textproduct: Chicago

KEY MESSAGES

- Moderate to high swim risk expected through early Sunday during the final weekend of beach season.

- Relatively cooler Sunday and Monday, although remaining seasonably warm.

DISCUSSION

Issued at 137 PM CDT Sat Sep 5 2026

After an unseasonably hot and humid stretch to start September, cooler conditions are beginning to filter over the area from north to south today. Moisture pooling along a cold front draped across the southern CWA combined with temps in the lower 90s south of the front is leading to heat index values around 105F. Those numbers have likely peaked as the front should clear the remainder of the CWA over the next couple hours. While surface convergence is fairly weak on the front, interactions with residual outflow boundaries from earlier convection over Indiana are providing an added focus early this afternoon. Cannot rule out an isolated strong to briefly severe storm through 2-3pm given a weakly capped, highly unstable, and slightly sheared environment.

Surface high pressure will expand across the Great Lakes region Sunday and Monday. Though temps will remain seasonably warm, drier air will advect in from the east through the day Sunday, with values falling into the 50s during the afternoon and continuing through Monday.

Mid-level ridging will build back across the western Great Lakes by Tuesday before deamplifying in response to a strong mid-level system crossing southern Canada on Wednesday. Daytime highs both days will push well into the 80s to perhaps near 90, but dew points are expected to be lower (mid to upper 60s) than the past few days.

A cold front associated with the southern Canadian system will slowly drag across the area Wednesday into early Thursday morning. Minimal deep-layer forcing and weak low-level convergence on the front suggest that the coverage of showers and embedded thunderstorms will be, at most, scattered. Would not be surprised to see a mostly dry frontal passage.

Like Sunday and Monday, cooler (but seasonably warm) conditions will follow the front late in the week before ridging again builds over the area and results in unseasonably warm conditions next weekend.

Kluber

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1220 PM CDT Sat Sep 5 2026

Key Messages:

- Low MVFR cloud bases gradually improve through the afternoon.

- Threat for some ground fog at DPA and RFD around, daybreak Sunday morning.

A deck of IFR to low end MVFR stratus continues to shift into the area on northeasterly winds early this afternoon. While this cloud cover is expected to persist this afternoon, bases are expected to gradually improve through the remainder of the day, with VFR conditions potentially returning at most terminals after 20Z.

Some patchy ground fog is then possible early Sunday morning at DPA and RFD. Otherwise, expect quieter weather during the day Sunday, with east-northeasterly surface winds.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 7 PM CDT this evening for ILZ032-ILZ033- ILZ039.

IN...Heat Advisory until 8 PM EDT this evening for INZ019.

LM...None.