959
FXUS63 KLOT 052015
AFDLOT

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

.KEY MESSAGES...

- There remains a low (20-30%) chance for a few snow showers and
  patches of freezing drizzle, mainly north of I-80 overnight.

- A clipper system will deliver a swath of snow in our region
  Saturday night into Sunday.

- Light lake effect snow may affect portions of northwest
  Indiana and northeast Illinois Sunday night into Monday
  morning.

&&

.DISCUSSION...
Issued at 215 PM CST Fri Dec 5 2025

Tonight through Sunday:

A low-amplitude upper-level wave continues to propagate
eastward across the Upper Mississippi River Valley, and is
generating a broad region of snow across southern Wisconsin, far
northern Iowa, and western Wisconsin. As the wave shifts
eastward this evening and overnight, it will gradually weaken
leading to corresponding weakening lift. Forecast soundings
continue to depict marginal temperatures for snowflake
production, and indeed, have noted a few intermittent reports of
freezing drizzle primarily across far northern Iowa this
afternoon. So, the inherited forecast calling for snow showers
and perhaps a few patches of freezing drizzle along and north of
I-80 through the overnight hours remains on track.

A narrow surface pressure ridge will slide through the area
tomorrow, leading to a brief period of quiet weather. With
little change in the airmass and snowpack between today and
tomorrow, as well as the expectation for lingering clouds, felt
persistence was the way to go is for high temps to be similar to
today and in the mid upper 20s.

Attention then turns to a clipper system due to race from the
northern Plains tomorrow and into the Great Lakes Saturday night
through mid-Sunday morning. CAM guidance seems to favor a
stronger, wetter, warmer, and further northward evolution of the
system, with forecast soundings depicting a 3 to 6 hour window
where lift would be maximized just beneath the DGZ (ideal). In
fact, forecast thermal profiles get precariously close to
transitioning toward a wintry mix or even rain with southward
extent as surface temperatures warm toward freezing. In all, the
stronger/northern scenario would lead to a quick hit of 3 to 5
inches of snow, especially along and north of I-80. Meanwhile,
global guidance and ensemble means remain more muted, less
amplified, and colder with temperatures remaining in the 20s and
the greatest lift displaced well beneath the DGZ (small
snowflakes). Such a scenario would lead to a broad swath of 1 to
3 inches of snow centered near and south of I-80.

For now, will gently nudge the forecast toward a blend of both,
leading to a swath of 0.15-0.3" of QPF with ratios near 12:1
translating of a broad swath of 2 to 4 inches of snow across the
area.

Borchardt


Sunday night through next Friday:

Surface high pressure will quickly shift across the western
Great Lakes Sunday night into Monday. The high will be somewhat
elongated SW to NE, which will favor NNE/NE low-level flow over
southern Lake Michigan and into northern Illinois through the
night. Likely development of a land breeze over Lower Michigan
will enhance the more northeasterly shift. This would normally
result in a favorable set-up for lake effect snow into far
northwest Indiana and northeast Illinois. However, there are
several limiting factors that will greatly reduce LES potential
during this time. Weak short-wave ridging will lower inversion
heights to or slightly below 5kft, roughly in the DGZ. Adding in
some low-level drying upstream, a marginal saturated cloud
depth only poking into the DGZ would yield lower SLR values and
snowfall intensity. Have included up to an inch of snow during
the night near the lake, with the potential for a more focused
LES band and/or mesolow (from enhanced land breeze convergence)
to produce slightly higher snowfall amounts.

The second system in the extended wave train across the region
will track eastward across Lake Superior Monday night. An area
of snow from strong low-level WAA and broad mid-level diffluence
may brush the northern CWA with minor accumulations, but the
lack of full top-down saturation this far south precludes
including higher totals or more widespread snow potential.

The next, and much stronger, system will swing across the
western Great Lakes late Tuesday night into Wednesday morning.
The CWA will be well within the warm sector, with robust low-
level WAA from Monday night through Tuesday evening pushing
temps above freezing. However, it remains to be seen how the
erosion of the upstream snowpack across Missouri and southern
Illinois modifies the air mass. With that said, thermo profiles
indicate that any initial snow (also possibly some freezing
rain) will quickly transition to predominantly rain over much of
the area through the night. Strong CAA will then change rain
back to snow Wednesday morning, with windy conditions and
scattered snow showers persisting through the day. Still a lot
of details to iron out this far out in terms of precip type, but
plan for at least some wintry precip and potential impacts
Tuesday evening and/or Wednesday.

Active weather will continue in the region through next
weekend, with the potential for a couple additional clipper-like
systems to bring wintry weather over the Great Lakes during
this time.

Kluber

&&

.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

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until midnight CST tonight for the IL and
     IN nearshore waters.

&&

$$

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