Rye, Harrison and Rye Brook are facing upper-90s heat with a 100-degree heat index Tuesday and Wednesday, prompting Westchester's health commissioner to warn residents to stay indoors and hydrated.

"Heat stroke and dehydration can surprise you," said Dr. Sherlita Amler, the county's health commissioner. "The elderly, young children and those with high blood pressure, heart disease, or lung conditions should be especially careful to avoid heat-related illnesses. High humidity and some medications can also increase a person's risk for heat stroke."

Humidity should ease Thursday and Friday before returning over the weekend, according to the National Weather Service. Gov. Kathy Hochul echoed the warning statewide Tuesday, noting extreme heat is the leading cause of weather-related deaths in the U.S. The state is monitoring the power grid as air conditioning demand climbs, with about 5,500 utility workers on standby in case outages hit.

Drink two to four glasses of water an hour, even without feeling thirsty, and skip caffeine, alcohol and sugary drinks — they speed up fluid loss. If you have to be outside, wear SPF 30+ sunscreen, a hat, and light, loose clothing, and take frequent shade breaks. And never, even for a moment, leave a child or pet in a parked car — interior temperatures can turn life-threatening within minutes.

Find a nearby cooling center through the state's Cooling Center Finder at health.ny.gov/environmental/weather/cooling — just call ahead, since some only open during active heat events. If nothing's listed nearby, even a few hours at an air-conditioned library, supermarket or mall can help your body recover.

Heat stroke shows up as hot, dry skin, shallow breathing, a rapid weak pulse, and confusion. Call 911 immediately if you suspect it, and start cooling the person down while help is on the way.

Residents can sign up for real-time weather alerts by texting "Westchester" to 333111.