Record heat wave threatens July 4 events across the United States

Peak heat indices could reach 115 degrees as dangerous, record-breaking heat blanketed much of the central and eastern United States for Independence Day, and the National Weather Service forecast the worst of it would continue through Friday before shifting into the eastern U.S. through the holiday weekend.

The Weather Prediction Center forecast high temperatures of 95 to 105 degrees, combined with humidity, would push heat index values to 100 to 115 degrees across parts of the East. More than 87 million people were under extreme heat risk, and more than 150 million were under heat alerts stretching from Kansas to Maine. Philadelphia was headed toward 103 degrees, just shy of its all-time July record of 104, while Central Park in New York City hit 100 degrees Thursday, its first triple-digit reading since 2012.

The heat already disrupted the July Fourth calendar. Philadelphia canceled its Wawa Welcome America parade, and Washington's Great American State Fair temporarily closed Friday afternoon.

America250 has been planning the largest synchronized Fourth of July celebration in U.S. history to mark the 250th anniversary of the signing of the Declaration of Independence. Its America's Block Party livestream was scheduled to begin at 9:45 a.m. ET on July 3, alongside a benefit show at the Los Angeles Memorial Coliseum and local celebrations across the country.

New Jersey Transit warned of delays of up to 30 minutes and select train cancellations because of extreme heat on equipment. Amtrak said Northeast Corridor trains could run at reduced speeds between 11 a.m. and 7 p.m. through Saturday. Severe thunderstorms were also possible across parts of the Midwest, Northeast and mid-Atlantic, with damaging winds, isolated tornadoes and flash flooding as fireworks displays and holiday travel moved into the evening.