textproduct: Chicago

KEY MESSAGES

- Episodes of showers and storms are expected in the general region this evening through tomorrow night. Repeated downpours may lead to localized flash flooding.

- Stormy conditions may continue this weekend and into early next week.

DISCUSSION

Issued at 112 PM CDT Tue Aug 4 2026

The Lower Great Lakes region is located on the western fringe of an expansive surface high pressure system centered on the northeastern US coast and downstream of an elongated frontal boundary extending from central Wisconsin through central Iowa and into the Southern Plains. Meanwhile, aggregate upper-level troughing is evident in GOES-19 water vapor imagery centered on the northern United States. Closer to home (more or less between the features of interest), it's a pleasant early August day with temperatures in the upper 70s to lower 80s, partly cloudy skies, and a southwesterly breeze.

A subtle upper-level shortwave continues to support clusters of showers and thunderstorms along the frontal boundary from central Nebraska to western Iowa at press time. This activity is poised to continue working eastward through the afternoon and should reach the Mississippi River by this evening. In spite of surface dew points in the upper 60s extending toward Lake Michigan, quality moisture through the entire convective layer wanes with eastern extent suggesting any storm this afternoon/evening may tend to weaken while moving across northern Illinois. Will maintain low-end (20-30%) chances for a thunderstorm across northwestern Illinois through the evening just to be safe.

Tonight, the broader upper-level trough is expected to shift eastward toward the Great Lakes as an embedded upper-level jet streak shifts into southern Ontario. Increasing low-level confluence at the nose of a 700mb low-level jet and upper-level diffluence in the right entrance region of the upper-level jet streak will support the development of new showers and storms across Iowa tonight. With time, at least some clustering should encourage eastward propagating into northern Illinois by and especially after daybreak. Confidence is hence increasing in a wet morning commute across the area with clusters of showers and storms. The intensity of storms may tend to weaken with eastward extent by mid-morning owing to the instability axis remaining focused near the Mississippi River, so am not currently anticipating a severe weather or flash flood threat in the morning. Coverage of showers and storms should then decrease altogether from mid to late morning as the low- and upper-level jet forcing weakens. Residual isentropic ascent along the southeastward-drifting front may nevertheless support a few showers in the general area through early afternoon.

During the afternoon and evening hours, yet another upper- level shortwave embedded in the aggregate upper-level troughing is poised to move into the Lower Great Lakes supporting another round of showers and storms. There remains lower than average confidence in the coverage and intensity of any afternoon/evening round of storms owing to impacts of early- day activity (that is, extensive cloud cover may limit the ability for meaningful instability to develop ahead of the wave). In scenarios where little to no instability is able to materialize, the afternoon/evening may be characterized by scattered showers and a few non-severe storms with a main threat of lightning. However, if more meaningful instability is able to develop, there remains concerns or a more widespread convective episode including a threat for flash flooding given ample synoptic-scale forcing within the high-PWAT regime. Any local clustering could support a water-loaded downburst threat as well, consistent with a Level 1/5 severe weather threat. Will maintain higher-end PoPs (70-90%) across the area tomorrow afternoon through the overnight hours and let future shifts make informed adjustments based on real-time trends.

Current thinking is that the repeated rounds of showers and storms along the frontal boundary should shove the effective instability axis well south of our area by Thursday morning especially if additional storms develop in Missouri Wednesday night. So, will cautiously cut down PoPs on Thursday to advertise low-end chances (20-50%) for additional thunderstorms largely focused south of I-80.

Friday onward:

As aggregate upper-level troughing shifts eastward toward the end of the week, a quasi-zonal to northwesterly upper-level flow pattern will materialize across the Upper Great Lakes region supporting additional waves of showers and storms. Some of these may be rambunctious as the eastern edge of an EML plume is repeatedly re-advected eastward atop a stalled frontal boundary draped across the Great Lakes. Naturally, confidence in when/where eventual thunderstorm clusters will roam is low at this range. With that said, the pattern matches conceptual models for threats for severe weather and flash flooding in the general region through the weekend and into early next week.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1244 PM CDT Tue Aug 4 2026

Key messages for the 18Z TAF period:

* A system of showers and embedded thunderstorms progged for Wednesday morning, with another round possible in the late afternoon/evening

* Periods of MVFR possible with the rain on Wednesday, especially at RFD

SW winds up to around 10 kt this afternoon will subside for the night. A system of showers with possible embedded thunderstorms will work across northern IL on Wednesday morning. Favored time of arrival is after 09Z at RFD and after 12Z at the Chicago sites. Periods of MVFR vsby will be possible with the showers. A break in precip coverage is expected during the early part of the afternoon before a second round arrives during the late afternoon into evening. Additional thunderstorms and periods of MVFR will be possible.

At RFD, the favored signal is for MVFR cigs to settle overhead with the morning precip and remain through the afternoon leading up to the next round. MVFR cigs may find their way over Chicagoland as well, but confidence there is lower.

Looking ahead, showers and a couple of storms may linger through the night and into early Thursday with a signal for low stratus and fog possibly developing Wednesday night.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.