181
FXUS63 KLOT 180004
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
704 PM CDT Fri Oct 17 2025

.KEY MESSAGES...

- Periods of showers and storms are expected late tonight
  through Sunday morning as a fall storm system moves through
  the area.

- Parts of the area may pick up much-needed soaking rainfall
  over the weekend. The highest rainfall totals (1-2+ inches)
  are currently favored to occur near and east of Interstate 55.

- A period of gusty northwest winds (gusts of 30 to 35 mph) are
  expected on Sunday. Additional periods of breezy winds are
  possible on Monday and Tuesday as another fall storm system
  moves through the region.

- Temperatures will be on a see-saw through the middle of next
  week.

&&

.DISCUSSION...
Issued at 314 PM CDT Fri Oct 17 2025

Through Sunday:

The mid-level ridge that has brought us the warm and dry
conditions the past few days continues to move eastward as
deepening trough moves into the Upper Midwest and Plains. That
said, expect the rest of our Friday afternoon and evening to see
increasing cirrus clouds with temperatures generally holding
steady in the mid to upper 70s before cooling into the low to
mid-60s overnight. The ongoing breezy south-southwest winds will
also persist through sunset before subsiding to around 10 mph
tonight. Though, some occasional 20 mph gusts may be seen
tonight as a low-level jet develops overhead especially in urban
areas where the atmosphere may remain more mixed.

Our attention turns towards the aforementioned trough entering
the Plains which is part of a developing storm system that will
be in control of our weather this weekend. The trough will
continue to advance towards the Midwest tonight and Saturday and
then begin to phase with the shortwave diving into the Mountain
West as it pivots into the Great Lakes late Saturday into
Sunday morning. At the same time, the cold front stretching from
eastern MN to OK will begin to move towards northern IL. A line
of showers and embedded thunderstorms have already developed
along this front in IA this afternoon and those showers are
expected to move into northwest IL towards midnight and then
swing through the rest of the area Saturday morning. Given that
these showers and storms will be outrunning the cold front and
entering an area of more limited instability, suspect that their
coverage may be somewhat scattered in nature as they move
across northern IL and northwest IN.

While the initial wave of showers/storms may be more hit and
miss, a second round of showers and thunderstorms is expected to
develop Saturday afternoon as the phasing trough pivots
overhead and a surface low begins to develop. Guidance continues
to vary on where exactly the low will take shape but the
consensus appears to be converging towards it forming over IL
and IN and then lifting into eastern Lower MI Saturday night.
Nevertheless, the strong forcing will generate a broad area of
showers across a good portion of the area Saturday afternoon
through Saturday night. Forecast soundings continue to show the
instability Saturday afternoon to be rather limited (around
200-400 J/kg) which should keep the severe threat low, but the
35-45 kts of shear and strong synoptic ascent may still allow a
isolated stronger storm to develop. If a stronger storm does
materialize the main hazard with them will be locally gusty
winds but a brief spin up cannot be completely ruled out
particularly south and east of I-57 closer to the better
instability.

In terms of rainfall, a swath of 1-2 inch rainfall amounts
continues to look likely along the northwest periphery of the
surface low. With the current forecast consensus it appears the
axis of highest rain amounts may stay more into northern IN and
Lower MI, but rainfall totals generally in the 0.5 to 1 inch
range are expected area wide. That said, the ongoing drought
conditions and long duration of the rain (36-48 hours) suspect
the threat of flooding is still very low. However, if some of
the stronger storms train over the same spots localized ponding
cannot be ruled out.

The surface low and the associated trough will begin to shift
east allowing drier air to filter back into the area. Therefore,
rain should taper from west to east Sunday morning leaving us
with a dry afternoon. Though, temperatures will be notably
cooler behind the system with highs forecast to be in the mid to
upper 50s Sunday afternoon. Winds will also be quite gusty on
Sunday as the lingering pressure gradient and cold advection
allow gusts to peak in the 30- 40 mph range Sunday afternoon.

Yack


Sunday Night through Friday:

An active upper-level pattern will continue next week with
several disturbances slated to quickly shift across the northern
CONUS. The next such wave is forecast to dive southeastward
across the Upper Midwest Monday through Tuesday. The ongoing
trend with this system has been towards both a deeper parent
upper-level wave and associated surface low developing somewhere
across northern Great Lakes region Monday night into Tuesday
morning.

Despite this deeper trend, moisture return remains a wildcard,
as this system will be following quickly on the heels of this
weekend`s disturbance, and it remains a bit unclear how much
boundary layer moisture/dewpoints will manage to recover prior
to the arrival of the trailing cold front. The GFS, which may be
over mixing the BL a bit, shows dewpoints falling into the
upper 30s/near 40 degrees ahead of the front, while most other
guidance suggests surface Tds remaining quite a bit higher. This
will have a considerable impact on the degree of precipitation
coverage with the front. At this point, suspect the GFS remains
a bit too dry, with the NBM- delivered chance PoPs looking
appropriate at this range.

The main item of note with this system looks to be a potential
for strong wind gusts, both in the warm sector and subsequent
cold advection regime. Warm sector southerly wind gusts could
push 40+ mph Monday afternoon, but will be dependent on mixing
depths which remain uncertain at this time. Then late Monday
night into Tuesday, as low-level lapse rates steepen behind
FROPA, an additional surge of west to northwesterly gusts will
also be possible.

Cold advection will send temperatures falling into the 30s and
40s Tuesday night/Wednesday morning. At this time, a widespread
freeze appears unlikely, but lows could push into the low/mid
30s with slackening winds late Tuesday night. Forecast
confidence lowers from the middle of the week onward. With that
said, ensemble model guidance is starting to exhibit a signal
for yet another system approaching the general area toward the
end of the workweek.

Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 700 PM CDT Fri Oct 17 2025

- Two windows for scattered SHRA with possible isolated TSRA:
  early Saturday AM and again Saturday afternoon.

- Widespread SHRA then lifts into the area Saturday night.

An axis of showers and isolated embedded thunderstorms currently
extends from NW Missouri through Iowa to SW Wisconsin. Model
guidance remains fairly consistent in this activity continuing
its gradual eastward progression toward the area through the
night. While shower coverage overall looks to be on the lower-
end (isolated to widely scattered) it felt prudent to include a
TS mention to account for any embedded lightning strikes.

A secondary more weakly forced batch of SHRA and isolated TSRA
may then develop early to mid afternoon, currently favoring
areas near and south of I-55 (MDW/GYY). Confidence in this
activity decreases with northwest extent (ORD). For now have
also accounted for this activity with PROB30s.

Winds overnight will remain south to southwest around 10 kt
though sporadic gusts in the upper teens to around 20 kt can`t
be ruled out as the low-level flow increases ahead of the
approaching weather system. Winds then turn breezy out of the
southwest with gusts in the mid-20 kt range late Saturday
morning into the afternoon. In the wake of the second round of
showers in the afternoon wind speeds may then ease some, which
could lead to a brief period of somewhat variable wind
directions into early evening.

Toward the end of the 30-hr TAF window, widespread showers are
expected to lift into the region Saturday night leading to
deteriorating VSBY/CIGs and a return of gusty winds.

Petr

&&

.MARINE...
Issued at 314 PM CDT Fri Oct 17 2025

- Gale Watch is in effect Sunday morning through the evening.

A fall storm system will move through the Great Lakes this
weekend bringing with it periods of showers and thunderstorms
and gusty winds. The strongest winds are expected to develop
late Saturday night and persist through Sunday evening with
northwest gales to 35 kts expected. These winds will also lead
to large waves especially in northwest IN and the eastern
portions of Lake Michigan.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch from Sunday morning through Sunday evening for the
     IL and IN nearshore waters.

&&

$$

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