textproduct: Chicago

KEY MESSAGES

- Showers will affect areas generally south of I-80 today and tonight, and especially south of U.S. 24 where rainfall could be heavy at times.

- A few thunderstorms are possible Saturday night into Sunday morning.

- Dangerous heat will build across the area next week, with afternoon heat index values over 100 degrees each day beginning Monday.

DISCUSSION

Issued at 241 AM CDT Fri Jun 26 2026

Early morning analysis shows a convectively-enhanced wave over southeast Kansas beginning a trek along a WSW-ENE oriented stationary front. This wave should continue follow along the front today, crossing the southern third of Illinois by this evening. This activity is somewhat farther south than earlier guidance indicated, supporting at least a downward trend in PoPs for areas north of the Kankakee River Valley. In fact, areas north of I-80 may remain dry. However, a substantial moisture gradient on the northern periphery of the incoming wave suggests the potential for a notably sharp QPF gradient over or just south of the southern CWA. South of the gradient, PWAT values around 2", a relatively warm profile (10kft deep moist layer warmer than -10C), and seasonably strong dynamics will yield high rainfall rates, and may result in a narrow corridor of heavy rain somewhere across central Illinois this afternoon and evening. Would not be surprised to see 1-2" amounts south of the U.S. 24 corridor by late this evening, but chances for higher impact rainfall look to remain around and south of I-74.

Lingering showers across the far southern CWA early Saturday should end by late morning. Saturday will then feature gradually clearing skies and highs around 80 (cooler lakeside).

Primary focus then turns to developing dangerous heat across the region next week. A building ridge over the central Great Plains early this weekend will drift ENE through Sunday night, with the core of the ridge settling over the lower Ohio River Valley for much of next week.

Developing convection toward the Mississippi River Saturday evening in response to weak isentropic ascent ahead of an approaching EML will encounter a gradually less favorable environment with northeast extent through the night. Given marginally favorable low-level moisture profiles and modest mid- level lapse rates, expectations are that elevated convection should survive at least into the southwest CWA overnight before fully decaying toward northeast Illinois by daybreak Sunday. Following any precip potential early Sunday, heat and humidity will begin to build with heat index values rising into the 90s during the afternoon.

Concerns for dangerous heat will quickly grow beginning Monday as a constant feed of Gulf moisture arrives over the area. Afternoon heat index values will likely rise over 100 beginning Monday and persist possibly through the entire week. Meanwhile, nighttime temps will struggle to fall much into the 70s, and may remain near 80 over the core of the Chicago metro for several nights. Early in the week, there looks to be little relief from lake breezes or upstream convection as substantial capping and the strength of the ridge deflect most activity well to the northwest over Minnesota and northern Wisconsin. By Thursday, weakening of the cap may allow more robust convective complexes to near from the north and provide some relief for northern portions of the area. If this does not occur, similar heat will likely continue heading into the Independence Day weekend.

Kluber

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 613 AM CDT Fri Jun 26 2026

Aviation forecast concerns for the 12Z TAFs:

- Scattered SHRA near/south of the Chicago terminals mid-morning through this afternoon. Farthest south KGYY has the highest chance (30-40%) of seeing rain at the airfield.

- Low-confidence (10-20%) for patchy MVFR ceilings off the lake later tonight into early Saturday for Chicago terminals.

Surface low pressure wave was analyzed near the OK/KS border early this morning, with a nearly stationary frontal boundary extending northeast and east through central IL/IN. The surface wave is expected to track east along the frontal zone today, spreading rain south of most of the terminals across primarily central IL/IN. Model trends have backed-off the northward extent of RA/SHRA, and while a few sprinkles can't be ruled out farther north, scattered SHRA are expected mainly south of KORD/KDPA and even KMDW (though precipitation may come close to KMDW). KGYY looks to have the greatest potential (30-40%) of actually seeing a period of -RA/SHRA at the airfield, and have indicated a PROB30 there along with an MVFR ceiling later this morning and afternoon from 15Z-21Z while maintaining a VCSH mention for KMDW. Thunder potential continues to appear quite low (<10%) given poor low-mid level lapse rates. VFR conditions are expected for all sites, though there is a low (10-20%) probability of some MVFR ceilings developing off of Lake Michigan later tonight into early Saturday morning for the Chicago terminals.

Surface winds are expected to be from the east to northeast through the period, generally 10 kts or less.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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