textproduct: Chicago

KEY MESSAGES

- Trending warmer and more humid with periods of showers and thunderstorms starting early Friday and continuing into next week.

- Low (level 1 out of 5) severe weather threat Friday PM along with a threat for locally heavy rainfall.

DISCUSSION

Issued at 217 PM CDT Thu Jun 4 2026

For Friday and Saturday the main forecast question is shower and thunderstorm trends, with greater emphasis on the showers given the overall marginal lapse rates and lack of strong forcing for convection. CAM guidance normally wouldn't be relied on too closely, but several models show similar trends and align with the overall concept of how things appear likely to evolve, so elected to follow a blend of those that appeared to reflect this thinking.

A cluster of showers with some embedded thunderstorms is expected to make its way into the area from Iowa early Friday ahead of a diffuse cold front and weak shortwave aloft. This activity should overspread the area through the morning into early afternoon at which point slight ridging aloft and movement away from the supporting surface boundary may allow for a decrease in coverage for later in the afternoon and evening. The exception may be in the southern half of our forecast area where an outflow boundary from the earlier activity could provide the focus for additional development.

Most locations on Friday and Friday night do not appear poised for a complete washout, but given an axis of PWATs exceeding 1.75" and the expected slow motion of this cluster, some areas could see some heavy rainfall. SPC also has us in a Day Two marginal risk but this would not appear to pose a widespread threat given the aforementioned marginal support for convection overall.

Later Friday night into Saturday the model guidance begins to diverge, but the general idea still appears to keep the best focus for continued showers in southern and western portions of the forecast area. The exception could be along a developing lake breeze that might be able to tap into the moist airmass and support an additional area of showers Saturday afternoon.

For Sunday into the first part of next week, model guidance is showing a narrow mid to upper level ridge building overhead, with a stationary front at the surface extending east from the Plains into the Ohio Valley. This boundary, wherever it happens to park, would appear to be the best focus for additional shower and thunderstorm activity, especially with support from afternoon heating and dewpoints in the 60s to occasionally lower 70s.

Lenning

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 641 PM CDT Thu Jun 4 2026

Forecast concerns include...

Chance of thunderstorms overnight through Friday afternoon. Mvfr cigs, possible ifr cigs, Friday morning. Gusty southwest winds Friday.

Scattered showers and isolated thunderstorms are expected to develop across northwest IL and at RFD in the predawn hours and then move east/northeast through the mid morning hours Friday. There is quite a bit of uncertainty for when this activity may develop and how far east it will move and whether it may reach the Chicago terminals. Opted to include vicinity shower mention for the Chicago terminals for this potential and then maintained tempo showers for the mid/late morning. However, some thunder mention may be needed during this time but confidence is too low. Best chance for thunder appears to be in the early afternoon and current tempo mention seems on track for now.

Mvfr cigs are expected to develop across northwest IL by daybreak and may extend as far southeast as the Chicago terminals by late morning. Cigs may continue to lower to ifr across northwest IL including at RFD though there remains uncertainty for how low cigs may drop. These lower cigs are expected to slowly lift Friday afternoon.

South/southwest winds will gust as high as 20kt early this evening, then diminish with sunset. Speeds may increase back into the 10-15kt range in the predawn hours. Gusts into the mid 20kt range are possible Friday morning but these higher gusts will likely be dependent on precip coverage. Maintained these higher gusts for now, but changes will be possible. Winds will trend more west/southwest Friday afternoon with some gusts still possible. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Air Quality Alert until midnight CDT /1 AM EDT/ tonight for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory from 10 AM to 7 PM CDT Friday for the IL and IN nearshore waters.