– The East Coast on Sunday sweated through another day of extreme heat and humidity as organizers in Boston canceled a benefit run, Delaware Civil War re-enactors got the day off and the New York Police Department implored residents to take it easy.

"Sunday has been canceled," the NYPD jokingly tweeted. "Stay indoors, nothing to see here. Really, we got this."

The central part of the country, meanwhile, enjoyed some relief as a cold front moved steadily southward and eastward across the country, bringing down the temperatures. But the cooler weather settling in Monday and Tuesday is also bringing severe storms packed with powerful winds and heavy rains that have already caused damage in the Midwest. The National Weather Service warns flash flooding might be possible in some areas.

From the Carolinas to Maine, daytime highs reached the upper 90s Sunday. Coupled with high humidity, temperatures felt as hot as 110 degrees Fahrenheit in places.

"There's no point being out," Washington, D.C., bus driver Ramieka Darby remarked while taking a quick break amid temperatures of nearly 100 degrees.

In New York City, where all eyes were on the power grid even before the hot weather following a Manhattan blackout last weekend, electricity company Con Ed reported scattered outages Sunday, the vast majority in the borough of Queens.

The city also directed office buildings to set thermostats no lower than 78 degrees through Sunday to reduce strain on its electrical grid. A day earlier, a commemoration of the 1969 moon landing planned for Times Square and an outdoor festival featuring soccer star Megan Rapinoe and musician John Legend were called off because of the heat.

In Boston, the Jimmy Fund 5K cancer benefit race and a popular Sunday market in the South End were nixed. Officials opened up city pools free to residents as the temperature topped 90 for the third day. Police in a Boston suburb playfully posted on Facebook, "Due to the extreme heat, we are asking anyone thinking of doing criminal activity to hold off until Monday."

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 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, a popular route for residents and beachgoers.

The heat even prompted Delaware officials to close Fort Delaware State Park, which served as a Union prison camp during the Civil War. Temperatures were simply too high for costumed interpreters, who wear wool garb, to work safely.