CHICAGO — Cook County residents will experience at least three days of dangerously hot conditions this week, officials have warned, as a heat dome builds over the Midwest and eastern U.S. and as Illinois is hit with its first heat wave of the summer. While chances for thunderstorms later in the week could cool off the area in time for the July 4th holiday, any reduced storm coverage could prolong the hot stretch.

Locals can expect hot, humid weather — like “air you can wear” — starting Monday, said Brett Borchardt, senior meteorologist with the Chicago office of the National Weather Service.

There is, however, already a moderate risk of extreme heat persisting through Sunday in parts of western, west-central and southern Illinois, according to the weather service’s prediction center.

Beginning around noon on Monday and lasting through at least Wednesday night, afternoon heat indices in parts of northwest and northeast Illinois will reach up to 105 degrees. The heat index combines atmospheric temperature and relative humidity to determine how heat feels to the body. Central and southern Illinois are forecast to hit heat indices as high as 108 and 110 degrees, respectively.

“Heat waves have a way of attracting thunderstorms,” Borchardt said. “And storms are nature’s sort of natural air conditioning. They literally cool temperatures.”

While there are indications that storms could roll into the area Thursday and Friday, he added, there is a “very real possibility” that the heat wave lingers on those days too if said storms don’t materialize.

“If we do get thunderstorms — with (that) amount of heat and humidity, they probably would be severe with damaging winds and flash flooding,” he said. “So it’s sort of a ‘pick your poison:’ dangerous heat or severe thunderstorms, in this kind of pattern.”

Daytime highs in and around Chicago will be in the 90s with no cooling effect from the lake, and residents will experience little relief as heat indices remain in the 90s until well after sunset and overnight indices stay over 75 degrees.

“A lot of folks focus on those afternoon temperatures and heat indices, but it’s really the overnight temperatures that can do a lot of damage. That’s when our bodies really struggle to cope, when we can’t cool off,” Borchardt said. “One way to think about it is, if it’s midnight or 3 a.m. and it’s still 75 degrees outside — that’s almost like a summer day.”

These kinds of sticky, muggy temperatures that persist into the night can pose a major risk to the health of anyone without access to effective cooling or adequate hydration. County officials are saying the anticipated heat will combine with “oppressive warmth at night,” leading to “hazardous conditions” — particularly for vulnerable populations like those who are pregnant, newborn babies, children, the elderly and those with chronic illnesses.

The city of Chicago typically offers cooling areas in six community service centers over periods of extreme heat, though these operate from 9 a.m. to 5 p.m. Monday through Friday. Residents needing relief from the heat are also encouraged to visit senior centers, park district fieldhouses, public library branches and City Colleges of Chicago during hours of operation.

It was immediately unclear from several news releases whether the centers would remain open over the July 4th weekend if the extreme heat lingered. A Tribune analysis found gaps in the city’s cooling network, including the fact that in recent years, some of these spaces have closed during hot stretches around weekends and holidays, including the Juneteenth holiday last summer.

Additionally, five of the community centers and all the senior centers were closed for a whole weekend during a heat wave in June 2025. And only the 22 Chicago Police district stations remain open 24 hours during heat waves, but advocates say homeless people, people of color and immigrants may not feel comfortable seeking shelter there.

Borchardt said the adverse health effects of heat are worse when they accumulate.

“A lot of times, the first day or two, we feel like we’re OK, maybe we’re able to make it to the pool or just get some relief in the heat,” he said. “But usually by days three, four and five of a heat wave, that’s when we really start to notice it impacting our bodies, and those are the most dangerous heat waves, the ones that go for a long time.”

Offering overnight and consistent relief for vulnerable populations, however, is becoming more imperative as climate change makes sweltering summer nights more common, extending human exposure to uncomfortable temperatures.

Warmer nights occur when the atmosphere is waterlogged. Clouds reflect incoming heat from the sun into space during the day, but after the sun sets, they absorb heat from the surface and radiate it back toward the ground.

Prolonged exposure to heat can lead to insufficient or poor sleep, compromising the immune system, increasing the risk for cardiovascular disease and diminishing cognitive performance. Heat-related illnesses like heat stroke can also be fatal.

In Chicago, overall summer average temperatures have warmed by 1.9 degrees since 1970 as Midwest summers become more humid, but that’s not the whole story: Average lows on summer nights have increased by at least 2.5 degrees in that same time.

While dry conditions drive up temperatures dramatically, moist air keeps them a bit lower but makes the heat feel worse to humans. This is because, just like greenhouse gases trap heat, moisture holds onto heat in the atmosphere. Rising temperatures, in turn, lead to rising humidity. For every 1.8-degree Fahrenheit increase in temperature, the atmosphere can hold 7% more water.

A curious contributor to humidity in Illinois air is corn sweat: Rising average temperatures from climate change are increasing the rates at which its millions of acres of corn crops release water vapor into the atmosphere as they mature in the summer and saturate the air with moisture. For context, 1 acre of corn can release 3,000 to 4,000 gallons of water daily, and Illinois planted 11.2 million acres of the crop in 2025.

Research has found that the abundance of skyscrapers and asphalt in Chicago traps high temperatures, amplifying the effects of heat by an average of 8.71 degrees, meaning the city has the potential to be that much hotter on a given day, particularly when temperatures are already high.

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