Area Forecast Discussion
Issued by NWS Chicago, IL

Home |  Current Version |  Previous Version |  Text Only |  Print | Product List |  Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
601
FXUS63 KLOT 181937
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 PM CDT Sat Oct 18 2025

.KEY MESSAGES...

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

- Much of the area will pick up much-needed soaking rainfall
  tonight into Sunday. The highest rainfall totals (1-2+
  inches) are expected to occur near and east of Interstate 55.

- A period of gusty northwest winds (gusts in excess of 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 237 PM CDT Sat Oct 18 2025

Through Sunday:

Showers and thunderstorms are starting to redevelop along a
pseudo dryline that is currently oriented along a Mendota to
Chicago line. Given this orientation and the continued southeast
movement of the boundary, still expect the greatest coverage of
showers and thunderstorms this afternoon to be along and
southeast of I-55 where a plume of better instability resides.
Recent aircraft soundings of of MDW do show cape values around
900-1000 J/kg which in combination with the 40-50 kts of shear
overhead may support some strong to near severe storms this
afternoon and evening. If any strong to severe storms
materialize the main threats will be gusty winds in excess of 45
mph and hail up to quarter size. However, there is a non-zero
chance that a few of these storms try to rotate especially if
they interact with some surface vorticity being noted along the
dryline. That said, the environment today is not really
favorable for tornadoes due to the somewhat limited instability
and weak low-level shear so if rotation is observed in storms
suspect it would mainly be a funnel cloud or landspout threat.

Heading into tonight, the cluster of showers and thunderstorms
ongoing across south-central MO will begin to pivot into north-
central IL and northwest IN as a surface low develops.
Additionally, the cold front in east-central IA will also be
moving through the area tonight. The interaction of these
features will allow showers to expand in coverage and fill in
across northern IL and northwest IN. Once showers fill in they
are expected to persist into Sunday morning as the front and
surface low move through the region. From a thunder perspective,
the instability should get pushed south of our area this
evening and thus limit the thunderstorm coverage but an isolated
rumble or two of thunder cannot be ruled out through at least
midnight. The showers tonight will bring a nice soaking rainfall
to the area with most expected to pick up anywhere from 0.5 to
1.0 inches of rain. Though, a corridor of high rain amounts is
expected to develop along the low`s deformation axis which looks
to reside across far eastern IL to northwest IN and into Lower
MI. Totals in this area could peak in the 1 to 2 inch range with
locally higher amounts (possibly near 3.0 inches) possible.
Normally rain amounts of this magnitude would be a concern for
flooding but with the long duration (12+ hours) and the
antecedent dry conditions the threat for flooding continues to
look low.

Showers will gradually exit the area from west to east Sunday
morning leaving us with a dry conclusion to the weekend.
However, the strong pressure gradient and deep mixing associated
with the cold advection will generate some gusty winds through
Sunday afternoon where gusts are expected to peak around 30-40
mph. Aside from the gusty winds, temperatures on Sunday will
also be notably cooler with highs only reaching into the upper
50s to around 60.

Yack


Sunday Night through Saturday:

The lingering tight surface pressure gradient will gradually
relax Sunday night into early Monday morning. With the upper-
level cyclonic shear axis pushing quickly east of the region and
lower near-surface dewpoints sloshing in from the west, knocked
a few degrees off the low temperatures early Monday morning.
With the slackening flow, can`t rule out some patchy frost
development west of the Fox Valley in/near sheltered/typical
cool spots.

The upper-level pattern will remain energetic in the wake of
the weekend storm system with a secondary upper-level trough and
associated surface low poised to dive southeastward into the
Great Lakes region in the Monday to Tuesday timeframe. Even with
the usual variance in the evolution of the low depicted among
guidance, a signal remains for a period of breezy southwest
winds on Monday (WAA ahead of the system) transitioning to
breezy west to northwest winds on Tuesday (CAA behind the
system). In scenarios where the surface low is strong (sub
1000mb), it is easy to envision peak gusts in either regime
tagging 40 mph, as hinted by 40kt of flow at the top of the
mixed layer in deterministic GFS and ECMWF forecast soundings.
This`ll be something to keep an eye on as we get closer. Unlike
the storm system coming through this weekend, chances for
meaningful rainfall with the secondary system will be limited by
meager moisture return and relegated to a quickily-moving band
of showers along the southeastward-surging cold front Monday
evening and perhaps a few showers lingering behind as the mid-
level cold pool associated with the trough slide overhead.

After a relatively warm Monday with highs in the mid 60s,
temperatures behind the front on Tuesday will be noticeably
cooler and in the 50s. We`ll have to watch for any embedded
shortwaves to move over the area along the backside of the
upper-level trough to support a diurnal flare in scattered
showers, as can happen this time of year. As the system pulls
away Tuesday night, temperatures will be poised to fall into the
upper 30s to lower 40s away from the lake. At this point,
suspect there will be enough cloud cover or lingering winds to
limit temperatures from really tanking out, and indeed latest
guidance has trended a bit milder with overnight temperatures
into Wednesday morning. The colder night could end up being
Wednesday night/Thursday morning. Still have some questions
related to how quickly the surface flow will diminish overnight,
but will need to keep an eye on this period for and increased
frost/localized freeze potential.

Forecast confidence lowers from the middle of the week owing to
spread within ensemble guidance in the treatment of an
eastward- moving upper-level low emanating from California.

Carlaw/Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1239 PM CDT Sat Oct 18 2025

Key Aviation Messages for the current TAF period:

- Showers and storms possible this afternoon, mainly along and
  south of a KVYS to KGYY line

- Showers and storms expand northward this evening,
  transitioning to just rain after midnight

- Gusty northwest winds in excess of 30 knots tomorrow morning

MVFR cigs are being observed across much of northern Illinois,
though VFR clearing has finally arrived at KRFD. Cigs are
expected to gradually break up and lift through the afternoon.
Southwest winds around 10 knots are expected through the
afternoon. An occasional gust to 20 knots is possible, but it
should be infrequent enough to keep it out of the TAF. Winds
will slowly turn to the south (potentially a short window to the
southeast) late this afternoon as a surface low approaches.

While there is a break in the rain at the time of this
discussion, attention is turning toward central Illinois where
new thunderstorms are forming and moving east-northeast. The
threat for showers and storms this afternoon should mainly be
along and south of a KVYS to KGYY line. A stray shower north of
there is possible which allows a PROB30 to be maintained at
KMDW, but the higher confidence is to the south which is why the
KGYY TAF now has a TEMPO for -TSRA this afternoon.

As a surface low deepens over Illinois this evening, widespread
expansion of rain and MVFR conditions across the area is
expected. With instability still present, VCTS was kept in the
TAF, though chances diminish toward and after midnight. There is
a chance for IFR vis during the heaviest rain, but confidence
was too low to add it to the TAF currently. Rain will slowly
dissipate tomorrow morning from west to east, with dry VFR
conditions expected in the afternoon.

Winds will turn to the northwest after midnight as the surface
low starts to move east. But as the pressure gradient tightens,
wind gusts will increase. Wind gusts in excess of 30 knots can
be expected toward daybreak, but there is a chance for gusts as
high as 40 knots for brief periods of times. Gusty conditions
will continue through much of the day tomorrow, though the
magnitude will gradually diminish through the afternoon.

DK

&&

.MARINE...
Issued at 237 PM CDT Sat Oct 18 2025

- A Gale Warning remains in effect from Sunday morning through
  Sunday evening.

A fall storm system will move through the Great Lakes tonight
into Sunday, bringing with it periods of showers and
thunderstorms and strong winds. The strongest winds are expected
to develop overnight tonight into Sunday morning and persist
through Sunday evening. Confidence remains high in 35 kt gales
being observed across all of southern Lake Michigan, and so the
Gale Warning is still in effect. Gales as high as about 40 kts
could be observed over Indiana nearshore waters and points
farther to the north and northeast. These winds will also lead
to very large waves, which will also be highest along the
northwest Indiana shore and across southeastern portions of Lake
Michigan.

Yack/Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning from 7 AM to 7 PM CDT Sunday for Winthrop Harbor
     IL to Gary IN.

     Gale Warning from 7 AM to 10 PM CDT Sunday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

Visit us at weather.gov/chicago