706
FXUS63 KLOT 051733
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1133 AM CST Fri Dec 5 2025

.KEY MESSAGES...

- Cold temperatures will be observed this morning before south-
  southwesterly winds become blustrier later today. Isolated
  blowing/drifting snow could not be ruled out in open areas
  this afternoon and evening.

- A mix of snow and patchy freezing drizzle may materialize
  tonight, primarily in areas north of I-80 (20-40% chance).

- A clipper-like pattern will bring regular opportunities for
  snow to the general region beginning Saturday night into
  Sunday and continuing through the upcoming week.

&&

.DISCUSSION...
Issued at 350 AM CST Fri Dec 5 2025

Through Saturday:

It`s a cold morning across the region today with most
observation sites in our forecast area reporting single digit
temperatures. Still monitoring the potential for freezing fog
development, but thus far, outside of (interestingly) some
narrow streaks of cooling pond-induced low clouds/fog, there
hasn`t been much else in terms of fog development as dew point
depressions have generally remained too high for that. There`s
still some time for fog to develop in southeastern portions of
our forecast area, but the window for that is slowly shrinking
with a stratocumulus deck rolling in from the west and the
expectation that surface winds will pick up towards daybreak.
Growing cautiously optimistic that we`ll be able to get away
without needing a fog headline, but will continue to monitor
satellite and surface observations over the next few hours to be
sure.

The south-southwesterly winds will creep upward even further
and even begin gusting later on today as the surface pressure
gradient continues to tighten between the departing surface high
pressure and the incoming surface low pressure trough/cold
front. A strong 900-950 mb temperature inversion and the
existing deep snowpack across the area should temper gust
magnitudes enough to keep the potential for blowing/drifting
snow limited during the daytime today. The strongest gusts (and
thus the greatest potential for blowing/drifting snow) today
may actually occur this evening as warm air advection
discourages the decoupling of the planetary boundary layer at a
time when the overhead low-level jet will be strengthening.
However, even during this time, BUFKIT momentum transfer output
using several different models isn`t too keen on gusts greater
than 25 kts occurring at ground level from the resultant
downward mixing of stronger low-level flow, so not planning to
highlight the potential for blowing/drifting snow any more at
this time.

Anyways, the south-southwesterly winds will help push air
temperatures up into the mid to upper 20s during the daytime
today. While a dip in temperatures may occur after sunset, it
appears that the strengthening warm air advection should allow
for many areas to warm towards and into the low 30s during the
late evening and overnight hours ahead of the cold frontal
passage.

Of greater note tonight will be the potential for a brief
period of wintry precipitation along/ahead of the front that
may include freezing drizzle. Low-level isentropic ascent along
this frontal corridor should help sustain a stratus deck that
appears will be deep enough to support precipitation. Model
guidance remains mixed on just how cold the top of this stratus
deck will get, which will dictate whether the presence of ice
nuclei will favor poor quality snowflakes as the dominant or
sole precipitation type, or whether the lack thereof will keep
any precipitation limited to drizzle (which would freeze upon
contact with most untreated surfaces given the presence of sub-
freezing air temperatures both during and in the lead-up to this
potential precipitation event).

In general, it appears that most of the 00Z and 06Z hi-res
model solutions have cloud tops reaching -10C, which would point
to snow occurring over freezing drizzle. However, some sounding
progs still have at least one hour when the stratus deck is
entirely warmer than -10C while still being deep enough to
support precipitation, which would favor freezing drizzle or
snizzle (a mix of freezing drizzle and snow) as the prevailing
precipitation type -- at least for a brief period of time. If
the aforementioned model guidance ends up being a degree or two
too cool with regard to cloud top temperatures, then that could
also be enough to tip the scales in favor of freezing drizzle.
There`s also the possibility that none of this matters because
the stratus ends being too shallow for precipitation to occur.
Since it still looks like we`ll be toeing the line between
having cloud ice or not (and between having precipitation or
not), maintaining a mention of both snow and freezing drizzle
in the grids while keeping PoPs capped in the chance range still
seems like the way to go until upstream observational trends
become apparent later today.

Any overnight precipitation should end by sunrise on Saturday.
Daytime temperatures will likely struggle to rise much in the
wake of the frontal passage, and accordingly are largely
forecasted to remain in the 20s with many areas likely setting
their high temperatures for the day during the early morning
hours tomorrow.

Ogorek


Saturday Night through Thursday:

Focus in the extended forecast period remains the continued
unseasonably cold temperatures for early December and the
beginning of a likely active clipper-like pattern setting up (upper
ridging across the western CONUS and upper troughing across the
Great Lakes). This will allow a series of disturbances to move
within the resultant northwest flow overhead leading to multiple
opportunities for snow somewhere in the general region.

The first of these systems arrives late Saturday evening
through Sunday morning. Moisture with this system is not
especially high, but a period of modest ascent through the depth
of the column (and within the DGZ) overnight would support
widespread steady light snow (rates up to around 0.25"/hr) and
generally 1-2" totals. Localized amounts up to 3" will be
possible, especially west of the Chicago metro where the forcing
is slightly more pronounced and for a bit longer duration.

While the recent bitterly cold weather has helped cool the lake
by several degrees (now upper 30s to lower 40s), the
unseasonably cold airmass overhead in the wake of the weekend
system will lead to modest lake induced instability supportive
of lake effect snow Sunday PM. There remains a lot of
variability in how this plays out, however, from the band
focusing mainly into northwest Indiana to potentially drifting
back to the west into the Chicago metro before ending Sunday
night. Confidence accordingly remains quite low in the details.
Stay tuned!

Beyond the weekend, additional waves move within the upper
level flow beginning Monday night and continuing off and on
through the end of the week. These generally favor a slightly
more northerly track compared this weekend with snow chances
increasing with northward extent, particularly into Wisconsin.
Given we are still several days out this could certainly change
with each subsequent disturbance! It is worth noting that
ensemble guidance remains fairly consistent in developing a
stronger surface low that tracks across the Upper Mississippi
Valley Tuesday into Wednesday (albeit with notable differences
in just how strong). Nevertheless, this could help lift warmer
air into the area (low-mid 30s) and support more of a rain/snow
mix scenario.

After the aforementioned potential warmer temperatures in the
30s midweek, temperatures then fall again in the wake of that
system, with forecast highs back in the teens and 20s through
the end of the week and lows in the single digits and teens,
paired with the continued periodic snow chances (20-40%).

Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1133 AM CST Fri Dec 5 2025

Key messages:

- Lowering CIGs mid to late evening with low end MVFR likely
  into Saturday morning.

- Couple hour window favorable for very light snow and drizzle
  tonight.


VFR Cloud cover will continue through the remainder of the
daylight hours today. However, CIGS are expected to lower as we
head through the evening hours, particularly after 04Z. Low end
MVFR is expected, and we cannot rule out some periods of IFR
CIGs overnight. A couple hour window also exists for some very
light snow (possibly briefly mixing with freezing drizzle)
tonight. The favored time window for this light precipitation
continues to generally be in the 03-09Z timeframe. While some
FZDZ cannot be ruled out with this precipitation tonight, the
chances and duration of it appear to low to include a formal
mention in the TAF at this time. Otherwise, expect MVFR CIGS to
linger into Saturday.

Southwest winds this afternoon will turn west-northwesterly
later tonight following the passage of a weak surface boundary.
Prior to the passage of this boundary, there may be a short
window this evening for marginal LLWS conditions as winds off
the surface increase up to near 40 kt. Currently, it appears
the magnitude will not meet true LLWS criteria, but we will
continue to monitor.

KJB

&&

.CLIMATE...
Issued at 335 AM Fri Dec 5 2025

Here are the current record low temperatures for Today,
December 5:

Chicago            Low
Friday 12/5      4 (2005)

Rockford           Low
Friday 12/5     -5 (2005)

- NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until midnight CST tonight for Winthrop
     Harbor to Wilmette Harbor IL.

     Small Craft Advisory until midnight CST tonight for Wilmette
     Harbor IL to Michigan City IN.

&&

$$

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