CHICAGO — Stifling heat and humidity will cover the Chicago area from Wednesday morning through Thursday evening, according to forecasters.

On both days, temperatures are expected to reach the mid to high 90s, with afternoon heat indexes between 100 and 110 degrees. The National Weather Service has placed a heat advisory in effect from 10 a.m. Wednesday to 10 p.m. Thursday.

Localized thunderstorms are possible throughout the advisory period, though the weather service predicts they will ramp up Thursday night into Friday and Saturday. Torrential downpours from those storms could result in some localized flash flooding and lower temperatures, according to the weather service.

This pattern of cold weather followed by warmer temperatures is a common occurrence in the Midwest at this point of summer, National Weather Service senior meteorologist Brett Borchardt said. The potential combination of high heat and humidity along with warm overnight temperatures creates concern about heat-related illnesses, he said.

“Even though we get heat waves every year, and we’ve already had a heat wave this year, it’s good not to let your guard down, just to practice those safety tips (of) limiting time outdoors, staying hydrated, even if you don’t think you need water, just continuously drinking water and fluids and checking in on neighbors and loved ones, especially those who may be vulnerable without air conditioning,” Borchardt said.

Overnight on Wednesday, heat indices may not drop below 90 degrees for more than a few hours in Chicago, according to the weather service.

“A lot of people focus on the highest temperature or the heat index during the day, but it’s those warm overnight, low temperatures that can be really dangerous, because their bodies can’t cool off,” Borchardt said.

A cold front is expected to return at the end of the week, which could aid in creating conditions for thunderstorms.

What to know

•Heat wave’s high humidity raises health risks in Chicago and across the country

•30 years after deadly Chicago heat wave, threats persist as climate change elevates risks

•Heat wave shows gaps in Chicago’s network of cooling centers

•Heat waves are hotter and cooling costs are rising in cities such as Chicago, studies show

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