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.

UPDATE

Issued at 838 PM CDT Thu Sep 3 2026

Updated evening pops for ending of near-term shower/thunderstorm threat, and cancelled northernmost tier of Heat Advisory a little early this evening with heat indices below criteria.

Isolated thunderstorm threat has ended as of mid-evening, with the loss of diurnal low-level instability with sunset. Attention now turns to the upper midwest (northern WI back into northern MN/eastern ND) where clusters of thunderstorms are ongoing in response to a mid-level wave tracking east-northeast across that region. This wave is expected to take on a more east-southeast propagation overnight, as the upper level ridge axis strengthens west of the upper Mississippi Valley. A very warm and humid late summer air mass remains in place south of a nearly stationary frontal boundary stretching from the upper Midwest to the lower Great Lakes, with a corresponding reservoir of instability along/south of the front. Latest available CAM guidance continues to display a bit of spread even in recent high- res initializations, though the general expectation is that ongoing convective clusters over the upper Midwest will eventually propagate southeast as one of more MCSs across WI, Lake Michigan and lower Michigan from later tonight through Friday morning. How far southwest convection builds remains uncertain, though certainly can't rule out some of the solutions which swipe far northern/northeast IL and northwest IN toward and after daybreak Friday. Have maintained inherited chance (30-40%) pops for this roughly along/north of a Rockford- Valparaiso line after 4-5 AM. SPC's latest update through 12Z Friday keeps a marginal severe risk (level 1 of 5) for wind across our northeast cwa for this potential. Most current guidance decreases shower/thunderstorm chances for the area mid- morning and beyond.

Have cancelled the Heat Advisory a little early this evening for the tier of counties from Lee to Will in IL and Lake and Porter in northwest IN. These areas were affected by outflow and onshore flow this afternoon, and heat indices had fallen below advisory criteria. To the south, heat headlines remain in effect into Friday evening with another day of 100-105 degree peak heat index readings expected. With the potential for convection and/or outflow (and possibly an afternoon lake breeze) across the far north and northeast parts of the forecast area Friday, no headlines are currently in effect. However, if convection or outflow does not extend as far south/southwest into the forecast area as expected, it is possible the advisory footprint could need to be extended back to the north.

Ratzer

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 /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1215 AM CDT Fri Sep 4 2026

Key messages for the 06Z TAF period:

* Possibility of showers and thunderstorms Friday morning

* Uncertainty in wind trends Friday morning

Thunderstorms are expanding in coverage across portions of central WI as of this writing. A few scattered showers and storms may attempt to spread south across northern Illinois during the morning. PROB30s were maintained for this potential at all TAF sites, although latest guidance has been largely dialing back on coverage, especially thunder coverage.

A passing shower or storm or two will also be possible during the late afternoon and evening. Confidence was too low at this time for a mention in TAFs.

Light SSW winds will prevail through the night. Early Friday morning, outflow from thunderstorms to the north may shift winds to NE at largely below 10 kt for a period during the morning at the Chicagoland sites. This latest TAF package reflects this wind shift, although confidence isn't terribly high. Direction may decide to stay SW through the morning hours, but if not, SW should return by early afternoon. Expect a shift to N/NNE early Saturday morning near the end of the 30-hr TAF period as a cold front drifts south across the area.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 7 PM CDT this evening for ILZ019-ILZ021- ILZ023-ILZ032-ILZ033-ILZ039.

IN...Heat Advisory until 7 PM CDT /8 PM EDT/ this evening for INZ010-INZ011-INZ019.

LM...None.