732
FXUS63 KLOT 090200
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
900 PM CDT Wed Oct 8 2025

.KEY MESSAGES...

- Patchy frost expected tonight in typically colder, rural
  locations, especially across northwest Indiana.

- A growing signal for some light rain on Friday, otherwise
  generally dry with seasonable temps through the upcoming
  weekend.

&&

.UPDATE...
Issued at 900 PM CDT Wed Oct 8 2025

No significant changes to the forecast made through Thursday
morning. A Frost Advisory remains in effect for northwest IN
from 3 AM until 8 AM CDT.

High pressure of 1032 mb remains centered over the northern
Great Lakes region at mid-evening. This will maintain light
northeast winds across the area, especially near Lake Michigan,
overnight. Farther inland however, better radiational cooling
will result in stronger decoupling of the boundary layer and
allow winds to become light/variable/calm. A dry air mass,
light winds and mainly clear skies will set the stage for a
chilly early fall night in portions of the area. Light, but
persistent winds off of Lake Michigan look to keep things
warmer across much of the Chicago metro and to some extent
across northern IL, where slightly higher dew points continue
to spread westward. In addition, patchy lake-induced stratocu is
expected to increase a bit later tonight north of Chicago
proper. This, and slightly stronger winds above the nocturnal
boundary layer inversion will also help to modulate stronger
radiational cooling there.

While interior parts of northern IL away from the core of the
metro will likely see some mid-upper 30s for overnight lows and
some patchy frost in spots, northwest Indiana away from the
immediate Lake Michigan shore appears to have the greatest
potential for more widespread mid-30s overnight lows and
greater frost coverage. Thus have maintained the current Frost
Advisory headline for that area.

Other than increasing partly cloudy sky cover over far northeast
IL near the lake later tonight and some hourly temp and dew
point tweaks per current observation trends, no significant
changes appear warranted at this time.

Ratzer

&&

.DISCUSSION...
Issued at 250 PM CDT Wed Oct 8 2025

Early this afternoon, a scattered cumulus deck is building off
the lake and extending SW across much of the CWA, while
elsewhere remains mostly sunny. Temperatures are nearing their
highs for the day in the lower and middle 60s. Localized cloud
cover may continue to develop over the lake through the night,
but coverage should be largely limited to communities near the
IL lakeshore. A big majority of our CWA will see mostly clear
skies through the night. Sitting comfortably within a broad
surface trough, winds tonight will also be light to near calm
with dry boundary layer air. This is a prime setup to favor the
cooler side of guidance overnight, which would result in lows in
the middle to upper 30s around the suburbs and outlying areas
while 40s are expected in the city and around the interior metro
area. The coolest conditions should largely be found around our
NW Indiana counties where temps may fall to just above
freezing. Similar conditions are expected on more of a localized
basis across rural areas of northern IL. Accordingly, made the
decision to hoist a Frost Advisory for our five Indiana counties
from 3AM/4AM to 8AM/9AM CDT/EDT where greater coverage of frost
is attainable away from the lake. Frost coverage will be more
patchy elsewhere and primarily confined to outside of the
Chicago metro.

Tomorrow will be very similar to today with highs in the middle
and upper 60s beneath mostly sunny skies. Attention in the
forecast then turns to a tightly wound upper low that will clip
the region on Friday. The low is expected to close up north of
the Great Lakes late Thursday and track southeast into the the
upper Ohio Valley by Saturday. The storm`s cold front will get
dragged across the CWA Friday into Friday night. Ensemble PoPs
around the area have been on a consistent upward trend run by
run, but probabilities for measurable precip remain considerably
low, largely in the 10 to 30% range. With the cooler mid level
air staying out of reach to our north and a dry PBL on forecast
soundings, instability will be low to absent. Moisture looks
rather shallow as well, but ample enough to stir up some light
showers amid some noteworthy omega ahead of the advancing low.
The NBM populated the forecast with only slight chances near our
north, which was maintained for now. Those may have to be
bumped up or expanded though in the coming days, but brief and
likely lighter showers are probably all we`re looking at for
Friday.

Friday could also feature a good breeze, especially in our
north and northwest, as we mix into a decent LLJ. There is
support for 20 to even near 30 mph gusts, but there is still
some uncertainty. For now, boosted the forecast from the NBM to
get some near 20 mph gusts up that way, but these may need to be
bumped even further as confidence grows. If the rain holds out
and winds perform to their fullest, a localized elevated fire
risk may even be in the cards in our west and northwest on
Friday.

For the past couple of days now, medium range deterministic and
ensemble guidance has been trending cooler behind the front on
Friday which may have implications for temperatures this weekend.
Forecast highs for Saturday and Sunday have been on a steady
downward trend. While there still is some uncertainty and
support for warmer conditions, highs in the 60s around the metro
are looking more likely with time, especially on Saturday.
Lower 70s may be more attainable across outlying areas. Sunday
should be a tad warmer with some southerly return flow kicking
in late Saturday. For Sunday the forecast is now calling for 50s
downtown through mid- morning and peaking in the middle 60s.

More uncertainty lingers into next week, but slightly warmer-
than-average conditions are favored for at least the first half
of the week. Another, much larger synoptic system looks to clip
the region early next week introducing slightly higher precip
chances to the forecast, but there remains no solid signal for a
healthy widespread rain over the next seven days.

Doom

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 618 PM CDT Wed Oct 8 2025

No aviation forecast concerns for the 00Z TAFs.

Surface high pressure was centered over the Great Lakes region
early this evening, producing modest northeast winds and mainly
clear skies across the TAF sites. Winds will become light and
variable at most locations after sunset this evening, but may
stay up a bit from the NE for ORD/MDW through late evening.
Synoptic southeast winds are then expected across the area
Thursday, as the surface high drifts slowly off to the east.
ORD/MDW may see a bit of a lake-enhanced shift to ESE in the
afternoon, but not enough change in speed/direction to detail in
TAFs.

The only cloud cover in the area is some patchy lake-induced
VFR stratocu off of Lake Michigan. Some FEW/SCT bases around
3500 ft will persist into this evening/overnight for the Chicago
terminals, but should decrease in coverage late as flow above
the boundary layer eventually veers more E/SE into Thursday.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Frost Advisory from 3 AM CDT /4 AM EDT/ to 8 AM CDT /9 AM EDT/
     Thursday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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