366
FXUS63 KLOT 081132
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
532 AM CST Sat Nov 8 2025

.KEY MESSAGES...

- A storm system will bring a period of rain from this afternoon
  through early Sunday morning. A transition to wet snow is
  expected overnight, with a low (20-30%) chance for a slushy
  inch or two of accumulation away from Lake Michigan.

- Lake effect snow may migrate from northeastern Illinois to
  northwestern Indiana Sunday morning and into early afternoon,
  with a low to medium chance (30-50%) for a quick inch or two
  of snow in Cook, Lake (IN), and Porter counties.

- Intense lake effect snow bands may move into northeastern
  Illinois or northwestern Indiana Sunday night into Monday. A
  Winter Storm Watch is in effect for Cook, Lake (IN), and
  Porter counties.

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

&&

.DISCUSSION...
Issued at 245 AM CST Sat Nov 8 2025

Today through Sunday (Rain Changing to Snow Period):

Recent nighttime microphysics imagery depicts a busy scene
early this morning with a potent upper-level shortwave
pinwheeling into the Dakotas, a second short wave moving across
central Nebraska, and blossoming low-level stratus across the
Lower Great lakes across a developing baroclinic zone. Recent
radar imagery from both KMKX and KLOT depict westward-expanding
returns indicative of rain showers streaming off Lake Michigan
associated with gradually increasing low-level easterly flow.
This morning is shaping up to be somewhat gloomy across much of
our area.

Over the next 24 hours, the Dakotas and Nebraska shortwaves are
expected to merge while moving into the middle Mississippi
River Valley. As this occurs, a relatively compact precipitation
shield is expected to develop along and to the north of the
center of low pressure across central Iowa. Deterministic and
ensemble model guidance remain in excellent agreement that the
system will then shift due eastward this afternoon and tonight,
leading to rain spreading from west to east across the area
(connecting with the lake effect rain showers closer to Lake
Michigan and transitioning to lake enhancement of the broader
precipitation shield in northeastern Illinois). As temperatures
(and more importantly, wet-bulb temperatures) fall after
sunset, a transition in precipitation type from rain to snow is
expected from northwest to southeast, taking longest near the
immediate Lake Michigan shoreline due to relatively warm onshore
flow.

With the precipitation type starting as rain and the ground
remaining relatively warm, continue to suspect that snow will
struggle to accumulate this evening even as air temperatures
fall. With that said, a region of relatively steep mid-level
lapse rates intersecting a strong and deep frontogenetical
circulation within the compact deformation region does raise
concern there could be a narrow/focused corridor where snow
rates are high enough to allow for a few inches of snow to stick
to the ground and even less traveled-upon roadways (call it a
20-30% chance). It`s tricky to pinpoint where such an axis may
materialize given the two shortwaves will be merging almost
directly overhead. We`ll let the day shift watch trends in
upstream observations and model guidance to inform any updates
to our gridded database.

As the system pulls away from our area early Sunday morning, a
trailing band of lake effect snow is expected to migrate
southeastward along the shoreline of Lake Michigan. Based on the
expected track of the surface low and orientation of the low-
level pressure field, do have some concern that the band may
last long enough at any given location to drop a quick inch or
two of snow, starting near Evanston and Chicago toward daybreak
Sunday shifting southeastward into northwestern Indiana by mid-
morning. If the band were to stall anywhere for a few hours
(particularly in northwestern Indiana), certainly can`t rule out
the need for a Winter Weather Advisory for a quick 2-4" of
snow. Confidence is too low on the evolution of the trailing
lake effect snow band to issue a Winter Weather Advisory at this
time, so will let the day shift take a closer look at that,
too. By Sunday afternoon, the lake effect snow band is expected
to shift eastward toward Lower Michigan, giving our area a brief
break in precipitation.

Borchardt

Sunday night through Monday (Impactful Lake Effect Snow
Period):

Sunday night and into Monday morning, a potent shortwave
embedded in the broader polar trough anchored by record-cold
mid-level thermal profile (500mb temperatures of -40C) is
expected to dive south-southwestward into the western Great
Lakes. With increasingly extreme lake instability by Sunday
night (marine airmass to 700mb water temperature differentials
pushing 35C degrees with effective equilibrium levels of
22-25kft), intense bands of lake effect snow are poised to
develop over Lake Michigan and point toward the lakeshore.
Exactly where the polar shortwave tracks will be paramount to
guiding where intense bands of lake effect snow push inland.

While there remains spread in the track of the shortwave, am
growing quite concerned in the growing consistency among
EPS/CMCE/GEFS members showing a track of the 500mb vorticity
core down or just west of the spine of Lake Michigan. When
paired with the aforementioned extreme lake instability, such a
track of the shortwave would support the development of a strong
lake-effect mesolow with both intense snow rates and
strong/enhanced northerly winds. The EPS/ECMWF suite in
particular has been steadfast in showing such an evolution with
the mesolow landfalling in northeastern Illinois early Monday
morning, accompanied by a 2 to 4 hour burst of intense snowfall
rates of >2"/hr and low visibility due to northerly wind gusts
in excess of 35 mph (highest lakeside). Thereafter, a lake
effect snow band would be poised to shift toward northwestern
Indiana. However, if the polar shortwave ends up tracking more
southeasterly along the eastern side of Lake Michigan, a north-
to-south oriented band of intense lake effect snow may develop
instead of a mesolow, and stall over northwestern Indiana
through much of Monday. If the latter scenario were to
materialize, snow rates and amounts could easily get out of hand
and support localized double-digit snow totals somewhere in Lake
or Porter counties in northwestern Indiana. Of course, some
combination of the above is in the cards, too.

All things considered, feel there is enough of a signal for
impactful lake effect snow characterized by intense snow rates
>2"/hr and gusty north winds >35 mph to justify the issuance of
Winter Storm Watches for Porter and Lake counties in
northwestern Indiana and Cook county in northeastern Illinois.
As confidence grows in where the polar trough will go and
exactly how lake effect snow bands will evolve, the Winter Storm
Watch or eventually upgraded areas may evolve (adding or
removing counties). Keep in mind that lake effect snow is a
highly localized phenomenon and can lead to very unequal
impacts. Much like thunderstorms, lake effect snow can hammer a
very localized area while keeping the rest of a county
relatively unscathed. So, being included in the Winter Storm
Watch is by no means an guarantee warning-level impacts will be
felt.

Finally, not to be forgotten, Sunday night will be the coldest
so far this season. The combination of overnight lows in the 20s
and blustery north winds will drive wind chills down toward the
single digits to mid teens by daybreak Monday.

Borchardt


Monday Night through Saturday:

The upper level long wave trough will gradually move eastward
Monday night through Tuesday. However, with persistent northwest
flow, lingering lake effect snow showers may remain through
Tuesday though the focus will be more on northwest Indiana.
Temperatures will slowly warm on Tuesday to allow for the
potential for it to be a rain snow mix, but any snow
accumulations should be minimal.

There is an upper level short wave that is expected to descend
out of Manitoba overnight into Wednesday morning. There is
fairly good agreement in ensembles presently that it should head
more toward the northern Great Lakes to inhibit any formal PoP
mention in the grids. Other than monitoring that that wave
remains to the north, the second half of the week looks fairly
dry and quiet (compared to how the week will start!). As a
surface high takes up residence near Georgia, warm air advection
into Illinois will help drive temperatures back up into the 50s
and potentially even 60s by Friday (so much for any snow
accumulations lingering....). Models are suggesting the
potential for another wave to move over the area next Saturday.
Being still over a week away, felt no need to adjust the roughly
25 percent chance PoPs that the NBM provided.

DK

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 532 AM CST Sat Nov 8 2025

Key Aviation Messages for the current TAF period:

- Light rain/drizzle and MVFR conditions linger through the
  morning for terminals closer to the lake

- Widespread rain this afternoon/evening with lower vis possible
  during heavier rain bands

- Moderate confidence in the timing of rain transitioning to a
  rain/snow mix after midnight, and then all snow closer to
  daybreak

- Northeast winds today becoming north-northwest after midnight.
  Wind gusts intensify Sunday morning with gusts in excess of 30
  knots around and after daybreak

Winds out of the northeast this morning are focusing some lake
effect rain showers over terminals near the lake. Rain rates
are fairly light with occasional drizzle at times. Cannot rule
out the occasional IFR level FEW/SCT cigs through 15Z, but cigs
should remain MVFR through today with occasional reduced vis
during steadier drizzle.

There could be a brief break in the light rain/drizzle late this
morning into the early afternoon, but the next weather system
passes through the region this afternoon evening providing
the next chance for widespread rain. Locally IFR vis is possible
during the heaviest bands of rain, but kept MVFR vis in the TAF
presently.

After midnight, temperatures will drop and wet, slushy snow is
expected to mix in with the rain. Eventually, closer to
daybreak, precipitation should a.) taper off away from the lake
(KRFD) and b.) transition to all snow closer to the lake
(Chicago terminals). There is moderate confidence in the timing
of the precipitation transition, but can be tweaked in later
TAF packages.

Additionally after midnight, winds will slowly turn to the north
and eventually northwest. The pressure gradient will tighten and
wind gusts will increase. Gusts 20 to 25 knots with occasional
to 30 knots is possible before daybreak. Then wind gusts are
expected to further increase to 30 knots, with occasional gusts
35 to 40 knots possible, tomorrow morning as snow slowly tapers
off in Illinois and focuses over northwest Indiana.

DK

&&

.MARINE...
Issued at 245 AM CST Sat Nov 8 2025

An area of low pressure will gradually deepen to 29.4 inches as
it moves eastward from northern Missouri to central Indiana on
over the next 24 hours. Additionally, high pressure over the
northern Plains will increase creating a strong pressure
gradient oriented across Lake Michigan. As a result, northerly
winds will increase quickly this evening along the Illinois
nearshore of Lake Michigan and spread to the Indiana nearshore
into Sunday morning. A period of gale force winds to 35 kt may
occur during the overnight hours into Sunday morning, before
diminishing slightly in the afternoon with occasional gusts to
30 knots. Another period of gale force winds to 35 knots may
occur Sunday night into Monday morning as a secondary system
moves southward down Lake Michigan. For these reasons, a Gale
Watch has been issued from midnight tonight through 6 AM Monday
for both the Illinois and Indiana nearshore zones of Lake
Michigan.

Bands of lake effect rain and snow are expected to develop
tonight and last through Monday across central and southern Lake
Michigan. Intermittent water spouts may occur in the most
intense bands of lake effect rain and snow.

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Watch from late Sunday night through Monday
     morning for ILZ103-ILZ104-ILZ105.

IN...Winter Storm Watch from late Sunday night through Monday
     afternoon for INZ001-INZ002.

LM...Gale Watch from late tonight through late Sunday night for the
     IL nearshore waters.

     Gale Watch from Sunday morning through late Sunday night for
     the IN nearshore waters.

&&

$$

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