587
FXUS63 KLOT 151834
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
134 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.

- Another period of showers and storms is favored in the general
  region Friday through the weekend.

&&

.DISCUSSION...
Issued at 134 PM CDT Tue Sep 15 2026

Per regional surface analyses charts, a stout 996mb surface low
is currently moving through central Ontario. A cold front
extends south of the low through central Wisconsin through
eastern Iowa. Meanwhile, showers and thunderstorms continue to
fester within a low- level warm air advection regime ahead of
the cold front from the central Plains through northern Illinois
and central Lower Michigan. Broad low-level confluence across
the Lower Great Lakes ahead of the front is leading to pooling
of low-level moisture as confirmed by surface dew points rising
into the upper 60s to lower 70s early this afternoon. A stout
temperature gradient is also present across the area (thanks to
morning clouds with northern extent), with current temperatures
ranging from the lower 70s near the Wisconsin state line to
lower 90s near the US-24 corridor. It definitely feels more warm
and humid out there today compared to yesterday.

Current expectations are that the continued warming of the
moistening boundary layer will support the redevelopment of
showers and thunderstorms by mid afternoon. Initial activity may
originate out of a remnant mid-level boundary currently
stretching just northwest of the I-55 corridor before expanding
near and south of I-80 by sunset as the aforementioned cold
front arrives and stalls across the area. Some 35 to 40 kt of
upper-level flow atop the instability axis (on the southern side
of an upper-level jet focused in the Upper Great Lakes) will
provide some means for localized clustering or even brief
supercellular structures this afternoon and evening supporting a
low-end (level 1 out of 5) threat for localized damaging winds
or even a funnel cloud.

After dark, a somewhat broad and unfocused but nevertheless
persistent low-level jet will develop and override the stalled
frontal boundary across the middle Mississippi River Valley. At
the same time, a broad region of mid-level warm air advection
across the central US ahead of a diving Pacific trough will
facilitate the northward expansion and strengthening of a mid-
level ridge into the Great Lakes, tightening the upper-level
geopotential gradient and associated jet (including upper-level
diffluence in the region). The net result should be an increase
in coverage of showers and storms along the stalled frontal
boundary across northern Missouri, southern Iowa, central
Illinois, and perhaps central Indiana overnight. It`s difficult
to say for sure where the primary axis of storms will be as
often observed activity often verifies some 25 to 50 miles
further south than depicted in CAM-guidance. With that said,
persistent low- to upper-level forcing within the 2.0"+ PWAT
regime will support torrential downpours and a threat for flash
flooding tonight (especially near the IA/MO border where low-
level convergence will be maximized). Will broad-brush mid-range
likely (55-75%) PoPs generally along and south of I-80 for the
overnight period.

Coverage of showers and storms should begin to wane after
daybreak as the low-level jet weakens and mid-level heights rise
(indicative of subsidence) along the northern flank of the
strengthening mid- level ridge. With that said, do see a signal
in CAM guidance for some sort of MCV or convectively augmented
wave to propagate eastward along the stalled frontal zone during
the late morning and early afternoon hours south of I-80.
Should that occur, showers and storms may continue unabated
through the afternoon hours (again, mainly near and south of
I-80). Confidence in the temperature forecast is hence pretty
low tomorrow. Will simply stick with the inherited forecast
calling for low 70s (north) to low 80s (south).

Conceptually, do think there may be another round of storms in
the general region tomorrow night through Thursday morning near
the terminus of yet another low-level jet and beneath the right
entrance region of the semi-perminent upper-level jet centered
over the Upper Great Lakes. Based on the forecasted location of
the terminus of the low-level jet across central Iowa, the main
focus for storms may end up extending from eastern Iowa
overnight and across northern Illinois by daybreak Thursday.
Will broad-brush mid-range chance (45-55%) chance PoPs across
the area in favor of refinement in later forecast packages. Like
tonight, there will be a threat for flash flooding tomorrow
night given the continued high PWAT environment.

By Thursday afternoon, ensemble guidance supports the idea that
the effective front will be well south of our area (potentially
aided by north to northeasterly flow on the southern side of a
a surface high pressure system moving across the Upper Great
Lake). Chances for showers and storms will return by Friday as a
lead shortwave propagates into the area ahead of the
aforementioned Pacific tough. At this point, can only offer
broad-brushed chance to low-end likely (40-60%) PoPs through the
weekend for what may end up as several rounds of storms in the
general region. Will note a heightened threat for flooding given
another period of PWATs approaching 2 inches overlapping strong
synoptic scale forcing.

Borchardt

&&

.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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