textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms ending this evening across the southern cwa.

- Wildfire smoke is expected to move in behind the front this evening and overnight resulting in reduced visibilities and poor air quality through at least Sunday.

DISCUSSION

Issued at 221 PM CDT Sat Jul 18 2026

A cold front is draped from central Lower MI to east-central IA and is progressing southward across northern IL and northwest IN. Scattered thunderstorms have developed along the front, especially in northwest IN, and that trend is expected to continue as the front moves through the rest of the area into this evening. Given the weaker wind shear present (around 25-30 kts) most of these storms are expected to remain sub-sever, but a few stronger to possibly severe storms cannot be ruled out particularly for areas south of I- 80 and east of I-57. The cold front and the associated thunderstorms are expected to exit our area between 7-8 PM CDT this evening with otherwise dry conditions expected for the rest of the night.

Behind the front, north-northeast winds will begin to usher in another plume of wildfire smoke. While the latest smoke guidance continues to show the concentration to be less than what was seen the past couple days, upstream observations are showing visibilities in the 1-3 mile range across northern MI and WI. So expect the plume of smoke to result in at least some localized reduced visibilities (most likely in the 2-5 mile range) through Sunday in addition to poor air quality. In fact, the IL EPA and IDEM have extended the Air Quality Alerts for our entire forecast area through Sunday night to account for the forecast unhealthy air quality. Aside from the smoke and poor air quality, the weather on Sunday will be otherwise tranquil with east-northeast winds around 5-10 mph and temperatures in the lower to mid-80s. Though with the onshore winds highs near the lake will only be in the mid to upper 70s.

Heading into Monday, a couple of shortwave troughs are forecast to be ejecting out of the northern Plains and diving across the Great Lakes. The leading shortwave is expected to move into the mid- Mississippi Valley early Monday morning and is currently forecast to have a cluster of thunderstorms associated with it. Should this storm cluster develop as most guidance (both deterministic and handful of ensemble members) suggests, it should traverse across MN and IA before dissipating. In its wake guidance suggests that the cluster will lay out an outflow boundary which will serve as the focus for additional showers and thunderstorms Monday afternoon and evening as the second shortwave pivots into the area. If the aforementioned track occurs as forecast then any new storms on Monday should develop mainly to the west of our area which in turn would give us another dry afternoon. However, if the cluster either fails to develop or the outflow is able to drift further east, then a the favored axis of showers and thunderstorms Monday afternoon/evening would materialize more into northern IL and maybe northwest IN. Given the deep layer shear around 30-35 kts, any storms that develop overhead on Monday could become strong to possibly severe. While most guidance has trended towards the western solution, there is still enough guidance members that show something moving through our area to warrant the maintenance of the 60-70% POPs offered by the NBM. That said, the thinking is that these POPs are likely too high and hopefully we can reduce them in future forecasts if the westward trend continues.

Regardless of what happens during the day on Monday, it does seem that a period of scattered showers may develop Monday night as the shortwave moves through the Great Lakes. By Tuesday any showers (or storms) should move out of the area as high pressure is expected to begin building in from the north. This high is then forecast to persist through much of next week which should keep any rain chances to a minimum. However, winds next week look to remain out of the east-northeast for most days which should keep daily high temperatures in the mid to upper 80s with overnight lows in the 60s. Unfortunately more rain chances do look to return heading into next weekend as another series of shortwaves dives through the region.

Yack

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 628 PM CDT Sat Jul 18 2026

The only forecast concern this period is the return of wildfire smoke and associated cigs/vis.

An enhanced lake breeze/cold front is moving west across northeast IL early this evening, shifting winds to the northeast. Speeds in the 10-15kt range will slowly diminish through the mid evening hours and some gusts into the 15-20kt range are possible for the next few hours. Speeds will gradually diminish under 10kts overnight and then increase back toward 10kt again Sunday.

The northeast winds are spreading wildfire smoke back into the area and the limited available guidance for smoke movement shows the wildfire smoke continuing to thicken this evening and especially overnight into early Sunday morning. While confidence is low, opted to go with prevailing 2sm during the overnight hours for the Chicago terminals and then lift vis back into mvfr Sunday morning. There is some potential for vis to improve back to vfr later Sunday afternoon into Sunday evening. If the vis does drop into the 2sm range, there may be a similar cig response as there was a few days ago, with mvfr cigs possible. Opted to included scattered mention for now. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until midnight CDT Sunday night for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103- ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Beach Hazards Statement through Sunday morning for ILZ006- ILZ103-ILZ104.

IN...Beach Hazards Statement through Sunday morning for INZ001- INZ002.

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...Small Craft Advisory until 4 AM CDT Sunday for the IL and IN nearshore waters.