Sept. 1 was the start of meteorological fall — but also the start of yet another heat wave in the Chicago area. A heat advisory was issued by Chicago's Office of Emergency Management and Communications for "dangerous heat" on Tuesday, Wednesday, Thursday and Friday.
Peak heat indices — reflecting how the temperature feels — are expected to reach 104 degrees Friday due to humidity before the warning expires at 10 p.m., according to the National Weather Service. Temperatures are expected to rise to 95 degrees in northern Illinois and northwest Indiana.
Is it unusual for September to be so hot?
This week’s high temperatures, the result of a heat dome moving into the area, will keep temperatures about 10 degrees above the normal average for late August and early September, said Zachary Yack, a meteorologist with the National Weather Service in Romeoville.
This heat wave is “somewhat unusual and a significant and potentially dangerous late-season event,” said Ashish Sharma, climate and urban sustainability lead at Discovery Partners Institute at University of Illinois Urbana-Champaign. “Chicago’s normal high in this period is about 80 degrees.”
But hot Septembers are not new. In 2015, the National Weather Service answered the question “How unusual is this September heat wave?”
The answer: not that unusual.
“Looking back at the climate statistics since 1980, there have generally been about 3 days per September with highs of at least 90 degrees. This does not happen every year, but is somewhat regular,” the NWS wrote in 2015.
Does it feel hotter because it’s been such a hot summer?
Maybe. In July, Chicago had two heat waves in which the temperatures topped 90 degrees.
“There is probably some heat fatigue for Chicagoans," Sharma said. "Many of us are mentally and physically tired of it.
“Chicago’s July was warmer than normal. This late burst of heat adds to an already demanding summer, especially when people were looking for more pleasant weather in September after schools are reopened."
The heat has taken a toll. In August, Cook County recorded its 13th heat death of the year.
A WBEZ and Chicago Sun-Times analysis of medical examiner data showed that Cook County has had more deaths related to the heat in 2026 than any other year in the last decade. Heat deaths previously peaked in Cook County in 2019 and 2022, with six deaths each of those years.
Does it feel hotter because it’s humid?
Yes. And maybe because of corn.
“When the air contains a great deal of moisture, sweat cannot evaporate as efficiently, and evaporation is the body’s primary cooling mechanism," Sharma said. "That is why an air temperature in the mid-90s can produce a heat index near 105 degrees and locally close to 110.
“The National Weather Service also points to moisture moving into the region and evapotranspiration from crops, sometimes called ‘corn sweat,’ as contributors to the high humidity."
Does it feel hotter because the weather has been more extreme?
Maybe. This summer, the Chicago area has been pummeled by heat waves, tornadoes, storms, flooding and wildfire smoke.
As of the end of August, 207 tornadoes have touched down in Illinois this year. The previous record of 142 was set just two years ago, according to Trent Ford, the Illinois state climatologist. This is the third year in a row that Illinois has experienced a triple-digit tornado season. The state’s average is closer to 50.
The Chicago area also has seen some of the most severe storms in recent years this summer, and they have caused damage to homes, widespread flooding, and for the first time, nearly filled the reservoirs meant to manage stormwater and flooding.
If anyone is craving fall sweater weather, they will have to grab some sunscreen instead.
