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 /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1152 PM CDT Mon Jul 13 2026

VFR conditions are expected through the TAF window. Winds are expected to be light and out of the west tonight, turning northwest after daybreak. There remains a signal for a weak lake breeze to develop once again. However, being so weak, there is another chance that it washes out prior to arriving at KORD (similar to Monday) keeping winds out of the west through the overnight.

DK

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.