CHICAGO (CBS) — Thunderstorms that moved through the Chicago area have delayed the arrival of a heat wave, with highs now expected to reach only the upper 80s to low 90s on Thursday, but extreme temperatures are still expected to arrive on Friday.

The National Weather Service originally issued an excessive heat warning from noon Thursday through 7 p.m. Saturday, but the start of that warning now has been pushed back to 10 a.m. Friday.

Friday and Saturday will bring the hottest conditions as temperatures will climb to nearly 100. Coupled with high humidity, heat index values are expected to be well over 100 both days, possibly reaching as high as 114.

This will be Lori Lightfoot’s first big test as mayor for handling a heat wave, and she said the goal is to make sure Chicagoans feel safe.

“We are completely unified in our city’s response and efforts to address this heat emergency. Today Chicago’s activating its extreme weather plan. As you can see, it’s all hands on deck,” Lightfoot said at a press conference with he heads of virtually every city agency to discuss preparations for the heat wave.

City officials urged residents to check on their neighbors, and to watch out for signs of trouble from the heat.

“The people most at risk would be the elderly, but also infants and children under 4, as well as people who are overweight or are on particular medications or have underlying illness,” Chicago Department of Public Health acting commissioner Dr. Allison Arwardy said.

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The number one thing you can do to stay cool is to stay hydrated.

The Chicago Fire Department warned people not to open hydrants to cool off. The spray can prevent passing cars from seeing you, and opening hydrants can lower water pressure when there’s a fire.

If you need help with the heat, you can call 311 for assistance, and if you see an unattended child or pet in a hot car, Lightfoot said get them out by any means necessary.

Many people were out at the city’s beaches Thursday morning, getting in their workouts before it gets too hot.

Even after the extreme heat moves in, the lakefront is sure to be a popular spot, where people will go to keep cool by taking a dip in the lake.

Lifeguards will be on duty at all Chicago beaches from 11 a.m. to 7 p.m.

The city also operates cooling centers at six Department of Family and Support Services buildings. Public libraries and 36 Park District field houses also can serve as cooling centers during normal hours.