437
FXUS63 KLOT 081727
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1227 PM CDT Wed Oct 8 2025

.KEY MESSAGES...

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

- Generally dry with seasonable temps through the upcoming
  weekend.

&&

.DISCUSSION...
Issued at 259 AM CDT Wed Oct 8 2025

Weak secondary cold front is moving across the area early this
morning. Lake effect stratocumulus has developed over the lake
in the wake of this boundary and is heading for the NE IL
shoreline. Water vapor imagery shows the upper trough axis
moving east of Lake Michigan now, with forecast soundings
showing crashing inversion heights this morning over the lake.
Given the lack of any showers now and conditions progged to grow
less favorable with time, no plans to introduce any lake effect
showers and sprinkles. Still expect lake effect stratocumulus
to stream in off the lake into NE IL this morning, possible
going mostly cloudy at times near the lake. Should see lake
cloudiness decrease this afternoon as land-lake stability flips
diurnally.

Clear skies (except perhaps some lake effect cloudiness
developing near the NE IL shore) and light winds tonight should
set the stage for strong radiational cooling. The typically
colder outlying, wind protected areas should see lows dip into
the mid 30s, which should set the stage for some frost tonight.
Northwest Indiana, away from the lake, will probably see some
of the coldest temps and best chance of frost given their
proximity to the sfc ridge. Have expanded the mention of frost
in the grids and suspect that the day shift may need to consider
a frost advisory for portions of our area tonight, particularly
our NW IN counties.

Attention then turns to the northwest flow clipper-like system
progged to dive southeastward into the western Great Lakes late
Friday into Friday night. The run-to-run swings in the
operational GFS and ECMWF in their handling of this system have
been quite dramatic over the past few days. Possibly due to the
system being onshore and better sampled by the upper air network
over Alaska and northwest Canada, the past few runs of the GFS
and ECMWF have been more locked into a much more vigorous,
compact closed mid-upper level low diving into the Great Lakes.
There`s still variability in the track and to a lesser extent
the intensity of this system, which plays a role into whether it
will bring any shower threat to our CWA or whether showers
remain to our north. In addition, this more vigorous closed low
is more likely to result in another re-enforcing shot of cooler
air this weekend. The time-lagged nature of the NBM still has
temps pretty warm this weekend, particularly Sunday, but if
latest model trends persist, then forecast temps for Sunday will
need to be lowered, possibly by 10F+.

Pretty fascinating and complicated evolution to the pattern over
the central and eastern U.S. over the weekend into next week.
Our vigorous clipper is one of the key players, the other being
the progged development of an extratropical coastal cyclone off
the East Coast. Medium range models, with pretty good support
from their respect ensembles, show the vigorous Great Lakes
clipper phasing with the coastal cyclone and becoming a massive
cut-off cyclone with the development of a blocking pattern
across North America. Medium range guidance tends to struggle
mightily with blocking patterns and with phasing cyclones, so
forecast confidence heading into next week is lower than
average, particularly with the placement of the key features.
Current ensemble means suggests we could be under an
unseasonably warm upper ridge much of next week, but any
westward shift in these features could lessen our chances of
another big warm spell.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1227 PM CDT Wed Oct 8 2025

VFR conditions are forecast through the period. Northeast to
east winds around 10 kt will continue through the afternoon.
Winds ease after sunset becoming light and variable overnight
and settle into a prevailing southeast direction after daybreak
Thursday.

Scattered to broken VFR lake effect clouds are expected at
times through this evening near the lake, otherwise mostly
clear skies are expected through the rest of the period.

Petr

&&

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

&&

$$

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