131
FXUS63 KLOT 151741
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1241 PM CDT Tue Sep 15 2026

.KEY MESSAGES...

- Waves of showers and storms are expected in the general region
  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 307 AM CDT Tue Sep 15 2026

The main focus continues to revolve around the timing and
favored placement of each wave of showers and thunderstorms
expected over the next few days.

A surface warm front will shift northeastward across the area
this morning, ahead of an approaching cold front from the Upper
Midwest. Meanwhile, a corridor of strong theta-e advection,
driven by a 45+ kt southwesterly low-level jet, is quickly
drawing deep atmospheric moisture (PWATs near 2 inches) from the
lower Missouri Valley into the western Great Lakes early this
morning. Associated strong isentropic upglide within this very
moist airmass is supporting a large region of showers and
thunderstorms from the Corn Belt into southern Wisconsin. This
activity will shift eastward, impacting much of northern
Illinois (especially near and north of I-80) through the
morning. Continued warm air advection may maintain widely
scattered showers through the afternoon across far northern
Illinois.

Widespread cloud cover and lingering showers will limit diurnal
warming today, especially over northern Illinois, reinforcing
an outflow-driven stationary front across the lower Great Lakes.
High temperatures should range from the upper 70s near the
Wisconsin border to around 90 well south of I-80. By late
afternoon, weakening convective inhibition may trigger renewed
scattered thunderstorms near the front, primarily south of I-80.
Access to ~40 kt of mid-level flow may promote localized storm
organization and water-loaded downbursts where convective
coverage is highest. For this reason, the Storm Prediction
Center maintains a Level 1 of 5 severe weather threat for areas
generally south of I-80 later today. Torrential rainfall will
also accompany these storms into this evening.

The surface cold front`s southward push is expected to slow or
stall across the southern half of Illinois tonight as its west-
to-east orientation aligns parallel to the mid- and upper-level
flow along the northern periphery of a broad subtropical ridge
centered over the southeastern U.S. With little pattern change
expected through Thursday, the front will remain near the
region, serving as the focus for multiple rounds of heavy-rain-
producing storms as several convective impulses move around the
perimeter of the ridge.

Uncertainty remains regarding the exact coverage of heavy rain
tonight into early Wednesday morning. However, storm coverage is
likely to increase south of I-80 this evening into early
Wednesday as additional convective waves track east-
northeastward along and north of the surface boundary. Combined
with a modest low-level jet (20-25 kt), this setup may support a
corridor of heavy rainfall near and south of I-80, particularly
where storms train over the same areas. Late Wednesday into
early Thursday, the stationary front is expected to stall or
retreat slightly northward as the subtropical ridge strengthens
over the Southeast. Additional subtle impulses moving around the
ridge along the boundary will likely trigger another round of
showers and thunderstorms. This period warrants close
monitoring, as a stronger southwesterly low-level jet response
could develop Wednesday night, increasing the potential for a
more focused corridor of training storms and an associated
flash flooding threat in portions of the area.

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.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1241 PM CDT Tue Sep 15 2026

Key messages for the 18Z TAF period:

* Isolated showers through the afternoon

* Thunderstorms possible this evening at GYY

* Additional rain and MVFR overnight into early Wednesday

Most of this morning`s rain has cleared the area, but isolated
pockets will fester through the afternoon, especially near MDW,
GYY, and RFD. A line of thunderstorms is anticipated to develop
along a passing cold front this evening. Coverage should remain
well south of most of the terminals, but there`s a chance it
could target GYY.

Another wave of rain will move across overnight into the
predawn hours, mainly south of RFD. An embedded thunderstorm or
two will be possible. Isolated showers may linger beyond dawn. A
period of MVFR appears likely with the rain, especially at MDW
and GYY, which may persist through the morning.

Meanwhile, SW to W winds this afternoon near or below 10 kt
will quickly veer to NE this evening. Expect NE winds near or
below 10 kt during the day on Wednesday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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