650
FXUS63 KLOT 041725
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1125 AM CST Thu Dec 4 2025

.KEY MESSAGES...

- Brief period of very cold weather through tonight.

- Series of clippers to bring periodic light snow chances to
  the region through the period. Confidence is not particularly
  high with tracks/timing of these features.

&&

.DISCUSSION...
Issued at 322 AM CST Thu Dec 4 2025

Arctic high pressure (1035mb at Yankton SD at midnight) is
building east-southeast across the Corn Belt early this morning,
in the wake of the sharp cold front which pushed through the
area Wednesday afternoon/evening. Temperatures continue to
steadily fall as northwest winds on the eastern periphery of the
high provide persistent low-level cold advection. Temperatures
are expected to dip into the negative single digits in spots
west of Chicago through/shortly after sunrise and the positive
single digits and low teens farther east. Coldest wind chills
are still expected to be around -15F early this morning, as
temps continue to fall but winds gradually diminish. The surface
ridge is progged to drift east across the forecast area this
afternoon under mostly sunny skies, further reducing wind
speeds. With a substantial early-season snow pack and shallow
mixing, temps will be slow to recover from this morning`s lows,
though sunshine should help. Guidance depicts a large range in
forecast highs for today, from the low-mid teens to the low-
mid-20s for afternoon highs for afternoon highs, and have
blended this to be a little warmer than our previous forecast.

With the surface ridge across the area early this evening,
we`ll see a quick drop in temps after sunset given the light
winds, mainly clear skies and the snow pack. Will likely see
readings drop into the negative single digits in places during
the evening, then become steady/slowly rise overnight as warm
advection develops (especially aloft) as winds turn south and
increase on the back side of the now-departing surface ridge.
Wind chills will likely bottom out around -15F (or even a bit
colder) during the night with increasing south winds offsetting
the slow rise in temps. Some guidance trends suggest the
potential for some low-stratus development after midnight as
warm/moist advection flow increases above the shallow boundary
layer. Temperatures are expected to reach the upper 20s and
ending this short bout of very cold arctic weather.

The first in what appears to be a series of clipper-type
systems is forecast to propagate southeast across the northern
Plains and upper Mississippi Valley Friday afternoon/evening.
12/18Z guidance from Wednesday had waffled a bit on the track
and precip footprint with this system, though 00Z guidance is in
general agreement in bringing the positive-tilt mid-level wave
across the area Friday night with the primary surface low
passing north of the area. Forecast soundings depict rather dry
moisture profiles which mainly saturate in the mid-levels
briefly Friday evening. Ascent is not particularly strong and
the DGZ depth not impressive, suggesting the potential for a
period of mainly light snow, with QPF and ensemble precip probs
also notably low south of the IL/WI border. Have maintained
30-40% pops generally north of I-80 for this - highest toward
the WI state line and north of it.

Guidance remains in decent agreement with another mid-level
wave tracking east-southeast across the central/northern Plains
on a slightly farther south track Saturday, which looks to have
a better potential for producing some light snow across parts
of the forecast area Saturday night into Sunday, albeit in a
weakening phase. The clipper parade continues onward into next
week, with ensembles continuing to highlight systems Monday
night into Tuesday, and again Tuesday night into Wednesday.
There continues to be ensemble support for a deeper surface low
to track north of the area in the Wednesday time-frame,
potentially producing warm enough thermal profiles in which
mixed precipitation could be an issue over our cold snow pack.
Snow cover should remain largely in place, given cooler than
average temperatures expected through the period.

Ratzer

&&

.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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