922
FXUS63 KLOT 092344
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
544 PM CST Sun Nov 9 2025

.KEY MESSAGES...

- A period of dangerous and intense lake effect snow is expected
  tonight into Monday in northeastern Illinois and northwestern
  Indiana. The peak coverage of lake effect snow should be
  between 11 PM and 10 AM Monday. Travel is not advised during
  periods of lake effect snow.

- A gradual warming trend, possibly accompanied by periods of
  breezy winds, is expected toward the middle of next week.

&&

.DISCUSSION...
Issued at 317 PM CST Sun Nov 9 2025

Through Monday:

DANGEROUS LAKE EFFECT SNOW EXPECTED TONIGHT INTO MONDAY:

The forecast message for tonight into Monday morning remains
largely unchanged from previous thinking. The only change to the
going headlines was to include Lake County IL in the Winter Storm
Watch.

An exceptionally favorable lake induced thermodynamic environment
continues to set up over Lake Michigan this afternoon as near
record cold 500 mb temperatures (~-40C) continue to move into the
western Great Lakes. Consequently, this will continue to support
the strengthening of a meso-low on the lake as it slides south-
southwestward towards the southern Lake Michigan shore mid to late
this evening. As this meso-low comes onshore, expect a very
intense arcing band of lake effect snow (likely accompanied by
thunder and lightning and locally stronger wind gusts) to quickly
shift onshore across much of the Chicago metro area and far
northwestern IN later this evening (after 9 or 10 PM).

While the band should weaken with westward extent towards the far
western Chicago suburbs, the most intense snow within this band
is likely to fall at rates of 2-3"+ per hour, which in combination
with gusty winds to 35 mph could result in some periods of near
white out conditions. The heaviest snow rates are expected to
largely remain along and east of the I-55 and I-355 corridors in
northern IL and parts of northwestern IN. These areas will
thus experience quickly deteriorating travel conditions after 10
PM this evening. Conversely, drastically lighter snow amounts and
rates are anticipated across the far western suburbs (towards the
Fox Valley), so travel conditions will likely vary considerably
with westward extent across the Chicago metro area.

Following this initial arcing band of intense snow, which will
last a couple hours overnight, the primary convergence axis will
be defected farther west towards the northeastern IL shore. This
will then act as the focus for a trailing north-northeast to
south-southwest oriented single intense band of lake effect snow
overnight. The most intense part of this band will become more
localized, likely resulting in significant variability in weather
conditions over just a few miles. Accordingly, some areas,
particularly along the northwestern IN shore, may experience a
several hour break in the most intense snow overnight. Ultimately,
however, this band of snow will gradually become more progressive
into Monday morning as northwesterly flow on the lake pushes the
main focus eastward along the southwestern and southern periphery
of the lake through Monday morning.

The potential for a more progressive movement into Monday morning
may help limit total snow accumulations from reaching the higher
end amounts forecast in some areas. However, with the extremely
heavy rate at which the snow will be falling, it will not take to
long to pile up amounts. Accordingly, some localized double digit
total amounts remain plausible. However, the overall foot print
of these extreme amounts may remain lower then our current
deterministic forecast advertises.

KJB

Monday Night through Sunday:

Any remaining lake effect snow across far northeast Porter County
will be ending early Monday evening as a fast moving ridge of high
pressure crosses the cwa Monday night. A weak disturbance will move
southeast in fast northwest flow early Tuesday morning with guidance
continuing to show an earlier arrival. While precipitation amounts
will be light, precip type will likely be all snow with the earlier
and colder air aloft.
Surface temps also appear to remain below freezing through mid
morning. Several runs and now some of their ensembles are showing a
dusting of snow accumulation across parts of northern IL. No changes
to the low chance blended pops, but if current trends continue,
higher pops for a dusting of snow accumulation may be needed.

Southwest winds will increase Tuesday morning as the gradient
tightens with gusts into the 30-35 mph range possible. Wednesday
also looks breezy with westerly winds gusting into the 30 mph range.

Generally dry and warmer conditions are expected for the end of the
week though a few showers may be possible as a warm front lifts
across the area Friday night into Saturday. While high temps may
already be in the upper 50s/lower 60s Friday, this warm front could
help push highs well into the 60s for much of the area for Saturday.
A deep trough moving across the western U.S. the develops into an
upper low next weekend and then slowly moves east across the region,
bringing the next chance of rain to the area. cms

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 544 PM CST Sun Nov 9 2025

Main Concerns:

- Timing of moderate to heavy wind blown lake effect snow and
  associated ceiling visibility impacts into Monday morning.

- Gusty northwest winds will likely prolong BLSN issues at the
  Chicago metro terminals until winds subside late day Monday.

Snow showers with some reduced VSBY at times should develop
around 03z. A burst of heavy at times lake effect snow probably
accompanied by some thunder will then pivot in off the lake
late this evening into northeast IL and then into northwest IN
early overnight. Forecast soundings suggest north-northwest
winds could also gust as high as 30-35 kt during the time of the
worst conditions, exacerbating runway issues due to blowing
snow.

There may be a brief lull in the snow intensity prior to another
(more narrow) intense lake effect band moving into northeast IL
in the pre-dawn hours and then shifting east into NW IN by the
mid to late morning. Opted to convert PROB30s for ORD and MDW
over to TEMPOs for LIFR conditions, though the narrowness of the
band does lower confidence a bit, especially at ORD. The
accumulating snow should end by 15-16z at ORD and MDW, with
continued BLSN knocking VSBY down some until sunset when wind
gusts subside.

Specific accumulations are always uncertain in lake effect
events, though GYY likely has the best chance of seeing amounts
upwards of 1 foot, while ORD and MDW end up in the 4-10" range
(probably higher at MDW). Despite it being an early season
event, the snow won`t be as wet as would normally be expected
due to the exceptionally cold air mass (ratios as high as 16:1).

Castro

&&

.MARINE...
Issued at 310 AM CST Sun Nov 9 2025

Periods of north to northwesterly gale force winds to 35 kt are
expected through Monday as winds funnel on the western side of
periodic bands of lake effect snow. A Gale Warning is in effect
until Monday evening. Waterspouts are possible where bands of
lake effect snow are prevalent.

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning from 9 PM this evening to 6 AM CST Monday
     for ILZ006.

     Winter Weather Advisory from 9 PM this evening to noon CST
     Monday for ILZ013-ILZ106-ILZ107.

     Winter Storm Warning from 9 PM this evening to noon CST Monday
     for ILZ103-ILZ104-ILZ105-ILZ108.

     Winter Storm Warning from midnight tonight to noon CST Monday
     for ILZ023.

     Winter Weather Advisory from midnight tonight to noon CST
     Monday for ILZ033.

IN...Winter Storm Warning from 9 PM this evening to 6 PM CST Monday
     for INZ001-INZ002.

     Winter Weather Advisory from midnight tonight to 6 PM CST
     Monday for INZ010-INZ011.

LM...Gale Warning until 6 AM CST Monday for the IL and IN nearshore
     waters.

&&

$$

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