631
FXUS63 KLOT 031930
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
230 PM CDT Wed Sep 3 2025

.KEY MESSAGES...

- Period of rain showers with a strong cold front this
  afternoon and early evening. A few embedded non-severe
  thunderstorms are also possible.

- A brief period of strong, gusty winds is possible late
  Thursday night into Friday morning.

- Below to well below normal temperatures are expected from
  Thursday through this weekend.

&&

.DISCUSSION...
Issued at 230 PM CDT Wed Sep 3 2025

Through Thursday:

A strong cold front will move across the area this afternoon
into this evening. Showers along with a few thunderstorms will
continue through this evening, ending from northwest to
southeast, with precipitation ending in the southeast cwa toward
late evening. Not expecting any severe weather with any
thunderstorms that develop, but brief heavy rain and gusty winds
will be possible with the strongest storms. There may be a few
lake effect rain showers for Porter County around daybreak
Thursday morning but for now have maintained a dry forecast.

The cloud cover and light rain through the morning kept temps
cooler across the northern half of the area with temps in the
60s to around 70 along/north of I-80. Temps will continue to
drop from here as the front moves through with low temps by
Thursday morning in the 40s for most areas, 50s for Chicago and
downwind of Lake Michigan into northwest IN. High temps will
rebound into the mid to perhaps upper 60s Thursday afternoon.

Westerly winds will shift northwest with the cold front with
gusts into the mid 20 mph range through mid evening. Wind speeds
will diminish overnight as a ridge of high pressure drifts
south of the local area Thursday morning. Westerly winds will
increase a bit Thursday with gusts into the 20 mph range
possible Thursday afternoon. cms

Thursday Night through Wednesday:

The main forecast concern is windy conditions late Thursday
night into Friday morning, followed by a lengthy stretch of
tranquil weather. A late fall to even winter-like mid-upper
level pattern across the Great Lakes and southern Canada will be
the driver of the cool airmass that will be with us through the
weekend. This pattern will relax by midweek next week, though
prolonged primarily dry conditions look to persist until later
next week or beyond.

An exceptionally deep and cold closed low is centered just
north of Lake Superior this afternoon (analyzed 500 mb heights
below 550 DaM are basically as low as it gets on KINL sounding
climo this time of year). Another strong short-wave diving
southeast out of northwestern Canada will constructively
interfere with the parent upper low and quickly dumbbell around
it Thursday-Thursday night. The result will be surface
cyclogenesis over the northern Plains on Thursday, quickly
tracking towards the northern Lakes early Friday at 990-995 mb.

This projected surface low pressure path is a bit farther north
than earlier model cycles, limiting the southward extent of
showers into our area Thursday night. Can`t rule out a few
showers or sprinkles across portions of far northern Illinois.
More notably, there`s still expected to be a rather impressive
low-level wind response associated with the fast forward
progression of the surface low and its trailing cold frontal
trough. This will particularly be the case over Lake Michigan
and points north and northeast of our area.

Following an evening lull in southerly winds when temps should
cool down into the 50s, pressure falls will overspread the area,
and we`ll start to at least occasionally tap into a stout
southwesterly low-level jet as temps rise overnight. Given the
overnight timing for the southwesterly winds and the surface low
path a bit farther north, the magnitude of southwesterly winds
overnight is a bit uncertain. Our official forecast will feature
gusts to 30-35 mph (mention of gusts up to 40 mph in messaging),
highest north of I-80. The cold front will sweep across the area
early Friday morning, likely followed by a couple hour "pop" of
west-southwest winds gusting to at least 30-35 mph through the
mid to late morning as pressure rises overspread the area in a
cold air advection regime.

The rest of the day Friday will be breezy and cool with highs
only in the mid to upper 60s I-80 and north and upper 60s to
around 70F south of I-80. If post frontal strato-Cu is a bit
more extensive than our partly cloudy forecast, that could mute
the warming some. High pressure nosing east from the central
Plains will bring a calm and chilly (40s outside of Chicago)
Friday night, followed by a partly cloudy, somewhat breezy, and
cool Saturday afternoon. Expansive high pressure will settle
southeastward Sunday into Monday, supporting continued chilly
overnights, while daytime temps trend upward to the lower-mid
70s Monday afternoon (upper 60s lakeside due to onshore winds).
Return flow on the backside of the slowly departing surface high
and rising 500 mb heights will bring a return to seasonable to
above normal warmth by midweek (low 80s inland of lake cooling),
though humidity levels will remain comfortably low.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1216 PM CDT Wed Sep 3 2025

Showers will continue to fester across the airspace at the start
of the TAF period within a weak WAA-regime. A secondary area of
showers and a few storms should develop along an approaching
cold front later this afternoon. While instability is fairly
scant and largely concentrated below -25C, will maintain the
inherited structure of the TAFs with prevailing SHRA and PROB30
groups for TSRA. A band of MVFR cigs may accompany the front
this afternoon, as well.

Winds will turn northwesterly behind the front this evening as
showers and MVFR cigs taper from west to east. Winds will ease
and may even become calm overnight as a surface pressure ridge
slides overhead. Periods of broken mid to upper-level clouds
will prevail through the overnight and Thursday morning period.

Borchardt

&&

.MARINE...
Issued at 230 PM CDT Wed Sep 3 2025

A Gale Watch is being issued for a period of strong southwest to
west-southwest winds late Thursday night into Friday morning.
The watch is valid from 1 AM to 10 AM CDT on Friday for all the
nearshore waters of southern Lake Michigan. This will be in
response to fast moving, deepening surface low pressure tracking
into the northern Lakes by Friday morning. A rapid ramp up in
winds is expected overnight, aided by unstable conditions from
the cool airmass over the lake. A strong cold front trailing
the surface low will sweep across the region early Friday
morning, resulting in a shift to west- southwest gales through
the mid to late morning hours. The nearshore forecast during
this period will advertise gales to 35 kt, though forecast
soundings suggest some upside potential, to 40 to perhaps
briefly 45 kt.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement from 7 PM CDT this evening through
     Thursday afternoon for INZ001-INZ002.

LM...Gale Watch from late Thursday night through Friday morning for
     the IL and IN nearshore waters.

     Small Craft Advisory from 7 PM this evening to 3 AM CDT
     Thursday for Northerly Is. to Gary IN.

     Small Craft Advisory from 7 PM this evening to 3 PM CDT
     Thursday for Gary to Burns Harbor IN-Burns Harbor to
     Michigan City IN.

&&

$$

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