textproduct: Chicago

KEY MESSAGES

- Hot conditions are expected for much of the week; though somewhat cooler near Lake Michigan.

DISCUSSION

Issued at 153 PM CDT Mon Jul 13 2026

An impressive mid-level ridge at over 600 dam over southern Minnesota this afternoon will elongate to the SE while slightly weakening over the area through Wednesday, setting the stage for a warm and humid week. While the humidity is expected to be less intense than what was observed leading up to Independence Day, several days with peak heat index values near or above 100 degrees are expected. Some relief via weak lake breezes is expected each afternoon, but would be confined to within only a few miles of shore.

There are competing factors adding uncertainty in the dew points over the next couple days. Diurnal mixing across a modest cap has resulted in slightly lower dew points compared to some guidance the past couple days. Tuesday's low-level profile does not look all that much different, albeit with overall weaker low-level flow within the surface ridge. Given added evapotranspiration, expectation is that dew points will run a few degrees higher in the low to locally mid 70s. Additionally, with near surface soil quickly drying out after our very wet stretch to start the month, dew points over the core of the Chicago metro may end up several degrees lower (mid to upper 60s) than outlying area. Ultimately, heat index values are expected to peak around 100 degrees Tuesday afternoon (lower 90s near the lake).

Stronger capping, another day with added evapotranspiration, and warmer low-level temps will likely yield the hottest day of the week on Wednesday. Heat index values are expected to peak in the 100 to 105 degree range (mid 90s near the lake).

The upper ridge will shift just far enough south to allow for a potential back door cold front to nose into northern Illinois Thursday afternoon and perhaps force an isolated thunderstorm. If this does not occur, heat index values will again reach around 100 degrees.

No heat-related headlines are currently expected, though a Heat Advisory may be needed areawide on Wednesday if higher dew points yield heat index values at or above 105 degrees. Also, the latest forecast keeps Cook County below Extreme Heat Watch/Warning criteria of two (three) days with heat index values at or above 105 (100) degrees for Tue-Wed (Tue-Wed-Thu).

Gulf moisture will finally begin advecting toward the area as the ridge quickly breaks down Friday and especially Saturday. Steering flow from the Lower Mississippi River Valley will direct any potential convective waves toward the area by Friday as a weak wave traverses WNW flow to the north, leading to the first notable increase in precip potential in several days. A more prominent mid- level wave within strong WNW flow aloft is then progged to bring a potentially larger convective complex across the Great Lakes region Saturday or Saturday night that could affect the local area. Continued broader NW flow aloft by early next week will yield cooler conditions with low-end thunderstorm chances.

Kluber

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1225 PM CDT Mon Jul 13 2026

VFR through the forecast period.

High pressure remains in control and will do so through the next few days. Main item of focus will be lake breeze development and evolution. For this afternoon, the lake breeze steadily pushing inland across the Chicago area and is expected to reach MDW toward 19z if not a bit sooner. Winds will turn easterly behind the boundary. Have sped up the arrival at ORD to around 2030z. Winds at GYY will be variable for a time but should turn steadily from the north or northeast in the next hour or two. Overall, speeds will remain on the light side below 8 or 10 kts behind the wind shift.

Light and variable winds this evening will increase slightly to the 5-8 kt range later Tuesday morning. Lake influence looks to take over earlier in the day at ORD/MDW/GYY. The lake breeze boundary will become better defined into the afternoon and may end up seeing a situation where the boundary lingers across ORD or even retreats slightly toward the lake during the afternoon. Regardless, wind speeds look to remain in the 4-7 kt range but the direction may fluctuate between east and northwest based on boundary positioning.

MDB

LOT WATCHES/WARNINGS/ADVISORIES

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

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

LM...None.