textproduct: Chicago

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.