textproduct: Chicago

KEY MESSAGES

- Patchy showers and light drizzle at times into Tuesday morning.

- Dry, but with below average temperatures for much of the remainder of the work week.

DISCUSSION

Issued at 135 AM CDT Mon Sep 21 2026

The surface cold front has settled south of the area across downstate IL/IN and the Ohio Valley this morning, while a broad area of surface high pressure was building across western Ontario and the far northern Great Lakes. Though the surface frontal zone is well south of the area, the forecast area remains beneath the northern periphery of the associated elevated baroclinic zone. Several minor mid-level vort centers are expected to propagate east along the upper level frontal zone today, which combined with lingering moisture and transient weak mid-level f-gen will result in persistent cloud cover and some light drizzle and spotty showers at times today. Breezy northeast surface winds gusting 25 to 30 mph will result in cooler than average temperatures in the 60s, and will also maintain high waves and hazardous swim conditions at our Lake Michigan beaches for the next several days.

Shower chances will continue (or redevelop) mainly west of the Fox Valley across northern IL tonight into Tuesday morning, as an upper level low drifts east from the Plains to the western Great Lakes. The eastward extent of these showers looks to be slowed/blocked by the continued feed of gradually drier low- level air thanks to the northeast surface winds.

The upper low will then gradually drift east-southeast across IN/OH and Tuesday night through Wednesday, while the southwestern periphery of the surface high to our north/northeast settles across the western Lakes and brings more substantially drier air to the area during the mid-late week period. Despite diminishing northeast winds, cooler than average temperatures are expected to persist with daytime highs generally in the 60s and nighttime lows in the 40s (lower 50s in Chicago). A gradual warm-up back into the low-70s looks to occur next weekend as medium-range guidance and ensembles develop upper ridging back into the upper Midwest. There remains a signal for precipitation chances to return to parts of the upper Midwest late in the weekend, though a deep upper trough along the East Coast may hold more meaningful chances west of the area until early next week.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 634 AM CDT Mon Sep 21 2026

Forecast concerns include...

Mvfr cigs through this afternoon. Chance of showers this morning. Gusty northeast winds through this evening.

Cigs have generally lifted into the 2kft range, or higher and mvfr cigs are expected to continue through early/mid afternoon before lifting from northeast to southwest across the area. There may be some brief periods of low mvfr cigs but not expecting these to become prevailing.

Scattered showers and drizzle will be possible this morning with any additional showers this afternoon expected to be west and southwest of the Chicago terminals.

Winds will remain northeast through the period with gusts into the lower/mid 20kt range through this afternoon. Gusts will begin to diminish with sunset and gusts may end for most areas overnight. cms

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 until 10 AM CDT this 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 10 AM CDT this morning for Burns Harbor to Michigan City IN.