570
FXUS63 KLOT 150553
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1253 AM CDT Tue Sep 15 2026

.KEY MESSAGES...

- Waves of showers and storms are expected in the general region
  tonight through much of the week.

- While some areas may not see much rain at all, others may
  experience repeated downpours with a threat for localized
  flooding.

- The next few days will also be marked by high humidity.

- A gradual pattern change toward cooler, less humid, and drier
  conditions is favored by next week.

&&

.DISCUSSION...
Issued at 129 PM CDT Mon Sep 14 2026

A 1024mb surface high pressure system continues to move through
the central Great Lakes leading to a generally quiet day.
Increasing cloud cover is muting the diurnal temperature rise,
though highs should still top off in the mid 70s most places
amidst a southeasterly breeze. Meanwhile, an early-autumn storm
system is moving along the US/Canadian border driving a large
area of warm air- advection driven showers and thunderstorms
across the Upper Mississippi River Valley. With a residually dry
airmass in place across the Lower Great Lakes on the
southwestern side of the departing high pressure system,
expectations remain that our area will remain dry through
sunset. With that said, cannot rule out a random sprinkle or two
this afternoon mainly near/west of I-39 and north of I-88.

Tonight, the northern Plains low will drive a surface warm
front northward into the Lower Great Lakes in advance of a cold
front sliding southeastward across the Upper Midwest. Strong
theta-e advection driven by an intensifying southwesterly low-
level jet will usher in a warmer and more humid airmass into our
region, pushing atmospheric column moisture (PWATs) near 2
inches. Although large- scale forcing will be modest through the
overnight hours, continued isentropic upglide in advance of the
cold front will support a blossoming region of showers and
thunderstorms later tonight from IA east-northeastward across
parts of northwestern IL into southern WI. This activity is
expected to shift over areas along and mainly north of I-80
after midnight tonight and will likely linger in the area
through Tuesday morning. The primary threat with these showers
and storms will be torrential downpours. Fortunately, the
steering layer flow tonight will favor faster storm motions (~40
mph), so any potential flash flood threat would likely remain
localized and tied to any potential areas of training storms.

Widespread cloud cover and lingering morning showers will limit
diurnal warming on Tuesday especially with northward extent
leading to the reinforcement of an outflow-driven stationary
front across the Lower Great Lakes. Highs should range from the
upper 70s near the Wisconsin state line to perhaps the mid 80s
near and south of the stationary front south of I-80. By mid-
afternoon, decreasing convective inhibition may trigger renewed
scattered thunderstorms near the front primarily south of I-80.
Access to 30-35kt of mid- level flow may promote localized
clustering and associated threats for water-loaded downbursts
where convective coverage is highest consistent with the SPC
Level 1/5 threat level for severe weather tomorrow.

Tomorrow night through Thursday, the stationary front is
expected to stall or modestly retreat back northward as a
subtropical ridge strengthens across the southeastern United
States. Subtle impulses parading around the perimeter of the
ridge along the stationary front will support episodes of
showers and thunderstorms in the general region, though forecast
confidence on exact timing and locations is lower than average
given the overarching pattern. Will have to watch for focused
corridors of torrential downpours with an associated threat for
localized flooding in this timeframe given PWATs hovering near
or just above 2". Also, could envision heat index values poking
toward 100 degrees near and south of the stationary front at
times. Unfortunately, this is a pattern that`ll have to be
forecasted day by day as individual waves and the location of
the stationary front can be parsed out. Finally, note that the
next 2 to 4 days do look wet for many but not everyone in our
area. In the absence of persistent large-scale forcing as well
as the presence of modest mid-level height rises, downpours may
be relegated to scattered coverage rather than enjoyed by all on
a widespread basis.

Extended guidance indicates weak mid-level blocking as a
subtropical low over the Gulf slows south of a weakening upper-
level ridge. Meanwhile, a upper-level trough moving across the
northern Great Plains on Friday may dig far enough south to
bring showers and thunderstorms to the area on Saturday.

Birk/Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1253 AM CDT Tue Sep 15 2026

Main Concerns:

- Gusty southerly winds (or LLWS) at the start of the period.

- SHRA and embedded TS early this morning. Additional SHRA will
  be possible at times this afternoon through tonight, though
  better TS chances should remain south of the terminals.

- Potential for MVFR CIGs at times later this morning into the
  afternoon and again tonight.

SHRA and TS off to the northwest (closest to RFD) will gradually
overspread the terminals early this morning, and then coverage
should wane by the mid to late morning. Maintained VCTS and
TEMPO TS mention, though adjusted the timing a bit later based
on radar trends. Additional adjustments may be needed with time.
Winds will veer to south-southwest from current gusty south-
southeast with the arrival of the rain, and can`t rule out some
outflow effects on the winds. There`s a signal (strongest at
RFD) for MVFR CIGs over northern IL as the morning convection
winds down.

Late this afternoon into this evening, winds will shift to
northwest as a cold front pushes across the area. Expect further
veering to north-northeast tonight. The northern extent of
additional convection this evening and overnight is uncertain.
Higher chances/coverage should favor near and south of I-80, and
as such included VCSH mention at GYY and MDW.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 7 AM CDT this morning for ILZ006-
     ILZ103-ILZ104.

IN...None.
LM...Small Craft Advisory until 9 AM CDT this morning for the IL
     nearshore waters.

     Small Craft Advisory until 7 AM CDT this morning for the IN
     nearshore waters.

&&

$$

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