The heat moved east while still affecting a swathe of the central section from Kansas to the Ohio Valley, the National Weather Service said.
In Newark, N.J., the mercury hit 108 degrees in early afternoon, the highest temperature ever recorded in the city, The New York Times reported. In Central Park, the nation's oldest weather station, the high was 104, the highest temperature in New York City since 1977.
At Dulles International Airport outside Washington and in Baltimore, the high was 105 degrees.
Christopher Vaccaro, an NWS spokesman, told the Times one measure of how hot the summer has been so far in much of the United States is that 1,400 temperature records were broken or tied in July across the country.
About two dozen deaths have been blamed on the heat wave.
New York Mayor Michael Bloomberg asked residents to set thermostats at 79 degrees to conserve electricity.
"Not having electricity would be a lot more uncomfortable," he said on his weekly radio show.
Saturday was expected to be uncomfortably hot and humid in the Northeast as well, with temperatures dropping slightly Sunday.
"The worst part of this heat wave is that lows at night will only drop into the 80s due to the extremely high humidity," senior meteorologist Henry Margusity of Accuweather.com. "This means there will be no time for people to cool off."