A “prolonged, dangerous heatwave” was sweeping across parts of the central and eastern United States on Thursday, with forecasters warning that extreme record-breaking temperatures are expected to continue into the holiday weekend for millions of Americans.

The National Weather Service (NWS) warned on Thursday that temperatures between 95F (35C) and 105F (40.5C), combined with high humidity, will push heat index values across parts of the region to between 100F and 115F.

“This will lead to widespread areas of major to extreme heat risk from the Midwest to the East oast” the NWS said. “Numerous daily temperature records are expected today Friday and Independence day, with some consecutive-day, monthly and all-time records possible.”

The agency said on Thursday afternoon that extreme heat is expected to “continue across most of the central and eastern US through Friday” then it will focus “across the eastern US through the Independence Day holiday weekend”.

“Peak heat indices of up to 115 degrees are possible” the forecasters added.

The NWS warned that “prolonged periods of extreme heat are among the deadliest weather hazards” in the US, “because impacts build over time”.

“Protect yourself by limiting time outdoors, staying hydrated, and ensuring access to A/C or cooling centers,” the agency said.

As of Thursday morning, numerous areas across the Mississippi valley, Great Lakes, Ohio valley, and north-east are under extreme heat warnings, which are issued when “extremely dangerous heat conditions are expected or occurring”.

In New York City, temperatures reached 100F in Central Park on Thursday afternoon, marking the park’s first triple-digit day since July 2012. Forecasters in New York warned that “dangerous heat and humidity” is expected through Saturday, with heat index values forecast to reach “as high as 115F”.

The NWS in New York also warned that thunderstorms are “possible Friday evening, and a few may become severe with damaging winds, heavy rainfall, and localized flash flooding”.

New York City mayor, Zohran Mamdani, urged residents to take precautions.

“Please stay prepared and informed as these dangerously high temperatures continue,” he said, adding that Friday’s heat index was expected to reach 110F. “Plan for extreme temperatures. Drink more water than usual. Reach out to vulnerable neighbors who may need help finding resources.”

Mamdani also announced that the city was deploying cooling vans staffed by medical personnel, who are set to “provide wellness checks, medical care, water and other supplies such as electrolytes and sunscreen, as well as transport to cooling centers or healthcare facilities”.

Kathy Hochul, the New York governor, also said on Thursday that the state’s electric grid was under “heavy strain as this heatwave drives up energy demand”. She encouraged New Yorkers to “voluntarily conserve power by setting your A/C between 75F–78F, avoiding unnecessary appliance use, and turning off lights and electronics when they’re not needed”.

“My team is coordinating with Con Edison, NYISO, and New York City leaders to reduce demand across the grid” she added.

Meanwhile, local media in New York City reported on Thursday that multiple shark sightings had caused intermittent closures at Rockaway Beach in Queens.

Across the mid-Atlantic, forecasters warned that “heat indices up to 110F are possible” on Thursday, and said that on Friday and Saturday, afternoon “showers and thunderstorms” are also possible “as heat persists” and “some storms could become severe”.

The NWS office serving Baltimore and Washington DC said on Thursday afternoon that additional extreme heat warnings and heat advisories had been issued for Saturday.

Muriel Bowser, the mayor of Washington DC, urged residents to factor the extreme temperatures into their Fourth of July plans.

“Consider the heat as you make plans for the 4th of July and America250,” she said, before encouraging residents to monitor weather alerts and safety guidance.

She advised residents to avoid strenuous outdoor activity, to drink plenty of water, dress for the heat, wear sunscreen and to limit time outdoors whenever possible.

Weather officials for parts of New Jersey and the Philadelphia area said that Thursday and Friday were expected to be the hottest days of the week there, but warned that “dangerous heat will continue through Saturday”.

Temperatures reached 102F in Philadelphia on Thursday, officials said. And it was reported earlier this week that organizers of the city’s “Salute to Independence Semiquincentennial Parade” have decided to shorten the route due to the extreme heat.

Weather officials for parts of Kentucky, Missouri, Illinois and Indiana, warned on Thursday the heat index value could get up to 110F that afternoon and again on Friday.

The heatwave in the US comes just days after parts of Europe endured unprecedented, record-breaking high temperatures, which brought dangerous conditions to parts of the continent.