textproduct: Chicago

KEY MESSAGES

- Quieter conditions with near to slightly above normal temperatures through Saturday.

- Waves of heavy rain producing thunderstorms possible in the general region early to mid next week.

DISCUSSION

Issued at 310 AM CDT Fri Sep 11 2026

Surface high pressure is centered overhead across the western Great Lakes early this morning. Light winds under this high may support patchy fog development before dawn, particularly west- southwest of the Chicago metro area. However, the main threat for dense fog will remain to the southwest across portions of northern Missouri into west-central Illinois. Any fog that develops will quickly burn off after sunrise.

The western Great Lakes surface high will slide eastward into New England through tonight. As it does, the surface synoptic flow will favor a south-southeasterly direction. However, wind directions are likely to turn more easterly across northeastern Illinois this afternoon as the flow becomes augmented by a lake breeze. Overall, today will be a pleasant early fall day for the area. Mostly sunny skies should warm inland locations to around (or just above) 80F this afternoon, with slightly cooler conditions expected lakeside. Humidity levels are expected to remain comfortable.

Warm air advection is forecast to ramp up across the area Friday night into Saturday as the low-level synoptic flow shifts south-southwesterly ahead of an approaching cold frontal trough. This will support a temporary bump in temperatures (into the mid-80s) and humidity on Saturday. The front should slide across the area late Saturday afternoon and evening. Currently, rain chances with this front look low (<20%) due to weak frontal and larger-scale forcing.

Northerly low-level flow behind Saturday evening's cold front will push a cooler air mass across the region for Sunday. Increased cloud cover and a chance of afternoon showers tied to an approaching mid-level impulse should keep high temperatures in the 70s across most locations. Rain chances then look to shift south and southwest of the area Sunday night as surface high pressure builds over the Great Lakes. While the current gridded forecast maintains 20-40% shower chances Sunday night, these may need to be lowered or removed if guidance trends hold. The first half of next week continues to look active across the region. Upper-level ridging will expand eastward over the southeastern CONUS while a shortwave trough axis migrates east- northeastward across the northern tier. As this occurs, a trailing surface cold front is expected to become quasi- stationary somewhere over the lower Great Lakes region through midweek, favoring multiple rounds of heavy rain-producing showers and thunderstorms. This pattern could result in a Maddox frontal heavy rain setup Tuesday into Wednesday, so this period will need to be monitored closely over the coming days.

KJB

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 609 AM CDT Fri Sep 11 2026

VFR through the period with no significant concerns.

Winds will become steady southeast by mid morning today. Expect lake influence to turn winds more easterly near the lake during the afternoon. There may be a brief period of closer to due south winds at ORD and MDW prior to the shift to east-southeast. Winds will then become light southerly tonight and southwest around 10 kt after sunrise on Saturday.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.