998
FXUS63 KLOT 180546
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 AM CDT Sat Oct 18 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 /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1246 AM CDT Sat Oct 18 2025

The primary aviation concerns are as follows:

- Threat for thunderstorms this morning, with highest coverage
  favored in the 10-14Z window across the C90 airspace.

- Development of SCT to BKN MVFR cigs this morning and lasting
  through the remainder of the TAF period.

- Threat for isolated thunderstorms between 19-22Z across the
  C-90 airspace.

- Expectation for rapid development of stratiform rain with
  embedded thunderstorms this evening, with relative highest
  threat for thunder near RFD between 03-06Z.

- Backing winds toward south to southeast this evening,
  followed by a northwesterly wind shift overnight.
  Northwesterly wind gusts may exceed 30kt by daybreak Sunday.

Discussion:

Recent radar and satellite imagery depicts an axis of scattered
showers and thunderstorms extending from MSN to TOP. The showers
and storms are tied to isentropic ascent being forced by a
35-40kt LLJ ahead of a mid-level shear axis slicing through the
Upper Midwest. While the axis will shift eastward through the
night, current thinking is that coverage at the terminals will
be highest at around daybreak as subtle wave currently passing
over MCI swings over the central ZAU airspace. Will lean on the
HRRR to refine the timing windows of the inherited TEMPO groups
to favor 10-14Z at any given terminal given it appears to be
resolving the subtle wave well. The main threat with
thunderstorms this morning will be lightning and brief drops in
visibility to 2-3SM.

Showers and storms should taper by mid-morning as the low-level
jet decays. Winds should remain southwesterly through the
morning, though developing stratus may tend to limit mixing
heights and thus the potential for frequent gusts. For this
reason, will cautiously remove gusts through the daylight hours
in the outgoing TAF package. Attention then turns toward an
upper-level shortwave currently lifting from New Mexico into
Texas and due to interact with a broader upper-level trough
digging southeastward toward the Midwest. As the shortwave
approaches, additional showers and storms should develop mainly
across the southern ZAU airspace. However, with forecast
soundings depicting minimal capping by early afternoon, do have
some concern there could be at least isolated thunderstorm
development as far north as DPA/ORD/MDW/GYY. So, will maintain
the inherited PROB30 groups for thunder in the 19-22Z timeframe.

As the southern-stream upper-level shortwave and northern-stream
trough merge directly overhead this evening, rapid cyclogenesis
is expected to take place over the central or eastern ZAU
airspace. As a result of area-wide pressure falls, suspect
winds will back in direction toward the south or even
southeasterly for a time this evening. As this occurs, broad
upper- level diffluence forced by couplet jet streaks will
support the rapid development and expansion of stratiform rain
and MVFR cigs this evening atop the terminals. In fact, would
not be surprised to see a few embedded thunderstorms in the
03-06Z timeframe given lingering instability as well as the
threat for mesoscale gravity waves (very unbalanced upper-level
flow). At this point, the signal for yet another period of
thunder appears relatively highest near RFD near the crashing
cold front and where the remnant instability should be
maximized, so will go ahead and introduce a TEMPO group for
thunder the tailing hours of the TAF.

Finally, a northwesterly wind shift is expected toward the end
of the 24-30 hour TAF window as the cold front sharpens and
moves through the area. While wind speeds should be somewhat
muted (10-13kt) initially behind the front, sustained and gust
magnitudes will increase markedly by daybreak Sunday with
frequent quests of 30-35kt, if not even higher.

Borchardt

&&

.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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