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 /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 626 AM CDT Wed Sep 2 2026

Key Aviation Messages for the current TAF period:

- Breezy southwest winds this afternoon

VFR conditions and southwest winds are expected to prevail through the TAF period. An isolated pop up shower today is possible, but the probability is too low for TAF mention. No major changes to wind trends through the forecast period. Breezier wind gusts around 20 knots are expected to develop region wide this afternoon before eventually diminishing around and after 00Z. The storms in Wisconsin are persistently developing outflows that move south; however, they continue to wash out around the Wisconsin state line. Trends will continue to be monitored, but no dramatic wind direction shift from the outflow has been added to the TAFs, though there is a second round of storms in Wisconsin overnight tonight that will be monitored. Speaking of wind shifts, models have backed away from any concept of a lake breeze developing. Lastly, models continue to suggest a short duration low level jet around 30 knots to develop tonight. As surface winds become light, if not calm, there is a chance for a brief window of low level wind shear (LLWS) at area terminals, but confidence is too low for formal mention in the TAF.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory from noon today to 9 PM CDT this evening for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Heat Advisory from noon CDT /1 PM EDT/ today to 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011- INZ019.

LM...None.