319
FXUS63 KLOT 280520
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1120 PM CST Thu Nov 27 2025

.KEY MESSAGES...

- Winter Storm Watch remains in effect for late Friday night
  through Saturday night for all of northern and central IL as
  well as northwest Indiana.

- Impactful snow is expected late Friday night through Saturday
  night. There is a >70% chance for 6"+ amounts and higher end
  travel impacts along and north of I-80.

- Well below normal temperatures will persist into or through
  next week, perhaps with additional chances for snow (Monday
  afternoon and night).

&&

.DISCUSSION...
Issued at 301 PM CST Thu Nov 27 2025

Through Friday:

Moisture channel loops reveal a fairly robust shortwave pressing
southward across southern Wisconsin. This feature will continue
into northern Illinois through this evening which will lead to
an increase in low-level cloud cover and an expansion of
flurries and snow showers. Based on upstream radar trends, have
increased PoPs and areal extent of flurries and snow showers
farther south and west through the forecast area. Outside of an
isolated dusting, not expecting impacts from this activity.

Lake effect snow showers may end up building across parts of
northeast Porter County tonight as colder air spills southward
out of Wisconsin. Lake induced ELs based on water temperatures
in the upper 40s are actually respectable--nearing 10 kft due
to cold 850 mb temperatures--but suspect much of this unstable
area won`t end up materializing due to dry air above 5 kft or
so. Regardless, have increased PoPs a bit this evening and will
show a little snow accumulation to account for some briefly more
robust snow showers overnight. Friday looks quiet as high
pressure briefly builds in across the region.

Carlaw


Friday Night through Saturday Night:

Accumulating snow with potentially significant travel impacts
(specifically due to busy holiday travel) will occur across the
area late Friday night through Saturday night.

A powerful trough about to move onshore of the WA coast will
dig southeastward to the central Rockies/Great Plains Friday
night while phasing with a weak subtropical wave currently over
the northern Baja of California. The combination of modest
Pacific moisture from both systems as well as strong low-level
moisture transport through the mid/lower Mississippi River
Valley and broad upper-level diffluence will yield a sizable
shield of precipitation across the central CONUS. As the phased
wave amplifies and transitions from a neutral to slightly
negative tilt, robust low-level WAA/isentropic ascent is
expected to bring intermittent waves of accumulating snow across
much of the forecast area very late Friday night through
Saturday morning. Varying orientations of low-level isentropic
ascent as well as some questions on the amount of initial
widespread saturation may ultimately result in light snow with
periods of moderate snow through the morning hours before deeper
forcing with the trough spreads a more expansive shield of snow
with higher rates over the area Saturday afternoon.

The increasingly favored track of the surface low from west-
central Illinois to far southern Lake Michigan late Saturday
afternoon and night indicates that snowfall quality will
diminish through the evening hours as low-level warming occurs
below an approaching mid-level dry slot. It is possible that
areas east of I-57 mix with or even briefly change over to
drizzle during the evening. The back edge of the system will
then depart late Saturday night through Sunday morning, ending
the broader synoptic precip potential.

Snow amounts and the need for corresponding headline changes
remain the biggest challenge with the system.

Snow-to-liquid ratios (SLR): Guidance as a whole depicts
average SLR values closer to 15:1. Lower SLR values from
typically better performing statistical output as well the lack
of a deep isothermal layer in the DGZ (both primary and warmer
secondary layers) suggest SLR values will settle more in the
9-12:1 range. Additionally, the track of the lower-tropopsheric
low directly across the forecast area and approaching mid-level
dry slot to the southwest will result in SLR values trending
lower with time later Saturday into Saturday evening.

QPF: While there has been plenty of noise from deterministic
and various ensemble members over the past couple days, the
broader envelope of ensemble solutions has been fairly
consistent with storm total QPF values in the 0.5 to 0.8" range.

Snowfall: Putting together the two above parameters, amounts in
excess of 6" remain likely for much of the forecast area. It
should be noted that both the QPF and SLR values from NBM values
are on the higher end of the ensemble envelope, and are
therefore producing substantially (3-5") higher snowfall than
the official forecast. Overall snowfall rates will average
around 0.5"/hr, with a period of rates up to around 1"/hr during
the afternoon hours.

Headlines: Have maintained an areawide Winter Storm Watch with
no new adjustments with this forecast. While some or all of the
watch will need to be upgraded to a Winter Storm Warning with
future forecasts, the overall low track directly over the CWA,
potential for lower SLR values (wetter snow), and longer
duration snowfall event (12-18 hours) suggest that some wrinkles
still need to be ironed out in the forecast this far out.


Sunday through Thursday:

A 1035-1040 hPa high building into the Great Plains impinging
on the low over Lake Huron and Georgian Bay on Sunday will
induce strong gradient winds and a modest 3-5mb/3hr pressure
rise over much of the area. Some shallow blowing and drifting
snow will likely persist for open areas conditional on a drier
character of the expected snow pack. Will include patchy
blowing snow in the forecast for now, with the greatest concerns
on N/S-oriented roadways in open areas west of the Fox River
Valley. Additionally, ongoing strong CAA with shallow stratus
under a steadily lowering subsidence will support some lingering
light snow showers or flurries well into the day Sunday. The
lower inversion will also limit lake effect snow intensity near
the lake in northwest Indiana, but additional minor
accumulations are probable for northeast Porter County.

The inversion should lower enough to erode remaining stratus
Sunday night as the surface ridge edges toward the area. As long
as stratus does not persist well into the night, diminishing
winds with a fresh snow pack will promote a rather chilly Sunday
night with lows potentially at or below zero across interior
northern Illinois. Very cold conditions will persist through
Tuesday as a reinforcing shot of colder air arrives on Monday.

A mid-level wave crossing the Ohio River Valley late Monday
afternoon and Monday night is now exhibiting decent phasing with
a sub-tropical system over the Lower Mississippi River Valley,
which supports increasing chances for light accumulating snow to
affect at least the southern CWA during this time. Resultant
higher cloud cover would ultimately tamper very cold conditions
Monday night, but if there is any clearing, sub-zero lows will
again be possible.

Deep troughing now anchored over eastern Canada will lead to
colder conditions through the remainder of the week, with some
energy from a Pacific NW trough bringing another period of
potential snow to the western Great Lakes Wednesday into
Thursday.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1120 PM CST Thu Nov 27 2025

- Widespread snow moves into the area late Friday night

For the remainder of tonight and through the daylight hours on
Friday winds will remain generally west northwest around 10-12 kt
with occasional upper teen gusts. Winds then ease after sunset,
becoming light southerly to at times variable Friday evening
ahead of the approaching winter storm.

Snowfall onset time remains a bit lower confidence as it will
take some time for the snow to work through the dry low-to-mid
levels initially. This is currently favored to occur during the
5-9Z timeframe into the Chicago terminals. Once snow begins,
deteriorating conditions are then anticipated with each passing
hour into the daytime hours on Saturday (beyond the current TAF
period). For now have introduced prevailing -SN with MVFR VSBYs
at 7Z for ORD/MDW with PROB30s for IFR VSBYs 10-12Z. While
snowfall rates prior to daybreak may remain under 0.5" per
hour, given the temperatures Friday night will be in the 20s,
any snow that does fall will readily accumulate on untreated
pavement.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Watch from late Friday night through late
     Saturday night for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108.

IN...Winter Storm Watch from late Friday night through late
     Saturday night for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 9 AM CST Friday for the IL
     nearshore waters.

     Small Craft Advisory until 3 PM CST Friday for the IN
     nearshore waters.

&&

$$

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