textproduct: Chicago

KEY MESSAGES

- Today will be another hot and humid day with heat indices around 105F.

- Several rounds of thunderstorms will move through the general area Thursday through Saturday. Some of these may be severe.

- A break in the hot and humid pattern may arrive by early next week.

DISCUSSION

Issued at 225 AM CDT Wed Sep 2 2026

A large heat dome remains stalled over the Lower Mississippi River Valley south of a slow-moving upper-level low currently in central Ontario. Quasi-zonal flow remains in place between both features and atop a stationary front across the central Great Lakes. A short-wavelength upper-level trough embedded in the zonal flow is currently supporting several clusters of showers and storms along the stationary front stretching from northwestern Iowa through central Wisconsin. Meanwhile, south of the front and indeed across our area, it is a warm and muggy night with current air temperatures in the mid to upper 70s (dew points in the low to mid 70s).

Based on the continued east-northeasterly trajectory of the aforementioned shortwave, current expectations are that all clusters of showers and storms along the stationary front will remain north of our area through the morning hours. Even if any outflow makes it to northern Illinois this morning, it should be shallow enough to erode fairly quickly. So barring any unexpected southward jogs in the thunderstorm clusters or stronger southward outflow pushes, today is setting up to be another hot and humid day. Compared to yesterday, mixing profiles today look a hair deeper which may modestly offset evapotranspirational enhancement to dew points. Regardless, will continue to lean on NAM/HRDPS guidance which shows dew points rebounding to the mid to locally upper 70s today. When paired with highs in the lower 90s, heat index values should approach 105F again. For this reason, did expand the inherited Heat Advisory to encompass the entire area. If mixing is more effective today, suppose a stray shower or storm may attempt to develop this afternoon though coverage should be isolated at best.

Tonight, the stationary front will begin a southward sag toward the Illinois and Wisconsin state line. Broad and largely unfocused low- level flow atop the front may encourage the development of a shower or thunderstorm near the frontal boundary at any time (not unlike what we saw the past few nights), so will carry low-end (20%) PoPs north of I-88 through daybreak. Otherwise, tonight is setting up to be a repeat of early this morning with seasonably warm overnight lows and continued muggy conditions.

Forecast confidence lowers Thursday and Thursday night. Deterministic guidance suggests an upper-level shortwave will round the ridge toward the Iowa/Wisconsin/northern Illinois vicinity toward daybreak accompanied by an uptick in coverage of showers and storms. Provided this occurs, at least some footprint of cloud cover and cooler temperatures would be expected across our area lasting into early afternoon. Thereafter, guidance is hinting at another upper-level wave approaching the Great Lakes by early afternoon which may lead to another round of storms in the general region lasting through the early morning hours of Friday. If convective coverage is minimized in the morning, the afternoon round could end up fairly problematic with a reservoir of moisture-laden instability and increasing deep- layer shear to support all modes of severe thunderstorms (including a threat for flash flooding). However if the footprint of morning showers and storms is vast, could also envision evolution of thunderstorms Thursday afternoon being pretty messy with a lower ceiling for intensity. At this point, see too many conditionalities to offer more than broad-brushed chance (20 to 40%) PoPs. Will also hold off on issuing any new Heat Advisory for parts of the area Thursday in favor of evaluating future model trends in potential convective coverage.

Friday into Saturday, ensemble guidance suggests the upper- level ridge will retrograde westward placing the Great Lakes into the precarious upper-level northwest flow regime. With the reservoir of moisture-laden instability remaining in place across the Great Lakes region, can easily envision several more rounds of (severe) storms in the general region through Saturday night. At least a brief break in the pattern (both with respect to humidity levels and temperatures) is anticipated early next week.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 110 PM CDT Wed Sep 2 2026

A mid-level ridge over the area will persist into Thursday. Expect SW winds to gust to around 20 knots this afternoon and up to 20 knots on Thursday.

Though VFR/dry conditions are favored through Thursday morning, scattered high-based/elevated SHRA with some TS over southern Wisconsin late tonight into Thursday morning may brush far northern Illinois. Have included PROB30 SHRA with an associated N wind shift during this time at RFD, but chances under 20 percent at the Chicago terminals support no mention in the TAF at this time.

A complex forecast then unfolds Thursday afternoon and evening as waves of convection to the north begin to push outflow boundaries into a generally uncapped environment across northern Illinois. Expectations are for isolated to scattered TS to develop on one of these boundaries or an afternoon lake breeze across northeast Illinois, possibly affecting the Chicago terminals as early as mid- afternoon. Any TS that develops could be strong to severe and produce strong and gusty winds. Have included PROB30 TS from 21-00Z (end of the forecast period), though periods of TS are expected to continue beyond this time and into Friday morning.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 9 PM CDT this evening for ILZ003-ILZ004- ILZ005-ILZ006-ILZ012-ILZ013-ILZ103-ILZ104-ILZ105.

Heat Advisory until 9 PM CDT Thursday for ILZ008-ILZ010-ILZ011- ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106- ILZ107-ILZ108.

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ Thursday for INZ001- INZ002-INZ010-INZ011-INZ019.

LM...None.