textproduct: Chicago

KEY MESSAGES

- Dangerous heat wave will continue through at least Wednesday with daytimes highs in the 90s and peak afternoon heat indices of 100 to 110 degrees.

- Dangerous heat may continue into Thursday, though forecast confidence is lower due to the potential effects from storms.

- Thunderstorm chances will begin to increase late in the week, likely bringing some heat relief for at least some areas.

- While likely not a complete wash out, there will be periodic chances for thunderstorm through the holiday weekend.

DISCUSSION

Issued at 1253 PM CDT Mon Jun 29 2026

A strong upper ridge over the Ohio Valley will move very little through Wednesday setting the stage for a continuation of the heat and humidity. Most guidance depicts 925mb temps warming another 1-2C Tuesday, so high temps will likely be a couple/few degrees hotter than today. A strong low level inversion has limited the depth of mixing today and forecast soundings suggest a similarly strong inversion Tuesday. This strong inversion limiting mixing plus the beginnings of evapotranspiration (ET) season has resulted in dewpoints higher than pretty much all available guidance today. We are about a week away from reaching 50% peak from corn, so while it is still early, it likely is still a contributing factor. Given this, I continued to trend dewpoints toward the high end of the guidance spectrum Tuesday, which could push heat indices locally to around 110F degrees in the afternoon.

By Wednesday, there are signals in the models that deeper mixing could result in somewhat lower dewpoints, but higher air temps. The net result may be heat indices a couple/few degrees lower than Tuesday, but with air temps potentially reaching into the upper 90s any slight improvement in heat indices will be more academic. If guidance ends up being too aggressive in mixing out dewpoints Wednesday, then another day with heat indices of 105 to 110F would occur.

The heart of the upper ridge is expected to drift eastward some Wednesday night into Thursday which would place our area precariously close to the proverbial "ring of fire" convection. Most available guidance keeps convection north of the area through much of the day Thursday, which would result in a 4th consecutive day of heat indices of 100-110F. This would trigger an Extreme Heat Warning area-wide, since the warning criteria includes 4 consecutive days of advisory level heat indices. Pondered the upgrade/extension into Thursday, but opted to hold off for now given some lingering uncertainties with respect to potential convection, convective debris cloudiness, and/or outflow potentially disrupting the heat Thursday. Guidance is very often too far north with convection/convective effects in these type of set-ups, so while the signal is pretty strong for another day of dangerous heat Thursday, still think it is possible that at least part of the area could see some relief Thursday due to convection and/or convective effects.

A more active pattern is expected Thursday night through the upcoming holiday with at least periodic chances for showers and thunderstorms. There will likely be many dry hours, but an MCS or two will likely traverse the region at some point this weekend.

- Izzi

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 630 PM CDT Mon Jun 29 2026

VFR conditions are forecast through the period. Gusty SSW winds this evening will ease with sunset but may remain at least sporadically gusty through the night. Winds then pick back up during the day on Tuesday, particularly in the afternoon, with gusts back into the mid 20 kt range out of the SW. Winds then ease again after sunset.

Petr

CLIMATE

Issued at 252 AM CDT Sun Jun 28 2026

.Daily Records for the Upcoming Week........

Chicago (KORD) Record High Record Warm Minimum June 29 97 (1954) 80 (1931) June 30 99 (1953) 80 (2018) July 1 103 (1956) 78 (1931) July 2 99 (1970) 79 (1911) July 3 102 (1949) 78 (1911) July 4 102 (2012) 80 (1921)

Rockford (KRFD) Record High Record Warm Minimum June 29 103 (1931) 75 (1931) June 30 102 (1931) 76 (2018) July 1 101 (1931) 76 (1970) July 2 99 (1910) 74 (1970) July 3 99 (1925) 75 (2012) July 4 102 (2012) 77 (1977)

NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 10 PM CDT Wednesday for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107- ILZ108.

Extreme Heat Warning until 10 PM CDT Wednesday for ILZ103- ILZ104-ILZ105.

IN...Heat Advisory until 10 PM CDT /11 PM EDT/ Wednesday for INZ001- INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 10 PM CDT this evening for the IL nearshore waters.