textproduct: Chicago

KEY MESSAGES

- Scattered showers and non-severe thunderstorms are expected through sunset this evening.

- Hot and humid conditions continue Saturday afternoon.

- Scattered thunderstorms are expected to develop along a cold front Saturday afternoon and evening, some of which could be strong to severe.

- Wildfire smoke may return to the area in the wake of the cold front on Saturday and continue into Sunday.

DISCUSSION

Issued at 305 PM CDT Fri Jul 17 2026

Showers with widely scattered thunderstorms continue this afternoon across the area. 18Z DVN RAOB shows ample instability and tall ELs in the region supportive of continued thunderstorm development. However, weaker lapse rates amidst weak flow through the column suggest they should maintain "airmass" or pulse-like shower/storm behavior which has helped limit the thunder coverage and lead to overall shorter duration with individual cells. Renewed development is largely tied to localized areas of convergence such as the lake breeze or any outflow boundaries from nearby decaying showers/storms. The general expectation is for this activity to continue through the remainder of the afternoon with coverage starting to diminish as we approach sunset this evening. Primary hazards include lightning, torrential rainfall leading to ponding in poor drainage spots and roadways, and gusty winds locally up to around 50 mph with the strongest storms.

Focus then turns to Saturday as a mid-upper level shortwave dives across the Great Lakes region. An associated surface cold front is forecast to dive across the local area from midday Saturday through early evening. This will serve as the focus for scattered shower and thunderstorm development. Increasing mid- upper flow with the incoming wave will support a damaging wind threat with the strongest storms. Some uncertainty remains with respect to how soon storms initiate along the front (between ~11am-1pm CDT) owing to an initial period of capping prior to the arrival of steepening lapse rates with the mid-upper wave. Coverage than increases through the remainder of the afternoon from north to south.

Sunday is expected to be dry with the area situated between systems. While Lake Michigan temperatures have warmed into the mid-upper 70s, slightly cooler northeast to east winds off the lake will help keep temperatures cooler than the past several days in the upper 70s to lower 80s.

We are monitoring the potential for another round of strong to severe thunderstorms late Monday into Monday night as another wave dives across the area. A moist and unstable airmass develops across the local area ahead of the associated cold front. Stay tuned.

Smoke:

Southwest surface winds have helped to shift the greatest concentrations of smoke north and east some and improve surface visibilities, however, air quality remains in the "Unhealthy to Very Unhealthy" AQI ranges for much of the area. Gradual improvement may occur through the evening and overnight hours. Smoke may return to at least portions of the area in the wake of the aforementioned cold front Saturday afternoon and evening, with smoke continuing into Sunday.

Petr

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 637 PM CDT Fri Jul 17 2026

Forecast concerns include...

Line of thunderstorms Saturday afternoon. Wind shift to northeast Saturday afternoon. Isolated showers this evening. Wildfire smoke possibly returning Saturday evening.

Isolated showers will be possible through sunset, mainly east and southeast of the Chicago terminals. An isolated thunderstorm will also be possible well southeast of the terminals.

A cold front will move across the area Saturday afternoon and a line of thunderstorms is expected to develop along/ahead of this front in the early afternoon. The earlier the storms develop, the farther north they will occur and opted to include prob thunder mention at RFD and maintained tempo mention for the rest of the terminals, though there is still uncertainty and storms may develop just south of ORD/DPA. This line of storms is expected to be fairly progressive, lasting 1-2 hours at any one location, as it moves south through the mid/late afternoon.

Westerly winds may gust into the lower/mid 20kt range ahead of the front/storms Saturday and then shift to the north/northwest in the mid/late afternoon. Though a shift to the north/northeast is expected for the Chicago terminals as the front moves south down southern Lake Michigan. Gusts into the 15-20kt range will be possible into Saturday evening.

With the return of northeast winds, the wildfire smoke that remains over the northern Great Lakes may return to the local area Saturday evening or Saturday night and given the expected strength of the winds, opted to include smoke mention Saturday evening for ORD/MDW. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT tonight for ILZ003-ILZ004- ILZ008-ILZ010-ILZ011-ILZ019.

Air Quality Alert until midnight CDT Saturday night for ILZ005- ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105- ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until 1 AM EDT Saturday for INZ019.

Air Quality Alert until midnight CDT Monday night for INZ001- INZ002-INZ010-INZ011.

Air Quality Alert from 1 AM Sunday to 1 AM EDT Tuesday for INZ019.

LM...None.