NEW YORK CITY (Reuters) - U.S. cities are breaking decades-old temperature records this week as a heat wave stretches from central to eastern portions of the country, the National Weather Service said on Tuesday, in what officials are warning could become a deadly weather event.

With roughly 80 million people from Indiana to New England under a heat advisory or excessive heat warning, New York Governor Kathy Hochul has activated the state's Emergency Operations Center in response to high temperatures expected to last until the weekend.

"This is a deadly event," she said, one day after the city of Syracuse hit 94 degrees Fahrenheit (34.4 degrees Celsius), topping a record from 1994. "We have seen blizzards, we have seen flooding, we had hurricanes, we had tornadoes. But this heat event is most likely to cause more deaths."

The state's beaches and public pools will open early, in time for people to enjoy them over the Juneteenth holiday on Wednesday, she said.

Chicago registered 97 degrees at Chicago O'Hare International Airport on Monday, which broke a previous record of 96 degrees set in 1957. Cleveland also set a record.

Detroit and Philadelphia, as well as cities in New Hampshire, Connecticut and Maine are also due for record temperatures in the coming days, NWS meteorologist Marc Chenard said.

While it is too soon to say if the heat is driven by climate change, this wave has come sooner than others. Central Maine is running 30 degrees above average, he added.

"It's kind of early in the season to be getting this long of a duration of heat wave for the Ohio Valley and New England," Chenard said. "That's really the main danger or risks from this event."

The sweltering -- and record -- heat, started just days before the official start of summer on Thursday, and is expected to last the rest of the week

Michigan Governor Gretchen Whitmer took to social media to ask that residents watch out for their neighbors, especially the elderly who are more vulnerable to the heat.

"Be sure to check on your friends, family, and neighbors to make sure they're doing all right," she wrote.

(Reporting by Tyler Clifford in New York and Rich McKay in Atlanta; Editing by Josie Kao)

By Tyler Clifford