textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions will persist with peak afternoon heat indices around 100 to locally 110F today and again on Wednesday.

- Chances for thunderstorms increase Thursday and Friday as the hot and humid conditions are pushed south of I-80.

- A break in the humidity may materialize this weekend.

DISCUSSION

Issued at 202 PM CDT Tue Sep 1 2026

The broad ridge of high pressure over the Ohio and Tennessee River Valleys continues to build into the southern Great Lakes this afternoon. Beneath the ridge, clear skies have allowed temperatures to warm into the upper 80s and lower 90s so far and with some diurnal heating still to go highs should top out in the mid-90s before sunset. Dew points have also been increasing into the mid-70s courtesy of some moist advection and evapotranspiration (corn/crop sweat) which as temperature reach the mid-90s should result in peak heat indices around 105F with localized readings around 110F. Thus, a Heat Advisory remains in effect through 9 PM CDT for all of northern IL and northwest IN.

Riding atop the aforementioned ridge in northern IA and southern WI is a shortwave disturbance which has lead to the development of some scattered showers and thunderstorms. With the building subsidence in northern IL, these showers and storms should remain north of the IL-WI line so expect dry conditions to prevail locally through tonight.

After sunset the loss of diurnal heating should allow dew points to taper some and allow temperatures to dip into the lower to mid-70s overnight. This in turn should allow for a brief period of relief tonight before diurnal heating commence Wednesday morning and results in another day of hot and humid weather. That said, with drier air forecast to be overhead Wednesday afternoon dew points may actually be a tad lower with the deeper mixing expected which in turn may keep peak heat indices more into the 100-105F range. While these heat indices are still dangerous, they are just under official criteria for another widespread Heat Advisory. With the uncertainty in dew points have opted to maintain the Heat Advisory from 12-9 PM CDT Wednesday for areas south of I-80 where heat indices are forecast to be the highest, but if confidence grows in higher dew points occurring then this advisory may need to be extended across the rest of the area. Regardless, rain chances on Wednesday look to be virtually zero given the strong subsidence and drier mid-levels keeping the atmosphere capped.

The ridge should begin to break down and shift south and east into the Mid-Atlantic Wednesday night into Thursday which will turn allow the parade of shortwaves rounding it's northern periphery to drift back over northern IL and northwest IN. These shortwaves in combination with the warm, humid, and unstable air mass overhead should result in an uptick in shower and thunderstorm coverage. While there is a chance that the initial shortwave move across southern WI Wednesday night could bring a few showers/storms to northern IL, it seems the lingering subsidence and capping may keep these storms to our north until a secondary shortwave ejects out of the central Plains on Thursday. Therefore, the better coverage of showers and thunderstorms locally looks to be Thursday afternoon and evening as this secondary wave pivots overhead. Given the steep lapse rates, decent MUCAPE, and 40+ kts of effective shear, any storms that develop on Thursday will have the potential to become strong to severe. Since most of the instability should be elevated it seems the main severe hazard will be damaging winds and maybe some large hail, but if storms can become surface based there should be some decent low-level shear in place to generate some supercells and possibly a localized tornado threat. For now the Storm Prediction Center has maintained their marginal (level 1 of 5) risk for all of northern IL and northwest IN which seems reasonable at this time.

Heading into Friday, another mid-level ridge is forecast to begin taking shape over the central CONUS which looks to result in the return of northwest flow aloft over the Great Lakes. While this flow pattern should help to break up the heat and humidity a bit, it will also keep chances for periodic showers and thunderstorms around through at least the end of the week. With uncertainty in guidance on how far east the new ridge will establish, confidence is low on where the greatest coverage of showers and storms Friday onward will be. That said, it does look as if some dry periods will be possible Friday and this weekend so make sure to keep an eye on the forecast if you have any outdoor plans. Regardless, temperatures should settle into the mid-80s for much of the upcoming weekend and with lower humidity keeping heat indices on par with the temperatures. Though, these readings will still be roughly 5-10 degrees above average for early September.

Yack

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 607 PM CDT Tue Sep 1 2026

There are no key concerns to highlight for the 00Z TAF period.

SW winds below 10 kt through the night will gust to around 20 kt for the late morning and afternoon on Wednesday before subsiding again for the evening. Expect VFR throughout the period.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 9 PM CDT this evening for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

Heat Advisory from noon to 9 PM CDT Wednesday for ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106- ILZ107-ILZ108.

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011-INZ019.

Heat Advisory from noon CDT /1 PM EDT/ to 9 PM CDT /10 PM EDT/ Wednesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.