005
FXUS63 KLOT 151150
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
650 AM 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 /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 650 AM CDT Tue Sep 15 2026

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

- Potential for MVFR flight categories at times this morning
  into the afternoon and again tonight into Wednesday morning.

SHRA and scattered TS gradually filling in this morning warrants
continued VCTS and TEMPO TS mention (until 15z at the Chicago
terminals). It does however appear that higher TS chances have
shifted south of RFD as of this writing. There`s a continued
signal for MVFR CIGs over portions of northern IL as the morning
convection winds down, particularly at RFD. Widely scattered
SHRA will be possible this afternoon ahead of an approaching
cold front.

Late this afternoon into this evening, winds will shift from
southwest to northwest as the cold front pushes across the
area. Expect further veering to north-northeast tonight. The
northern extent of additional convection this evening and
into Wednesday morning. Higher chances/coverage should favor
near and south of I-80, and as such included VCSH mention at GYY
and MDW, along with a PROB30 for TS at GYY this evening. MVFR
CIGs appear probable at MDW and GYY late this evening into
Wednesday morning, with lower confidence elsewhere. Aside from
precip. caused VSBY reductions, 3-6SM BR may develop,
particularly at GYY.

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
     and IN nearshore waters.

&&

$$

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