OZY Fest, a hybrid music-lecture-food festival that had been expected to draw tens of thousands to the Great Lawn of Central Park over the weekend, had been canceled, as had the New York City Triathlon. Both were called off at the request of city officials.

Extreme heat blanketed much of the country, stretching as far as the Great Lakes and the Texas panhandle. Authorities in Washington ushered homeless people into shelters as the heat wave approached. In Boston, officials declared a heat emergency through the weekend, opening cooling centers and anticipating a rise in 911 calls. In South Dakota, a busy interstate was shut down for hours after the pavement buckled under the heat.

Meteorologists have predicted a miserable weekend, as a dreadful mix of soaring temperatures and high humidity will create heat indexes as high as 115 degrees in some places. Relief is not expected until Monday, when rain is forecast for a large swath of the country.

New Yorkers were worried that the electricity grid would falter and plunge them into muggy darkness, as it had in parts of midtown Manhattan last week. On Friday, ConEd’s president, Tim Cawley, said the utility was “very confident” that the power grid would withstand the heat.

The threat of the heat was serious enough that Mayor Bill de Blasio declared a state of emergency and cleared his presidential campaigning schedule. “This is a very, very difficult situation,” de Blasio said Friday. “Everyone’s got to take it seriously.”

By Saturday, National Weather Service meteorologists said, temperatures in Manhattan could reach 97 degrees. They cautioned it could feel as high as 111. Officials said it was the hottest stretch of weather to hit the region in a summer that had been marked by relentless rain.

New York City officials said they had set up nearly 500 cooling centers for those without air-conditioning to cool off.

This article originally appeared in The New York Times.