textproduct: Chicago

KEY MESSAGES

- Isolated to scattered thunderstorms may develop along a cold front into this evening. Locally heavy rainfall is possible with these storms.

- Drier weather is in store Monday into Tuesday before storm chances return midweek.

DISCUSSION

Issued at 104 PM CDT Sun Aug 16 2026

The primary forecast concern this afternoon and evening is the potential for isolated to scattered, heavy rain-producing thunderstorms ahead of an approaching cold front. This front, currently located across southern Wisconsin, will move southward into northern Illinois during peak heating this afternoon. Despite mostly cloudy skies, occasional breaks will support modest surface heating and the gradual destabilization of the very moist, prefrontal airmass. Accordingly, as capping continues to weaken this afternoon, isolated showers and storms are expected to develop. Given limited surface convergence along the boundary, storm coverage should remain isolated to widely scattered for the next few hours. However, coverage may increase along the front late this afternoon and into the evening as forcing for ascent strengthens in response to a remnant MCV moving east out of southern Iowa. If this occurs, it would likely impact portions of the Chicago metro area and northwestern Indiana between 4 p.m. and 9 p.m. While the threat of severe weather remains low due to poor mid-level lapse rates, some locally gusty surface winds up to 50 mph could occur, particularly where storms cluster. Of greater concern are the torrential downpours likely to accompany these storms, which could result in localized flooding, especially if multiple storms track over the same areas of the Chicago metro.

We will see an end to our chances for showers and thunderstorms with the passage of this front this evening. Surface high pressure will settle over the Mid-Mississippi Valley on Monday. Lower humidity and slightly cooler temperatures (highs around 80) will provide a brief reprieve from our active weather pattern of late, before another round of convection arrives late Tuesday afternoon and night. This next system, driven by a cold front and shortwave digging into the Great Lakes, brings a renewed threat of strong storms and heavy rain Tuesday night. Quieter, cooler weather (temperatures in the 70s) returns for the second half of the work week.

KJB

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1225 PM CDT Sun Aug 16 2026

Key messages for the 18Z TAF period:

* Isolated to widely scattered showers and storms possible this afternoon outside of RFD. Best chance late afternoon.

* A period of MVFR cigs possible Monday afternoon.

A scattered system of showers and thunderstorms is expected to move across N IL during the mid-late afternoon. While coverage looks to mostly favor areas south of the terminals, chances call for a PROB30 for MVFR thunder. A few isolated showers cannot be ruled out prior to the main body of precip during the early-mid afternoon. The rain should move away by 00-01Z. Coverage looks to stay south of RFD altogether.

MVFR should continue to gradually lift/scatter through the early part of the afternoon. Should be widely VFR by mid- afternoon, although any passing precip may reintroduce brief periods of MVFR. There is a signal for a period of MVFR cigs during the late morning and afternoon on Monday. Kept TAFs VFR during this period owing to a lack of confidence, but look for potential MVFR in future issuances.

Meanwhile, WNW winds near 10 kt will occasionally gust above 15 kt. They'll subside and go NNW for the overnight, flopping to NNE in the early morning. Wind speeds should stay largely below 10 kt throughout the day.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

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