968
FXUS63 KLOT 290531
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1131 PM CST Fri Nov 28 2025

.KEY MESSAGES...

- A Winter Storm Warning remains in effect for the entire area
  from late tonight through Saturday night.

- Impactful snow will move into the area late tonight into early
  Saturday morning. There is a >90% chance for 6"+ snow amounts
  and higher-end travel impacts along and north of Pontiac to
  Kankakee to Valparaiso line. The highest snowfall rates and
  worst conditions are expected Saturday afternoon into early
  evening.

- Increasing chance (60%) of light accumulating snow late Monday
  afternoon and night.

- Well below normal temperatures will persist through next week.

&&

.UPDATE...
Issued at 1011 PM CST Fri Nov 28 2025

There have been no significant changes to our going forecast
and thoughts for the incoming winter storm.

The only real change of note made for the evening forecast
update was to delay the start time of the snow by about an hour
or two. The 00Z RAOBs from DVN and ILX both sampled a fairly
stout wedge of dry air below 700 mb. This dry air, which has
been getting funneled northward into the region by a surface
high centered over the Ohio River Valley this evening, was
greatly impeding the descent of snowflakes in eastern Iowa
earlier this evening with the KDVN radar nicely depicting this
dry layer at work as radar echoes had struggled to make
eastward progress towards the radar site earlier. This dry
sub-cloud layer will continue to gradually be eroded --
primarily via top-down saturation -- over the course of the
night, but with the primary 700 mb f-gen axis remaining
displaced to our west, snow will likely struggle to accumulate
in most locations until the better upper-level forcing support
arrives late tonight into tomorrow morning. Therefore, felt
comfortable delaying the onset of PoPs by a bit and shaving a
couple tenths off of forecast snow accumulations in some areas
during the overnight hours.

Otherwise, some of the uncertainties discussed in the afternoon
discussion below are still apparent in the broader suite of 00Z
model guidance that`s still rolling in. While these
uncertainties could play a role in some finer-scale aspects of
our forecast (e.g. how far north the cutoff between snow and
drizzle/rain will protrude, who sees snow totals that are
towards the upper bounds of our snowfall forecast ranges vs. who
see totals that are closer to the lower bounds of those ranges,
etc.), the bigger picture and overall forecast message remains
unchanged. Broad synoptic-scale forcing for ascent will bring an
expansive snow shield across the region late tonight through
tomorrow, resulting in an extended period of accumulating
snowfall that will snarl post-holiday travel (particularly
during the afternoon hours, when the highest snowfall rates are
still expected to occur) and culminate in widespread snow
accumulations of 6 or more inches across the region. The going
segmented Winter Storm Warning still appears to reflect our
overall forecast thoughts well, so have made no changes to that.

Ogorek

&&

.DISCUSSION...
Issued at 255 PM CST Fri Nov 28 2025

Through Monday:

All eyes are on the expected widespread early season heavy snow
event across the region beginning late tonight and continuing
into at least Sunday morning.

The parent trough responsible for the incoming system is
currently analyzed near the Grand Tetons. The trough will
continue on an ESE/SE trajectory through tonight as it phases
with a remnant subtropical wave now over the far southern
Rockies. The resultant amplification of the trough will swing
across the far western Great Lakes while exhibiting a slight
negative tilt by Saturday evening. The kinematic and
thermodynamic set-up continue to become more impressive as the
event nears. Loose coupling of a departing upper jet streak over
the western Great Lakes and a strengthening 130 knot jet streak
entering MO and southern IL late Saturday afternoon will
provide plenty of upper-level support. Meanwhile, decent height
falls and expansive mid-level diffluence will spread across the
region. Stepping downward, guidance has shown a substantial
trend toward a sharpening 700 hPa frontogenesis axis while also
exhibiting increasing mid-level isentropic ascent below 7C/km
600-400 hPa lapse rates. Lastly, robust low-level moisture
transport of 50 knot southerly winds will provide a continuous
feed of moisture into the system. Needless to say, the continued
favored low track from the IA/MO/IL triple point to over
Chicago would not climatologically support a large snow-maker in
late November. But with the cold post-Thanksgiving airmass in
place and abundant forcing noted above, we are trending toward a
decent snow event for much of the forecast area.

There remain a few varying fail modes for widespread heavy snow
across the area. First, initial deep saturation on the western
extent of a rather dry airmass centered over the Tennessee
Valley will likely result in top-down saturation under only the
strongest ribbons of low/mid-level isentropic ascent and
frontogenesis. So as snow begins to spread eastward late tonight
and into Saturday morning, narrow bands of moderate to heavy
snow may be interspersed within a broader area of light snow.

Second, the steep lapse rates aloft will support borderline
upright convection above the impressive 700 hPa f-gen band, with
a decent amount of CSI present wherever upright convection does
not develop near and north of the band. In this case, a narrow
band of very heavy snow would settle over west-central Illinois
through the morning and early afternoon before lifting ENE over
much of the CWA later in the afternoon and early evening. This
would possibly cut down on snow totals through the morning and
limit the higher snow rates of 1"/hr or higher to a <6hr window
mid-afternoon into early evening over northern Illinois.

Finally, the more northern track of the surface low and
negative tilt of the trough suggest that a rapidly advancing dry
slot will cut off much of the better snow production, even
transitioning to drizzle, for much of the southeast half of the
CWA by early Saturday evening. Like the second point above, this
would limit the window for the higher snowfall rates to only
around 6 hours or so.

Regardless of all that said above, the combination of deep
moisture and very strong forcing will result in a period of
heavy snow with very hazardous travel conditions Saturday
afternoon into early evening. Additionally, the northern low
track now means that stronger SE winds with gusts over 25 mph
will become prevalent and create some blowing and drifting snow
concerns in open areas during the afternoon and early evening.
Snow totals areawide still look to fall in the 6 to 10 inch
range with a likely embedded band of more than 10 inches
somewhere over the area. The entire area remains in a Winter
Storm Warning late tonight through much of Saturday night.


Sunday and Sunday Night:

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 3-6mb/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.

Kluber


Monday Night through Friday:

The primary forecast focus beyond this weekend`s winter storm
is the increasing potential for another round of accumulating
snowfall during the Monday afternoon-Monday night. The multi-
model consensus this morning continues to advertise a secondary
shortwave dropping south through the Great Basin Sunday night
and eventually ejecting across the Central Plains on Monday as a
positively-tilted trough. This orientation would result in a
fairly progressive system overall, but intensifying mid-level
frontogenesis is forecast to result in an expanding region of
generally light snow across parts of our forecast area. A
significant north-south spread exists persists in the guidance
and ensemble output today, but in general, the favored location
for fgen-enhanced snowfall appears to be setting up somewhere
across the southeastern two-thirds of the forecast area.

Main concern during this period is that snow would be falling
into a cold airmass with surface temperatures in the upper teens
to mid 20s which would easily result in slick/hazardous travel
even with modest snowfall amounts. The other aspect that has our
attention is some degree of near-upright/convective instability
in recent model guidance above the main frontogenetic
circulations which could end up locally-enhancing precip rates.
At this time, liquid amounts are generally a quarter inch or
less which would support perhaps 2-4 inches of snowfall given
generally modest ascent through the DGZ and and cold surface
temperatures. Too much uncertainty at this point to pinpoint the
main threat area, but something we`ll be keeping a close eye on
over the coming days.

Increased cloud cover may end up tempering overnight lows/wind
chills a bit more than the currently-advertised NBM grids, but
cold conditions will nonetheless prevail next week. A
reinforcing shot of cold air is generally forecast to arrive
midweek as a roughly 1040 mb arctic high slides southward across
the central CONUS, perhaps with an additional round of snow or
at least flurries as the boundary layer saturates into the base
of a very deep dendritic growth zone.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1130 PM CST Fri Nov 28 2025

- Snow overspreads the region late tonight/early Saturday
  morning and continues through Saturday evening.

- CIGS and VSBYs will steadily deteriorate through Saturday
  morning. Periods of LIFR VSBYs are expected, particularly
  Saturday afternoon/early evening (localized VLIFR possible).

- Snowfall rates will average around 0.5"/hr but will peak
  around 1"/hr Saturday afternoon.

Tonight-Early Saturday morning:

Light snow/flurries may begin to move into the area as early as
6-8Z but it will take time for the snow to work through a
lingering dry layer. Because of this, opted to delay the onset
of MVFR -SN by an hour or two at all sites with the 3Z
amendments and have maintained those changes. VSBYs are expected
to steadily deteriorate to IFR (1-2 SM) as we approach daybreak
Saturday around 12Z (10Z at RFD). Expect steadily deteriorating
conditions through the remainder of the morning hours. Can`t
rule out occasional periods of LIFR VSBYs and IFR CIGs by mid-
late morning which may warrant TEMPO groups with later updates.

Light SE winds will gradually increase through the night with
gusts to 20-25kt expected by mid-late morning.

Saturday afternoon:

This is the window for the highest snowfall rates (up to 1"/hr)
and have accordingly maintained prevailing 1/2 SM VSBYs from
18-00Z. Localized dips to VLIFR also cannot be fully ruled out
but confidence remains too low for a formal TAF mention.

Saturday evening/night:

As the low center moves into the area winds will ease paired
with decreasing snowfall rates (0.25"/hr). This may also
coincide with a brief mix or full changeover to light drizzle
paired with lowering CIGs and VSBYs back down to LIFR to
potentially locally VLIFR. As the surface low moves through
overnight winds will return to southwest and then northwest,
becoming increasingly gusty again (gusts up to 20-25 kt) by
daybreak Sunday.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning from 3 AM Saturday to 6 AM CST Sunday for
     ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-
     ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108.

     Winter Storm Warning from 3 AM Saturday to midnight CST
     Saturday night for ILZ033-ILZ039.

IN...Winter Storm Warning from 3 AM Saturday to 6 AM CST Sunday for
     INZ001-INZ002.

     Winter Storm Warning from 3 AM CST /4 AM EST/ Saturday to
     midnight CST /1 AM EST/ Saturday night for INZ010-INZ011-
     INZ019.

LM...Small Craft Advisory from 9 AM Saturday to 10 PM CST Sunday
     for the IL nearshore waters.

     Small Craft Advisory from 9 AM Saturday to 7 AM CST Monday for
     the IN nearshore waters.

&&

$$

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