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 /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 629 PM CDT Sun Aug 16 2026

Forecast concerns for the terminals...

- Scattered showers and thunderstorms possible, mainly south of I-80, through mid-evening.

- Northeast wind shift at the Chicago area terminals Monday morning with a lake breeze.

A cold front is progressing across far northern IL this evening and will continue to drift through the rest of the area this evening. Ahead of the cold front, a shortwave trough is pivoting out of southeastern IA and has lead to the development of some scattered showers and thunderstorms in western IL. The expectation is for these scattered showers/storms to stay south and west of the terminals and mainly be an issue for areas near and south of I-80 (roughly a VYS to VPZ line). Though, given the trajectory of the ongoing showers/storms in Lee County, IL cannot rule out a shower or storm getting to DPA, MDW, or GYY between prior to 03z. Regardless, once the cold front pushes through this evening any showers/storms should get pushed south of northern IL and northwest IN giving as a dry night.

Winds will remain breezy (speeds around 10-12 kts) out of the west-northwest through the evening before becoming solidly northwest behind the front. While winds should diminish overnight, speeds look to remain in the 5-7 kt range overnight into Monday morning.

Dry conditions are expected to prevail through Monday with otherwise VFR conditions. However, guidance continues to be adamant on a plume of lake effect clouds developing over the Chicago terminals and GYY which could result in a period of MVFR (2000-2500 ft) ceilings. With moisture depths on the forecast soundings looking modest confidence on MVFR ceilings is low so have opted to maintain the SCT025 mention for now. That said, if the MVFR ceilings do materialize cannot rule out an isolated sprinkle with them either. Otherwise, expect north winds around 5-10 kts to prevail through Monday afternoon inland with northeast winds at the Chicago terminals and GYY. Though wind directions will veer towards a southwesterly direction late Monday afternoon and evening.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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