164
FXUS63 KLOT 050950
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
350 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 low that tracks across the upper Mississippi Valley
Tuesday into Wednesday system (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 continued periodic snow chances (20-40%).

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1108 PM CST Thu Dec 4 2025

Southwest winds under 10 knots at TAF issuance will gradually
increase this morning as a surface high exits eastward. Expect
gusts over 20 knots late this morning through this evening. A
cold front will then shift winds WNW with gusts around 20 knots
overnight.

Generally clear skies combined with the light winds and
existing snow pack have allowed a strong and shallow nocturnal
inversion to develop. BR is becoming more evident across
northwest Indiana, with the potential for IFR (or lower)
visibility and ceilings to form and expand/advect toward the
southern Chicago metro through the early morning hours. Mixing
of very dry air atop the inversion (roughly at 1kft) will
somewhat impede the northward progress of fog or stratus, and
VFR stratus shifting eastward across northwest Illinois will
also limit BR/FG development farther north over the main Chicago
terminals. Have maintained a mention of VFR BR at MDW/ORD, but
will need to continue to monitor for inclusion of lower
ceilings/visibility if fog quickly develops to the south and
southeast over the next few hours.

Moisture and warm-air advection ahead of an approaching cold
front this evening will likely promote MVFR ceilings mid-evening
through the overnight hours. IFR ceilings with flurries or
light freezing drizzle cannot be ruled out for a few hours
centered around midnight/06Z just ahead of the cold front, but
chances remain too low to include at this time.

Kluber

&&

.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 from noon today to midnight CST tonight
     for Wilmette Harbor IL to Michigan City IN.

&&

$$

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