762
FXUS63 KLOT 071734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1134 AM CST Fri Nov 7 2025

.KEY MESSAGES...

- A passing storm system will deliver rain to the area Saturday
  afternoon in early Sunday morning. Snow may mix with rain
  along the backside of the system, though little to no
  accumulations are expected.

- An early taste of winter is expected Sunday and Monday with
  highs in the 30s and lows in the 20s.

- Periods of lake effect snow with localized impactful
  accumulations are expected in the vicinity of southern Lake
  Michigan Sunday morning through Monday.

&&

.DISCUSSION...
Issued at 238 AM CST Fri Nov 7 2025

Through Tonight:

A southwest to northeast oriented band of showers is slowly
moving through northern Illinois toward northern Indiana at the
time this discussion was published. A narrow axis of 100-500
J/kg of MUCAPE has accompanied this band as it has moved into
Illinois, with the higher amount of instability to the
southwest. Isolated rumbles of thunder have developed at times.
The more organized line of thunder is over La Salle county and
stretching to the southwest. That portion is expected to remain
along and south of I-80. However, this line of showers and
storms are traveling around 50 mph, so by the time the sunrises,
most of the showers should already be exiting the forecast area
to the east.

Other than some straggling light showers/sprinkles just after
sunrise, drier conditions are expected to for much of the day.
Cloud cover will diminish into the afternoon and temperatures
will slowly rise into the upper 50s and low 60s. Winds will
remain breezy through much of the day with gusts around 20 to 25
mph.

Cloud cover is expected to return in the late afternoon as
another mid level wave quickly passes overhead. Models have
slowed the progression of the wave, so the forecast was adjusted
for isolated to widely scattered showers to arrive mostly after
6 PM, though satellite and radar trends will be monitored if
that has to be adjusted back earlier. Most of the showers should
be focused in far northeastern Illinois before moving out over
the lake and toward northwest Indiana into Saturday morning.
Temperatures will tumble overnight. Areas closer to the lake
will remain in the low 40s, but upper 30s can be expected south
of I-88, and mid 30s north and west of the Fox Valley.

DK


Saturday through Thursday:

The main story from this weekend into early next week is an
early taste of winter as a lobe of the polar vortex gets
dislodged southward into the Great Lakes.

Saturday into Sunday, a pair of upper-level shortwave troughs
embedded on broad northwesterly flow are expected to race
southeastward from the northern Plains and into the Lower Great
Lakes. Ensemble model guidance remains in fair agreement that
the pair will at least partially phase while passing overhead,
allowing for the development and passage of a surface low
pressure system from roughly northern Missouri through central
Indiana.

As the systems approach, increasing easterly flow off Lake
Michigan and marginal lake instability should allow for
scattered rain showers to develop across far northeastern
Illinois Saturday morning. More widespread rain is expected by
mid-afternoon as frontogenetical forcing increases within the
developing deformation band on the northern side of the passing
storm system. With temperatures remaining in the mid 40s to
lower 50s throughout daylight hours on Saturday, precipitation
type will remain rain. As the system matures Saturday evening
and into Sunday morning while moving into Indiana, cooler and
drier air will advect into far northern Illinois causing wet-
bulb temperatures to fall below freezing. As a result, snow may
begin to mix with rain from north to south along the backside of
the deformation band of precipitation. With relatively warm
(and wet) ground temperatures and lowering precipitation rate
intensities, any snow that does fall Saturday evening should be
more of a novelty than impact with little to no accumulation
expected. (Note the first snowflakes in our area typically fall
around October 31, so we are due).

The main lobe of the polar vortex will swing into the Great
Lakes early Sunday and into Monday. With predominantly northerly
surface flow and increasing over-water instability, would
expect periods lake effect snow down the spine of Lake Michigan
starting as early as Sunday morning along the backside of the
departing system and lasting through Monday. In fact, with lake
to 850 mb temperature differentials pushing 20 to 25 degrees
and effective equilibrium levels rising toward 20 kft, deep
layers of convective latent heating may lead to compact mesolow
trains and the associated "braided challah" look to lake effect
bands at times. In addition, any embedded upper-level shortwave
passing over the lake may support the development of larger,
more organized mesolows (particularly in the Sunday night to
Monday timeframe). With this in mind, confidence in exactly how
and where lake effect snow bands evolve at this point is on the
lower side. Even so, will cautiously introduce snow
accumulations in Lake and Porter counties in northwestern
Indiana keeping in mind the envelope of evolutions include
accumulating snow making it as far west as Chicago. In all, the
Sunday to Monday timeframe remains one to watch for localized
impactful lake effect snow in the vicinity of southern Lake
Michigan.

For those away from Lake Michigan, the main weather story will
be a period of well below average and winter-like temperatures
with highs Sunday and Monday struggling to climb out of the 30s.
Sunday night will be the coldest night so far of the season
with overnight lows falling to the low to mid 20s (upper 20s
lakeside). When combined with blustery northwest winds, wind
chills overnight and into Monday morning will fall toward the
single digits in outlying areas!

Tuesday onward, ensemble model guidance advertises the
northeastward retreat of the lobe of the polar vortex. Assuming
any trailing upper-level shortwaves in reestablished
northwesterly flow don`t delay the similarly northeastward-
retreat of the upper-level cyclonic shear axis, do agree with
the general idea of temperatures quickly rebounding toward
average values Tuesday into Wednesday. In fact, Tuesday and
Wednesday may need to be watched for periods of strong winds
owing to the tight pressure gradient between conglomerate low
pressure in east-central Canada and a seasonably strong surface
high along the Gulf Coast.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1134 AM CST Fri Nov 7 2025

Forecast concerns include...

Gusty westerly winds through sunset this evening.
Wind shift to north this evening then northeast tonight.
Chance of showers Saturday morning.
Rain and mvfr cigs Saturday afternoon.

Westerly winds will gust into the mid 20kt range through sunset
and then quickly diminish. Directions may briefly turn
west/southwest for a few hours and then will shift northerly as
a frontal boundary moves across the area. Directions will turn
northeasterly tonight and then easterly Saturday.

A few showers are possible during the predawn hours through mid
morning Saturday, mainly near Lake Michigan and at the Chicago
area terminals. Current prob mention remains reasonable for this
potential and time period. Some mvfr level cloud cover is
possible with these showers, but will be dependent on how
widespread the shower activity becomes.

Rain will spread across the entire area Saturday afternoon,
possibly in the 17z-18z time period for RFD and by 21z for
ORD/MDW. Prevailing mvfr cigs are expected as the rain begins
and possibly low mvfr or ifr by sunset Saturday evening. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 PM CST this afternoon for the IL
     and IN nearshore waters.

&&

$$

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