textproduct: Chicago

KEY MESSAGES

- Another hot and humid day is in store Friday with peak heat index values of 100 to 105F, warmest south of I-80. - Torrential rain-producing storms with a threat for flash flooding may impact far northern Illinois late tonight into early Friday morning, though confidence is higher that they remain in Wisconsin.

- There is another chance for showers and thunderstorms Friday evening.

- A break in the pattern will arrive by Sunday with lowering humidity values and temperatures.

DISCUSSION

Issued at 307 PM CDT Thu Sep 3 2026

An outflow boundary has pushed south to the I-80 corridor early this afternoon. Northerly flow behind the boundary has moderated temperatures across far northern Illinois (primarily north of I-88), allowing for the early cancellation of heat headlines. While extreme heat has ended north of the boundary, expect conditions to remain humid through this evening.

Heat headlines remain in effect south of I-80, where afternoon temperatures are reaching the low to mid-90s with peak heat indices around 105. Additionally, the Heat Advisory has been extended through Friday for areas generally along and south of a line from Mendota, IL, through Kankakee, IL, and Rensselaer, IN.

The threat for additional showers and thunderstorms in our area the remainder of the day overall look low (20-30%), and of low aerial coverage, owing to nebulous large scale forcing near the mid-level ridge axis. If any storms are able to develop this afternoon, their favored placement will be near the outflow boundary from far east central IL into northern IN. Otherwise, we are looking to have a dry but muggy evening.

Tonight,an upper-level short-wave trough will track from the northern Plains across the Upper Midwest, driving a complex of thunderstorms across the lower Great Lakes. While the main complex may stay northeast of our area, renewed convection is plausible Friday morning along its southwestern flank as a southwesterly low-level jet interacts with a composite outflow boundary. This setup could support storm training and a localized flash flood threat, though confidence remains low with the highest potential north of our area. Regardless, there is a 30-50% chance of scattered showers and thunderstorms across far northeastern IL and northwest IN between 2 am and 10 am Friday.

Following morning activity, capping should maintain dry conditions through Friday afternoon. We are monitoring for scattered thunderstorms developing across southern Wisconsin late Friday afternoon or evening along a cold front, which would likely shift southeast into our area Friday night. The SPC has included our area in a Level 1 of 5 severe threat due to the potential for locally strong wind gusts.

Surface high pressure building into the Great Lakes this weekend will end our hot and humid spell, at least for a few days. Temperatures will trend toward the upper 70s near the lake to the low 80s inland by Sunday. A moderate to high swim risk is possible at Lake Michigan beaches late Saturday through Sunday due to persistent onshore flow and choppy conditions.

KJB

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 102 PM CDT Thu Sep 3 2026

TS N and E of the Chicago terminals earlier this morning pushed a robust outflow boundary across northern Illinois. N/NNW winds gusting up to around 20 knots will gradually weaken while steadily veering this afternoon, with directions already beginning to favor just E of N at ORD/MDW. The main TS potential will remain south of all terminals this afternoon, but have maintained PROB30 TS for another couple hours as isolated taller SHRA and perhaps a rogue TS cannot be ruled out over northeast Illinois.

Winds will continue to slowly veer tonight, likely settling SSW under 10 knots overnight. A mid-level disturbance will then induce elevated TS across Wisconsin and southern Lake Michigan tonight. It is unclear how far SW the TS will extend, so have maintained PROB30 TS roughly in the 12-15Z window for the Chicago terminals. Even if TS does not directly affect the terminals, strong outflow is expected to produce a NE wind shift sometime early to mid morning. Like today, winds should then slowly veer through the day, and possibly settle SSW by late afternoon.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 9 PM CDT this evening for ILZ010-ILZ020- ILZ106-ILZ107-ILZ108.

Heat Advisory until 7 PM CDT Friday for ILZ019-ILZ021-ILZ023- ILZ032-ILZ033-ILZ039.

IN...Heat Advisory until 9 PM CDT this evening for INZ001-INZ002.

Heat Advisory until 7 PM CDT /8 PM EDT/ Friday for INZ010- INZ011-INZ019.

LM...None.