textproduct: Chicago

KEY MESSAGES

- Hot conditions will continue through the week with high temperatures generally in the upper 80s to 90s and overnight lows in the 70s.

- A period of reduced visibility due to smoke may occur on Thursday generally along and northeast of Interstate 90.

- An isolated storm possible on Thursday, with increasing storm chances Friday and then again late Saturday afternoon into Saturday evening.

DISCUSSION

Issued at 205 PM CDT Wed Jul 15 2026

High pressure continues to reside across the Great Lakes and Upper Midwest regions which has set up another warm and humid summer afternoon. Despite the hazy skies aloft and some diurnal cumulus, temperatures have warmed into the upper 80s and lower 90s thus far and should warm another few degrees before all is said and done. The exception will be near the lake where a lake breeze is oozing inland and should keep temperatures a bit cooler in the mid-80s. Dew points have also been mixing down once again which is keeping peak indices around 100 degrees which is solidly below local heat advisory criteria. That said, with the lack of wind these temperatures will feel oppressive so be sure to stay hydrated and take frequent breaks if out and about.

Tonight, temperatures will once again cool into the lower to mid-70s overnight with light and somewhat variable winds. There will also be increasing smoke coverage, especially aloft, as the plume that has been blanketing northern WI and MI drifts southward. While hi-res guidance continue to depict some surface smoke concentrations making it into portions of northern IL and northwest IN, the inland and southward extent of the smoke remains unclear. That said, any surface smoke would result in periods of reduced visibility (possibly as low as 2-3 miles) in addition to poor air quality. Thus, the IL EPA an IDEM has extended the Air Quality Alerts to include most of our IL and IN counties (except for Kankakee, Livingston, Ford, Iroquois, and Benton counties) through Thursday night.

Some concentration of wild fire smoke is expected to persist through the day on Thursday as north-northeast winds aloft persist and the high pressure center begins to drift into the southeastern CONUS. Despite the smoke, temperatures are still expected to warm into the lower 90s for much of the area. However, east-northeast winds near the lake will keep temperatures in the low to mid-80s there. With cooler temperatures and slightly lower dew points, peak heat indices will be more in the 95-100 degree range. Furthermore, there also remains a low (15-20%) chance for some isolated thunderstorms as the shortwave currently over the southern Mississippi Valley drifts north. With deep moisture return ahead of the wave expected to be rather limited, the lingering subsidence from the high, and recent forecast soundings showing some capping in place suspect most areas will remain dry. Heading into Friday, the aforementioned high will be well out of the area allowing the northwest flow aloft to pivot another weak shortwave into the Great Lakes in addition to the lingering wave to our south. The forcing from these two features in combination with deeper moisture is expected to result in a broader coverage of scattered showers and thunderstorms on Friday especially during the afternoon and hours. Given the weak deep layer shear, thunderstorms on Friday should remain sub- severe. Though, the humid air mass overhead will allow any storms to be capable of locally heavy downpours which could result in ponding issues if storms propagate over the same areas for extended periods of time. While a brief break in the rain is expected Friday night into early Saturday, another (and much deeper) shortwave is expected to dive across the Great Lakes Saturday afternoon which will result in an additional period of scattered showers and thunderstorms. Similar to Friday, the kinematic environment looks rather weak so not expecting much in the way of severe weather but locally heavy downpours will be possible.

In the wake of the Saturday wave, high pressure is forecast to build in for Sunday and result in a period of more tranquil weather. Temperatures will still be on the seasonably warm side with highs in the mid to upper 80s, but with winds expected to be out of the east- northeast cooler conditions are expected near the lake. Unfortunately the quiet weather doesn't look to last long as more shortwaves are progged to pivot through the region early next week and bring with them additional chances for showers and thunderstorms. Regardless, temperatures will remain in the near to above average range with daily highs in the 80s to lower 90s and overnight lows in the upper 60s to 70s.

Yack

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1255 PM CDT Wed Jul 15 2026

Key aviation forecast messages:

- Potential lake breeze this afternoon at ORD/MDW/GYY

- Low confidence in wind trends on Thursday which may impact timing of any VSBY reductions in smoke

West winds around 5-10 kt are ongoing early this afternoon across the area. We are monitoring the lake breeze yet again today with continued lower confidence on its inland extent through early evening. Have maintained the 20Z MDW and 21Z ORD timing for an east wind shift with this update but it remains possible that it stalls just east of the terminals again. Winds then ease with sunset, becoming light and variable through early Thursday morning. Confidence in prevailing wind directions during the day on Thursday remains low, however, with guidance varying from either west southwest to east northeast This would also have implications regarding the timing of any potential VSBY reductions associated with the wildfire smoke to our north which may drift into portions of the area at times on Thursday, though any potential MVFR/near-IFR VSBYs may not arrive until late Thursday afternoon/early evening in the wake of a reinforcing lake breeze.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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

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

Air Quality Alert until 1 AM EDT Thursday for INZ019.

LM...None.