

.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.

&&

$$

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