204
FXUS63 KLOT 202331
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 PM CDT Sun Sep 20 2026

.KEY MESSAGES...

- Showers and drizzle to gradually diminish in coverage late
  this evening. Pockets of light showers/drizzle may continue at
  times tonight through midday Monday.

- Below normal temperatures through the end of the week.

&&

.DISCUSSION...
Issued at 1230 PM CDT Sun Sep 20 2026

A mid-level shortwave/convectively augmented MCV continues to
shift east directly overhead early this afternoon. At the
surface, a synoptic cold front is pushing southward, with gusty
northeasterly winds developing in its wake. As this MCV/mid-
level wave presses eastward, the greatest coverage of steady
showers will gradually push east into Indiana through late this
afternoon. The lightning threat has ended for locales north of
I-80, as the main elevated instability axis continues to move
southward. Can`t rule out a few bonafide thunderstorms
developing across our far south (Ford - Benton counties) this
afternoon along the cold front. Forecast effective inflow layers
are very shallow, greatly limiting available SRH, but deep
layer shear values near 35 kts could potentially support a few
mini supercells. The window for this will be very brief, perhaps
3-6 PM (if that), with the main potential focusing east and
south of our forecast area.

Since low-level moisture won`t really go anywhere tonight,
pockets of light showers/drizzle will be possible into Monday
morning as additional vorticity maxima meander overhead. Cloud
bases may lift sufficiently Monday afternoon to bring an end to
lingering precipitation, but additional disturbances drifting
atop the frontal zone may result in sporadic sprinkles/light
precip chances into Tuesday. Persistent strong northeasterly
winds will result in a prolonged period of dangerous swimming
conditions at area beaches due to high waves and strong
currents.

Thereafter, we`ll get a notable injection of drier air from the
east as high pressure settles across the Upper Midwest and
Great Lakes region through the mid and end of the week.
Seasonably cool and pleasant conditions look to prevail during
this period, with high temperatures generally in the 60s with a
very modest warming trend into week`s end. Upper-level blocking
looks like it`ll hold most precipitation chances to a minimum,
at least until next weekend when a small signal for
precipitation chances may return.

Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 630 PM CDT Sun Sep 20 2026

Key aviation forecast messages:

- Showers and drizzle at times through midday Monday, greatest
  coverage this evening with associated IFR CIGs and MVFR VSBYs.

- Breezy northeast winds gusting 20-25 kts through the period.

Showers and drizzle remain persistent across much of the area
early this evening and is likely to continue through much of the
evening, with associated IFR CIGs and MVFR VSBYs. While at
least patchy drizzle/spotty showers can`t be ruled out through
the remainder of the overnight hours, coverage is expected to
decrease after ~3Z as low-level ascent decreases some (VCSH).
Given residual low-level moisture in place across the area,
coverage of drizzle may increase toward daybreak Monday through
late morning as low-level ascent increases again. Have
introduced PROB30 groups for -SHRADZ from 10-16Z to account for
this for now.

Winds will remain breezy out of the northeast with gusts in the
20-25 kt range expected through the period, highest near Lake
Michigan.

VFR conditions are currently forecast into Monday afternoon,
however, if patchy drizzle is slower to diminish, CIGs may
remain MVFR through the end of the period.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Wednesday evening for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Wednesday afternoon for INZ001.

     Beach Hazards Statement through Monday morning for INZ002.

LM...Small Craft Advisory until 7 PM CDT Wednesday for the IL
     nearshore waters.

     Small Craft Advisory until 10 AM CDT Wednesday for Calumet
     Harbor IL to Burns Harbor IN.

     Small Craft Advisory until 7 AM CDT Monday for Burns Harbor to
     Michigan City IN.

&&

$$

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