714
FXUS63 KLOT 041945
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
145 PM CST Thu Dec 4 2025

.KEY MESSAGES...

- Tonight will be very cold with overnight lows ranging from -5
  to +5F. Typical cold spots may make a run for -10 to -15F.

- Patchy but dense freezing fog may develop tonight, especially
  in open or low-lying areas. Where freezing fog develops, hoar
  frost may lead to slick spots on untreated surfaces.

- Friday will start cold and calm, before breezy southwesterly
  winds develop by mid afternoon. Isolated blowing/drifting snow
  cannot be ruled out.

- A mix of freezing drizzle and snow may materialize north of
  I-80 Friday night (20-30% chance).

- A clipper-like pattern will become established this weekend
  into early next week with regular opportunities for snow in
  the general region.

&&

.DISCUSSION...
Issued at 145 PM CST Thu Dec 4 2025

Tonight through Friday night:

The center of a surface pressure ridge will continue to slide
through the region this afternoon and settle across the Ohio
River Valley tonight. Clear skies and a healthy snow pack at
sunset will set the stage for quick decoupling and ideal
conditions for efficient radiational cooling tonight, supporting
overnight lows dropping like a rock. (It`s probably not a good
sign that some areas already have dew points ranging from 5 to
10 below). Will go ahead and explicitly forecast overnight lows
ranging from -5 to +5F, keeping in mind that typical cold spots
(Rochelle, Aurora) may make a run for -10 to -15F. The record
low temperature for the calendar date of December 5 will be
threatened at Chicago (4F set in 2005) and Rockford (-5F, also
set in 2005).

Gradually increasing warm air advection along the backside of
the high pressure system in tandem with the very cold surface
temperatures will set the stage for the development of a strong
low-level inversion tonight. With forecast soundings depicting
trapped low-level moisture beneath the inversion (partially a
consequence of sunshine facilitating sublimation at the very top
of the snowpack today), concern is rising that shallow but
dense freezing fog will develop tonight, especially in sheltered
areas and near rivers. The main limiting factor for fog appears
to be some 20-25kt of flow right at the top of the inversion,
which may mechanically mix dry air downward toward the surface.
However, given the very cold forecasted overnight low
temperatures, do think that the near surface layer may end up so
stable it resists the downward entrainment of any dry air. So,
will introduce patchy freezing fog across the entire area, save
for near Lake Michigan. If freezing fog does develop and become
dense, the deposition of ice (hoar frost) would be likely on all
surfaces (trees, fences, roadways/bridges/overpasses). If/where
temperatures verify on the colder end of the spectrum, a
saturated DGZ may develop literally at ground level, perhaps
supporting suspended ice crystals (ice pillars). In all, tonight
will be more typical of the middle of winter in the northern
Plains than early December in the Great Lakes.

Tomorrow, a surface low will move across northern Wisconsin and
act to tighten the low-level pressure gradient across the Lower
Great Lakes. Mixing into the strengthening low-level wind field
will be stunted initially by the strong morning inversion,
though by early afternoon, do expect southwesterly winds to gust
20 to 25 mph. In scenarios where mixing is most efficient,
gusts in the 30 to 35 mph range cannot be ruled out by early
evening. If such strong winds were to materialize, would have to
watch for areas of blowing and drifting snow in open areas (a
quick check outside confirmed the top layer of snow has become
quite powdery). Highs will rise toward the mid to upper 20s.

Tomorrow night, weak isentropic ascent ahead of an approaching
cold front will induce the development of low stratus across the
region. Forecast soundings indicate that stratus thickness may
become sufficient to produce precipitation at some point early
Saturday morning, mainly near and north of Interstate 80. With
in-cloud temperatures appearing to be warmer than -8C (unsupportive
of cloud ice), precipitation type may tend to favor freezing
drizzle of snizzle. So, will tuck in a mention of freezing
drizzle into the forecast, ending by daybreak Saturday.
Temperatures look to hover in the mid to upper 20s all night, so
any freezing drizzle would be prone to sticking to any
untreated surface.

Borchardt


Saturday through next Thursday:

This weekend through the middle of next week, we`ll find the
general region sandwiched between a blocking upper high off the
central Pacific coast and a persistent upper low spinning over
Canada`s Hudson Bay region. This will result in a series of
upper trough axes training across the central CONUS and lock the
Midwest into a NW to zonal upper flow pattern. And with a rich
atmospheric river feeding onshore the PNW and Alberta coasts,
such a setup will drive a procession of mid-level impulses off
the lee of northern US and Canadian Rockies and down across the
Midwest presenting frequent opportunities for snow. As my
colleague puts it, "hop aboard the clipper express!"

The first in this series of waves comes Saturday evening into
Sunday morning. Low pressure will track across central IL and
likely bring a good clip of snow showers to the local area.
Forecast soundings from around the CWA feature saturation and
decent forcing for ascent through and on either side of the DGZ,
which should support a widespread push of snow north of the
storm track. Heavy banding doesn`t appear to be a big concern
with this system, but with the mid-level jet immediately to our
north, there is a signal for some moderate shearing deformation
on the storm`s northern flank which could offer a narrow
corridor of relatively higher totals north of the storm track.
This signal is much stronger out in IA before weakening into our
area. Accordingly guidance strongly favors higher totals toward
and west of the Mississippi and lesser totals locally. Ensemble
and deterministic guidance likes a largely 1 to over 2" range
around a majority of our CWA with fair support for a little less
than that, and almost no support for anything more than 3".

Bands of lake effect snow look to develop in the storm`s wake
Sunday afternoon into early Monday, but it`s unclear how this
potential will impact us locally. The ENS maintains more BL
moisture into Sunday night and has the low level wind field more
veered making it more bullish on the lake-effect potential,
particularly for the IL side, than the GEFS. Latest
deterministic Euro even resolves an explicit mesolow around the
bend of the lake which would also help steer snow toward the
Chicago. Probs from the GEFS are lower around the lake and are
focused toward northwest IN. Meanwhile, the GEPS gives a look
closer to that of the ENS. This doesn`t appear to be an overly
impactful lake-effect setup, but the potential is there for
localized areas to maybe see upwards of an additional inch or
two, likely much less than that on a more widespread basis.

Guidance then resolves a one-two punch of clipper systems
tracking through the region in quick succession early next week;
the first being Monday night into Tuesday and the other slated
for Wednesday. Uncertainty grows quickly from this point as
these systems will be in the process of phasing with the Hudson
Bay upper low making them especially tough to get a good handle
on. A lot of run-to-run model variability exists with these
systems, and even individual members of any given ensemble
system offer a wide variety of outcomes following this weekend.

Monday night-Tuesday, the clipper and highest PoPs are favored
to track just to our north with a surface cold front moving
across our local area. Moisture and forcing may be tightly wound
around the low to the north and ensemble PoPs drop off pretty
quickly with southern extent through the CWA. Surface high
pressure to the SE may also impinge on precip chances into our
CWA; the GFS is most bullish on this drier outcome. Snow showers
appear likely for at least northern portions of the CWA.

Wednesday`s clipper is also favored to track to our north
across WI, although there is even more variability with this
storm. Medium range guidance agrees that enough warm air will
infiltrate the system to offer rain or a wintry mix to those
roughly near and south of the storm track, and there`s a good
chance that includes at least a portion of our CWA. Additional
opportunities for lake-effect snow may present themselves on the
backsides of the these systems next week, although details are
very fuzzy for the time being.

Despite the uncertainties in precip expectations, there is fair
agreement on temperatures through the middle of next week,
particularly among daytime highs where there is more spread in
low temperature guidance. Below-normal conditions will persist
with daytime temperatures in the upper teens to mid 20s Sunday
and Monday. Sunday night into Monday morning looks particularly
cool, similar to what we saw this morning. There`s a bit of
spread, but single digits appear likely outside of the city and
interior Chicago metro, with single digits below zero very
attainable, especially across outlying areas. Winds look rather
light which should keep the wind chill factor from getting out
of hand. We`ll trend warmer for Tuesday and Wednesday, but
likely still below normal with lower and middle 30s favored.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1126 AM CST Thu Dec 4 2025

Main aviation concerns include:

- FEW to BKN MVFR stratocumulus clouds at ORD/MDW/GYY through this
  evening

- Threat for freezing fog at DPA/MDW through the overnight
  hours

- Increasingly breezy southwest winds on Friday.


Discussion:

Winds at the start of the TAF period will be light (5kt or less)
and generally from a northerly direction as the center axis of a
surface pressure ridge slides through the region. Winds should
settle on a southwesterly direction this evening along the
backside of the ridge, and remain so through the remainder of
the TAF period.

As a consequence of a modest 925mb northeasterly steering flow,
lake-effect stratocumulus clouds based between 2500 and 3000 ft will
continue to drift westward off Lake Michigan this afternoon and
get in the vicinity of ORD and pass over MDW/GYY through the
evening. Skies should clear after sunset.

Tonight, the combination of clear skies, high surface pressure,
and a deep snowpack will provide ideal conditions for the
development of an unusually strong low-level inversion. With
forecast soundings depicting trapped moisture beneath the
inversion, do have concern that patches of shallow freezing fog
will develop overnight, particularly at outlying airports. In
these kinds of situations, observations can vacillate between
P6SM and 1/4SM on the order of minutes as the shallow fog ebbs
and flows around ASOS platforms. In addition, where fog is
densest, surface deposition of ice would occur on all surfaces.
Given the expected strength of the low-level inversion, any
freezing fog that develops may be prone to sticking around
through an hour or two after daybreak in spite of increasing
low-level flow atop the inversion. In all, felt the course of
least regret would be to introduce TEMPO groups for 2SM BR at
RFD/DPA centered on midnight when the signal for fog is
strongest, keeping in mind that periodic much lower visibility
is very much possible.

After daybreak, mixing induced by the sun should allow for any
lingering freezing fog to erode. The initial strength of the
inversion will relegate mixing into the low-level wind field
behind the departing high pressure system to mid to late
morning onward, after which frequent southwesterly gusts in the
20-25kt range are expected. In fact, a few gusts up to 30kt
cannot be ruled out before sunset Friday as a low-level pressure
gradient tightens in response to a surface low moving across
northern Wisconsin.

Borchardt

&&

.CLIMATE...
Issued at 1126 AM Thu Dec 5 2025

Here are the current record low temperatures for Friday,
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 3 PM CST this afternoon for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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