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DOWNTOWN BROOKLYN — Wednesday’s heat was awful, but it was just a hint of what’s to come. It’s why the city declared a heat emergency through the weekend, and is urging people to stay in air conditioning, if at all possible, through the 100-plus degree heat that will keep the Tri-state sweltering through Monday morning.

The heat emergency that Mayor Bill de Blasio declared on Wednesday morning is expected to make a variety of resources available to help New Yorkers stay cool under very trying circumstances.

“This is an exceedingly unusual situation,” said Mayor de Blasio at a news conference at the Office of Emergency Management headquarters. “The last time we went over 100 degrees was in 2012.”

His heat emergency declaration opens up more than 500 cooling centers at libraries, NYCHA community centers, recreation centers at city parks and senior centers. Also, sprinklers and fountains in city parks will be left on until dark, and city pools will have extended hours to 8:00 p.m. through the weekend.

Mayor de Blasio also singled out subway platforms as a particular concern. Temperatures can get very high and relief can take a long time, particularly on the weekends. The mayor said it was of greatest concern with temperatures expected to be at their highest — above 100 degrees — on Saturday and Sunday.

“I worry a lot about people on the subway platforms,” he said, referring to the 5.7 million people who ride the subway daily.

Even passengers on the Grand Central 7 train platform, where there are ceiling fans and where incoming and outgoing trains create an indoor breeze, complained about the heat on the platform being stifling.

“Each day, it gets hotter and hotter, because there’s no ventilation,” said one passenger.

Meanwhile, the MTA on Wednesday afternoon demonstrated for local media its methods for maintaining, repairing and testing air conditioning systems on its subway cars.

Still, on Wednesday, this reporter ended up twice, in opposite directions, riding in subway cars with no A.C.

Even when train air conditioning is working, it blows hot air through the roof of each car, which, in turn, fills subway stations, pumping up their temperatures all the more.

It’s yet another reason why Mayor de Blasio included subway heat in his emergency recommendations.

“If you don’t need to go somewhere, don’t go somewhere,” the mayor said. “It’s kind of like we say in blizzards,” he continued. “This is now the heat equivalent of a blizzard, if we’re going up above a hundred degrees. Don’t go out if you don’t need to go out. It’s not business as usual.”

“So normally you would go into the subway and you wouldn’t think of standing there for 15 or 20 minutes,” he said. “It’s gonna feel really bad down there in this situation. If you don’t have to go, don’t go.”