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As extreme heat descends on New York City, a visitor takes advantage of the cool spray of the fountain at Washington Square Park on Friday. Photo Credit: Malibu Rum

The hottest weekend of the summer has arrived.

A heat wave is on track to hit New York City Friday through Sunday, with temperatures that will feel in excess of 100 degrees.

Friday’s high will hit 90, but the National Weather Service warns it’ll feel like it’s 99 degrees outside. Temperatures are only expected to increase from there, with highs in the upper 90s on Saturday and Sunday. But thanks to the heat index, it’ll feel like it’s nearing 110.

An excessive heat warning is in effect through 8 p.m. Sunday and peak heat index values are expected between 2 and 5 p.m., daily, the weather service said Friday.

Mayor Bill de Blasio declared a heat emergency and signed an executive order directing large office buildings to conserve energy.

"This is a very, very difficult situation. Everyone has to take this seriously," de Blasio said on Friday. "We have not seen weather like this in at least seven years."

The average high temperature for the city this time of year is 84 degrees, NWS meteorologist John Murray said.

Officials warned New Yorkers to stay indoors and use air-conditioning. And for those without it, the city has activated some 500 cooling centers that will be open from 8 a.m. to 5 p.m. through at least Sunday.

The city will also keep swimming pools open longer, from 11 a.m. to 8 p.m., and city-run beaches will be open and staffed with lifeguards until 7 p.m. Friday to Sunday.

Health Commissioner Dr. Oxiris Barbot said that it’s important to “be a buddy” and check in on those most vulnerable to heat-related illnesses, which can kill.

“As the city’s doctor, I want to warn every New Yorker that hot and humid weather like what we are experiencing today and through the weekend can be deadly,” Barbot said. More than 80 percent of heat stroke deaths in recent years involved people in their homes without air conditioning, she added.

New Yorkers were asked to set their thermostats to 78 degrees, a temperature that officials said will lower health risks while reducing strain on the electric grid.

Concerns about the ability of the city’s power network to handle a surge in demand still lingered Friday following last Saturday’s blackout in midtown that left nearly a quarter-million people without power.

Temperatures in New York City are expected to hit the upper 90s this weekend, the National Weather Service said. Photo Credit: Marcus Santos for Newsday

Con Edison has opened an emergency command center to monitor the power grid this weekend, and de Blasio said a representative from his office will be there through Sunday.

"They are giving us consistent information on the capacity levels," de Blasio said of his discussions with Con Edison, however he reiterated that he’s not fully satisfied with the answers he’s received about last weekend’s outage. "This is a situation where, sure trust is hard to come by."

New Yorkers were also cautioned to pass on booze and caffeine and instead drink plenty of water, as well as keep a careful eye on children and pets, especially if heading outdoors.

Anyone feeling the effects of the heat, such as trouble breathing or a rapid heartbeat, is urged to call 911.

The start of the workweek is expected to bring a slight reprieve from the intense heat, with a 50 percent chance of rain and a high near 87 degrees.