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 /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 108 PM CDT Thu Jun 4 2026

Key aviation forecast messages:

- Periods of SHRA and TSRA return Friday

VFR and dry conditions are expected through tonight for most of the area. Winds will be breezy out of the southwest with gusts around 20 kt through this afternoon then ease with sunset.

A cluster of showers and isolated embedded thunderstorms is expected to approach the area into Friday morning from Iowa. An initial arc of spotty showers may develop ahead of this feature, reaching RFD potentially as early as 9-10Z and the Chicago area terminals closer to 11-12Z with ceilings very gradually decreasing through the morning toward MVFR in their wake. As we get later into the morning toward midday shower coverage is expected to increase along with the potential for thunder. Have shifted the TS mention a bit later, to the early to mid afternoon hours, for the Chicago terminals based on a slower arrival of slightly more favorable instability. Heavy downpours (rainfall rates >1"/hr) and locally gusty winds are the primary concerns with these storms in addition to occasional lightning strikes. Guidance continues to trend toward the shower and thunderstorm potential focusing mainly south of I-80 into the evening and overnight hours.

Winds will quickly increase out of the southwest around 25 kts ahead of the storms Friday morning then become more variable in their wake and easing into the evening hours.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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

LM...None.