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 /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 653 PM CDT Tue Aug 4 2026

Aviation forecast concerns for the 00Z TAFs:

- Two periods of SHRA/TSRA are expected during the period, one toward/around daybreak Wednesday and another in the afternoon and early evening.

- Period of MVFR ceilings likely for KRFD Wednesday.

A slow-moving cold front stretches from near Green Bay southwest to near Kansas City early this evening. Deeper moisture return has been limited along the front, with a relatively narrow axis of instability limited to along/just ahead of the front. Shower and TSRA chances are expected to gradually increase across the area into Wednesday morning however, as the instability axis shifts slowly east ahead of the front and a mid-level disturbance tracks across the region. This looks to bring SHRA/TSRA initially into KRFD toward daybreak and into the Chicago area around/just after sunrise. Thunder potential with this first round appear greatest at KRFD, where a tempo has been maintained. For the Chicago terminals, most high-res CAM and CAM ensemble guidance suggests a decreasing lightning trend with eastward extent as the initial forcing begins to outrun the instability axis.

Most guidance is in decent agreement in a lull in precipitation during the late morning/midday hours, before another somewhat more robust disturbance arrives during the more diurnally favorable afternoon/early evening hours. By that time, the instability axis along the front looks to be in position to favor the Chicago metro TAF sites for TS potential and have indicated a period of tempo TSRA while limiting the KRFD forecast to a prob30. The TS threat should eventually shift east of the terminals by mid-evening, though some spotty showers may linger as the surface front moves slowly through. KRFD may have a prolonged period of MVFR ceilings during the day Wednesday, left from morning precipitation and lingering within the moisture axis within the frontal zone.

Wind wise, modest south-southwest winds are expected much of the period, though winds may become light/variable later Wednesday as the slow-moving front stalls somewhere near/across the Chicago area and back into MO.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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