textproduct: Chicago

KEY MESSAGES

- There will be periods of showers and thunderstorms Wednesday into Wednesday night, locally heavy rainfall is possible and could lead to some flooding concerns.

- Seasonably warm and mostly dry conditions are expected late in the week into the upcoming weekend.

- A return to a potentially more active northwest flow pattern is expected next week.

DISCUSSION

Issued at 313 AM CDT Tue Aug 4 2026

Seasonably strong west-southwesterly mid and upper-level flow will overspread much of the Midwest into the Great Lakes tonight into Wednesday as a large upper trough slides eastward across the north central CONUS. As it does, a north-northwest to south- southwest oriented surface cold front will gradually shift southeast from the Upper Midwest today, into the Great Lakes on Wednesday. This front will be the focus for a few rounds of showers and thunderstorms through the midweek period as forcing is augmented by several smaller scale impulses tracking east- northwestward along it. The threat for a few strong to severe storms cannot be ruled out in our area, particularly on Wednesday as the front shifts into IL and IN. However, potentially of more concern is the threat for an episode or two of very heavy rain producing showers and storms in and near our area given the axis of deep rich moisture pooling along the front (precipitable water values near 2").

There continues to be uncertainty with the exact timing of these waves of showers and storms in our area, owing largely to the timing of, and extent of convective enhancement to the individual smaller scale impulses translating through the larger scale trough. However, there continues to be a signal for convection to develop with one such wave near the frontal boundary later today into this evening west of our area across IA. This activity may then shift east-northeastward into parts of northern IL tonight into early Wednesday morning, though likely in a weakening state. Nevertheless, lingering cloud cover and rain showers Wednesday morning over parts of northern IL could keep afternoon heating/destabilization tampered. Accordingly, this could potentially impact the favored placement for the next round of convection.

Another round of developing convection is probable into Wednesday evening/night in association with another impulse forecast to sweep eastward across the Mississippi Valley into the lower Great Lakes. If cloud cover and lingering rain from any early morning convection linger through the afternoon and tamper destabilization enough, it remains plausible that this next round of convection could become favored farther south (potentially south of I-88), though how far south this may be is unclear at this time. Nevertheless, wherever this evening round of showers and thunderstorms sets up, very heavy rainfall and possible flash flooding will be a concern, particularly if storms train northeastward over the same areas along and north of the frontal boundary.

While the Wednesday night convection could push the effective front south of the area into Thursday, forecast guidance suggests that another impulse will track into the region Thursday afternoon. If this wave is strong enough, it could result in the front lifting back northward and stalling over portions of my CWA. If this occurs, an additional wave of heavy rain producing convection would be possible Thursday afternoon and evening. Confidence on this is low at this time, however.

Mainly dry and seasonably warm weather is expected late in the week through potentially most of the upcoming weekend. However, ensemble guidance favors the Hudson Bay low/trough to remain a dominant feature through the weekend before ejecting south- southeastward into the upper Great Upper Lakes early next week. Accordingly, the presence of broad upper troughing across the Great Lakes amid subtropical ridging across the southwestern CONUS will support a belt of strong northwesterly mid and upper- level flow atop another southward shifting surface cold frontal boundary early next week. Building instability in the vicinity of a boundary will thus support increasing chances for convective episodes (some possible severe with more heavy rain) in the area early next week.

KJB

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 635 AM CDT Tue Aug 4 2026

Forecast concerns include...

Chance of showers northwest IL this afternoon/evening. Chance of thunderstorms early Wednesday morning.

Some patchy/shallow fog formed in the predawn hours and will quickly dissipate soon. Light south/southwest winds will increase to 10kt by mid/late morning and then diminish under 10kt with sunset, possibly turning more southerly.

There remains a chance for a few showers, possibly a thunderstorm across northwest IL and RFD this afternoon into this evening, but confidence remains too low to include any mention with this forecast. There will be a better chance for showers and scattered thunderstorms in the predawn hours Wednesday morning through mid morning Wednesday across parts of the area, but there remains uncertainty for coverage and timing. Despite the uncertainty prob mention seems reasonable from this distance though changes are possible as trends emerge later today into tonight. Depending on the precipitation coverage, mvfr cigs are possible Wednesday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

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