789
FXUS63 KLOT 100537
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1137 PM CST Sun Nov 9 2025

.KEY MESSAGES...

- A period of dangerous and intense wind blown 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 heaviest lake effect snow.

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

&&

.UPDATE...
Issued at 944 PM CST Sun Nov 9 2025

Have expanded the winter weather advisory to include Ford and
Benton Counties, otherwise no changes to the many, many
headlines in effect. Also, no big changes to the forecast grids
this evening.

Convective allowing models (CAMs) often struggle with the exact
details of lake effect snow, much like they do with specifics of
summertime convection. This was the case this evening with the
movement and timing of the lake induced/enhanced meso-low, which
moved inland into northern IN hours earlier than most CAM
guidance had been indicating. The faster and farther east track
of this mesolow has resulted in CAMs being wrong in their
depiction of a large initial westward surging lake effect snow
plume well inland into northeastern IL late this evening.

Rather, this mesolow has become nearly stationary over north
central Indiana with a coherent, intense single band of lake
effect snow extending northwest then north up the spine of the
lake from this mesolow. Surface observations depict a sharp
surface trough extending northwest from the mesolow across
southern Lake Michigan. This trough axis has been steadily
moving southward down the lake, with the strong low level
northeasterly flow in the wake of this trough helping push the
lake effect plume steadily westward.

As this trough continues to push south of the lake, anticipate
that westward progression of the lake plume should push it into
northeast IL between 11 pm and midnight. Confidence is low in
how far west this plume will get into NE IL before becoming
quasi-stationary for a time, then likely shifting back eastward
toward and across northwest IN Monday. Really not putting a ton
of stock in CAM guidance at this point, so oncoming midnight
shift will be nowcasting this band through the night based on
observational trends.

No change to the magnitude of instability, which is confirmed by
a special 0030Z Valparaiso University Sounding from this
evening. Still anticipate snowfall rates of at least 2-3 inches
per hour within the heaviest elements of this band. Depending on
how long the band becomes quasi-stationary, it is certainly
possible that we could see isolated totals pushing or even
exceeding a foot before the band begins shifting eastward.
Current trends would seem to suggest that Lake or Cook County IL
could be that westward terminus that sees some of the heavier
totals.

Where ever the band is during rush hour tomorrow morning, travel
conditions will be very difficult to nearly impossible.

- Izzi

&&

.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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1137 PM CST Sun Nov 9 2025

Key Aviation Messages for the current TAF period:

- Moderate to heavy wind blown lake effect snow creating drastic
  impacts down to VLIFR conditions. Main uncertainties are for
  any "improvements" between bands of snow

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

The initial push of snow that dropped vis down to 3/4SM at
Chicago terminals is pushing southwestward toward KPIA. However,
the next band of snow is right along the lake shoreline at the
time this discussion was published. Currently conditions at
Chicago terminals are VFR, but with the next band about to move
onshore, the TAFs were sent with worse conditions due to the
expectations that things will deteriorate quickly after 06Z.
This band will be have more vigorous snow rates and strong gusty
winds. Low vis is expected, but may exceed expectations and
become sub-1/4SM. Additionally, buoys have already reported
gusts to 31 knots, and while 35 knots is in the TAF, it is
entirely possible that gusts touch 40 knots at times. It is
possible the the TEMPO from 06Z to 09Z is a hair long, but have
moderate confidence that there could be 3 hours of really
intense snow rates.

Looking at regional radar, there is a long lake oriented band
off the Wisconsin shoreline moving southward. This band will be
"Round 2" as it moves into Illinois. There will the expectation
is that there will probably some time of "break" in the intense
snow before it moves southward (longer at KMDW and KGYY). Can`t
rule out some quick VFR conditions, but felt more prudent to
leave IFR/MVFR conditions for ongoing snowfall. Nevertheless, a
second round of intense lake effect snow is expected before
daybreak. The is slightly lower confidence in the timing of this
band. In fact, we may not know the exact timing until the
scheduled 09Z amendments when we can see "the white`s of its
eyes". But again, vis and cig reductions down to LIFR if not
brief VLIFR is possible once again. With this band more focused
closer to the lake shoreline, terminals farther inland like
KDPA should still see some light snow, but more VFR or
occasional MVFR conditions.

Most guidance has the band of lake effect snow moving eastward
toward northwest Indiana after 14Z (lower confidence in the
timing of its exit east). While the precipitation may slowly
taper off with a return to VFR conditions, northwest wind gusts
20 to 25 knots will prevail allowing providing the risk for
blowing snow from accumulated snowfall to continue into the late
morning/early afternoon. Its possible the timing of BLSN ending
is a little long, but wind gusts are not expected to truly
subside until at or just before 00Z. VFR and winds around 10
knots and subsiding are expected after 00Z.

Near KRFD...
A flurry or two is possible through daybreak, but otherwise
snow free conditions are expected. The main impacts will be
from gusty northwest winds up to 25 knots (cannot rule out
isolated to 30 kt through daybreak), likely subsiding late
afternoon Monday.

DK

&&

.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 until 6 AM CST Monday for ILZ006.

     Winter Weather Advisory until noon CST Monday for ILZ013-
     ILZ106-ILZ107.

     Winter Storm Warning until noon CST Monday for ILZ103-ILZ104-
     ILZ105-ILZ108.

     Winter Storm Warning until noon CST Monday for ILZ023.

     Winter Weather Advisory until noon CST Monday for ILZ033-
     ILZ039.

IN...Winter Weather Advisory until 1 PM EST Monday for INZ019.

     Winter Storm Warning until 6 PM CST Monday for INZ001-INZ002.

     Winter Weather Advisory until 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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