The central part of the country, meanwhile, enjoyed some relief. A cold front was steadily moving southward and eastward across the country, bringing down the temperatures in the Upper Midwest and Great Lakes, according to the National Weather Service.
But portions of the Central Plains and Mississippi Valley and much of the East Coast were still expected to see temperatures approaching nearly 100 degrees Fahrenheit. The cooler weather settling in Monday and Tuesday will also bring severe storms and heavy rain that could cause flash flooding and produce damaging winds, the agency warned.
The Carolinas up to Maine were expected to see the highest temperatures Sunday. Daytime highs were expected in the mid-to-upper 90s, which, coupled with high humidity, could feel as hot as 110 degrees Fahrenheit (43 degrees Celsius).
Boston has opened up city pools free to residents this weekend as the region could see temperatures again approach 100 degrees, and area police posted a tongue-in-cheek request on their Facebook page.
"Due to the extreme heat, we are asking anyone thinking of doing criminal activity to hold off until Monday," Braintree police wrote as the high temperatures set in Friday. "Conducting criminal activity, in this extreme heat is next level henchmen status, and also very dangerous."
New York City has directed office buildings to set thermostats no lower than 78 degrees (26 degrees Celsius) through Sunday to reduce strain on its electrical grid.
In Philadelphia on Saturday, several hundred people were evacuated from a retirement community because of a partial power outage that officials say may have been heat-related.
Elsewhere in Pennsylvania, nine firefighters were treated for heat exhaustion and six transported to a hospital for treatment while fighting a house fire in sweltering conditions. The Strinestown Fire Company said all of the firefighters were released by the time Saturdays Conewago Township blaze was extinguished.
In New Hampshire, rescue crews helped rescue a 29-year-old hiker after he was overcome by the heat in the White Mountain National Forest on Saturday.
In New Jersey, the Oceanic Bridge over the Navesink River was closed Saturday evening after it got stuck open. Monmouth County officials say heat caused expansion of the metal encasing the drawbridge, which is a popular route for residents and beachgoers.
The heat wave is starting to break in the northern reaches of New England.
A Canadian cold front brought a series of thunderstorms Saturday evening that dropped temperatures across northern Vermont and upstate New York. A heat advisory remains in effect Sunday for southern sections of Vermont, New Hampshire and Maine, where heat indexes could still top 100 degrees.
But in many parts of the country, its not expected to get much better when the sun goes down. Temperatures are expected to remain at or above the high 70s overnight (26 degrees Celsius).
Inland, strong wind and rain were expected to persist Sunday in the Midwest, and a cold front stretching between the Central Plains and the Great Lakes region is forecast to move south. But in addition to lower temperatures, the cold front is expected to carry showers and thunderstorms, which could lead to heavy rainfall and flash flooding in the Midwest.
Storms knocked out power to hundreds of thousands of residents in Michigan and Wisconsin Saturday.
Experts have warned residents in affected areas to limit their time outdoors. The risks are greatest for young children, the elderly and the sick.
On Saturday, the heat wave canceled events across the affected region.
In New York, authorities scrubbed a Times Square commemoration of the 1969 moon landing and an outdoor festival featuring soccer star Megan Rapinoe and musician John Legend.
Horse racing tracks from Maryland to upstate New York cancelled races and pushed back others, including the $1 million Haskell Invitational that went off after 8 p.m. at Monmouth Park in New Jersey.
In Chicago, a 5K run in Grant Park was nixed.
And baseball fans broiled at big-league ballparks across the country. At Great American Ball Park in Cincinnati, "Let It Snow" and other winter-themed songs blared through the stadium PA.