textproduct: Chicago

KEY MESSAGES

- Additional showers late this afternoon and evening mainly well south of I-80.

- Small chance for some spotty showers Thursday, better chances on Friday.

- Moderating temperatures through the end of the week and into Saturday.

DISCUSSION

Issued at 1240 PM CDT Tue May 5 2026

A secondary impulse, attendant increase in upper-level jet divergence, and enhanced mid-level f-gen will result in another increase in showers south of I-80 this afternoon and evening. Any MUCAPE will be relegated well to the south across parts of central and downstate Illinois, so no thunder mention appears necessary in the gridded forecast. With activity expected to diminish overnight, there is some potential for a little shallow fog development along a diffuse confluence axis south/southeast of the Kankakee River. Low confidence in sufficient clearing, combined with a lack of a deeper near-surface hydrolapse precludes any fog addition at this time, however. Finally, temperatures may meander into the low/mid 30s across interior northern Illinois tonight. Given mixed-out dewpoints this afternoon, have not added a frost mention at this time.

Forecast guidance (including the NAM) depicts deep boundary layer mixing developing on Wednesday. Have nudged winds/gusts up a bit with mean boundary layer flow progged to be in the 15-20 kt range, and commensurately nudged afternoon dewpoints down closer to some of the more well-mixed guidance (HRRR/RAP). Given the stronger flow, it seems like any lake breeze should end up tied pretty close to the Illinois shore, with somewhat deeper inland movement across Lake/Porter counties.

High pressure will build in just to our south Wednesday night. It looks like some mid and high cloud cover will be present over the region, although this may end up being somewhat brief/ephemeral. With the pressure gradient relaxing overnight, looks like there's a potential for temperatures to drop pretty quickly through the evening. Have introduced some patchy frost mention in the grids with the expectation of dewpoints slowly recovering overnight. If cloud cover doesn't end up impeding OLR too much, temperatures could drop near/below freezing on a localized basis.

Thursday will feature another day with deep boundary layer mixing, and forecast guidance event depicts the development of a little surface-based instability during peak heating. A series of sheared-out mid-level perturbations are also forecast to drift across the region during the afternoon and evening, pushing a seasonably cool 500 mb airmass (mid -20s C) overhead. Could envision some isolated/spotty high-based shower development during the afternoon as a result.

Somewhat deeper moisture and more coherent synoptic scale forcing will arrive on Friday, resulting in another period of mainly light shower activity, potentially lingering into Friday evening. The next shortwave trough looks to be fast on the heels of Friday's with warm air advection in advance of the next wave looking to provide milder temperatures Saturday. That will be followed by another chance of showers Sat night with a frontal passage, followed by a return to cooler temps Sunday into Monday.

Carlaw/Izzi

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 607 PM CDT Tue May 5 2026

There are no significant aviation weather concerns anticipated through the period. VFR conditions will prevail through the period, with only some higher high CIGs at times. Otherwise, winds will abate below 10 kt early this evening and may even trend more towards an easterly direction for a few hours before turning back northerly later this evening and overnight. Winds will then favor a northwesterly direction during the day Wednesday, with some occasional gustiness up around 20 kt expected in the afternoon.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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