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(CNN) —  

It was close to 90 degrees Friday afternoon when the subway lines in part of New York stopped running and stations became packed with frustrated travelers.

The metro area, home to 20 million people, is one of several major population centers under an excessive heat warning that lasts until Sunday night, making it an especially stressful time to have to find alternative ways home or out to eat.

The subway problems were eventually resolved after a computer problem was fixed, but New York Mayor Bill de Blasio was steamed.

“This kind of meltdown during a heat wave is UNACCEPTABLE,” he tweeted. “The (Metropolitan Transportation Authority) owes every single New Yorker an explanation for this. We’ve known about this dangerous weather for DAYS. There’s no excuse for why they aren’t prepared.”

New York, which declared an emergency because of the heat, is not alone in sweating this weekend out.

Over the next few days, more than 85 % of the lower 48’s population will see temperatures above 90 degrees Fahrenheit, CNN meteorologist Dave He nnen said, and more than half will see temperatures higher than 95 degrees.

About 195.7 million people were under a excessive heat watch or warning or a heat advisory Friday, according to the National Weather Service. The alerts stretched from New Mexico to Maine.

Track the extreme heat

The heat has the potential to break records and turn deadly as temperatures climb over the weekend all along the East Coast and through the Midwest.

CNN

East Coast

High temperatures are expected to scorch the major cities of the East Coast.

Why the heat index matters more than the temperature in this heat wave

Boston, New York, Philadelphia, Baltimore and Washington were under excessive heat warnings on Friday. That kind of warning is issued when the combination of heat and humidity is expected to make it feel like it is at least 105 degrees for a prolonged period.

De Blasio declared “a local emergency due to the extreme heat” in New York. In an executive order that covers 9 a.m. ET Friday to 11:59 p.m. ET Sunday, the mayor is ordering office buildings 100 feet or taller to raise thermostats to 78 degrees in an effort to conserve energy. He also encouraged residents to set their thermostats higher to help reduce energy use.

“We are about to enter a heat emergency, and must do all we can to keep New Yorkers safe,” said de Blasio. “The city government is limiting its energy use to reduce strain on the electrical grid, and now private office buildings will also have to do their part.”

With extreme temperatures through Sunday, the New York City Triathlon has been canceled. The organization says it’s not safe for the athletes or the spectators.

The city is opening about 500 air-conditioned “cooling centers” in public facilities. Public pools will be open an extra hour (until 8 p.m.) from Friday through Sunday, And the city will set up portable drinking fountains at busy pedestrian areas through Sunday.

A construction worker stops to cool off in the water fountains at Canal Park in Washington.
Mark Wilson/Getty Images North America/Getty Images
A construction worker stops to cool off in the water fountains at Canal Park in Washington.

Also canceled in the region: Saturday’s card of horse racing at New York’s Saratoga Race Course, and all races Saturday and Sunday at the Maryland Jockey Club in Laurel – where the heat index will reach 110 to 115.

“The health and safety of our horses and jockeys is our highest priority,” said Maryland Jockey Club President and General Manager Sal Sinatra.

Midwest

Major cities across region – including Chicago, Cincinnati, Cleveland, Detroit, Indianapolis, Minneapolis and St. Louis – are under excessive heat warnings.

In the Wisconsin capital of Madison, where the high temperature Friday was 93 degrees with the heat index of 109, fires at two electrical substations knocked out power to more than 13,000 customers Friday morning – an inopportune time for residents wanting air conditioning relief from the heat.

Detroit took extra measures to make sure residents can get relief from the sweltering heat. The city will open recreation centers on the weekend to give access to air-conditioning, according to a news release.

The mayor’s office and the Detroit Police Department are also checking on homeless people to offer them transport to cooling centers this weekend, the release said.

Detroit officials are urging residents to drink plenty of water, reduce outdoor activities, eat light and check on family and neighbors.

Heat wave made worse by climate crisis

June of this year was the hottest June on record for the world, according to the National Oceanic and Atmospheric Administration. It isn’t clear yet how July will fair in terms of breaking records, but temperatures for this week are high.

Experts say the heat wave is only made worse by the ongoing threat of climate change. According to last year’s National Climate Assessment, the number of hot days in the US is increasing.

Heat waves have also increased in frequency, rising from an average of two per year to six per year in the last five decades. The threat is especially pronounced in the Northeast, where “the frequency, intensity, and duration of heat waves is expected to increase” due to the climate crisis.

The heat wave is canceling other sporting events around the nation, including horse races in New York state and Maryland.

CNN’s Jason Hanna, Robert Shackelford, Rebekah Riess, Rob Frehse, Devon Sayers, Evan Simko-Bednarski, Dylan Miettinen an Brandon Miller contributed to this report.