Heat wave forecast: Dangerous temps to continue into July 4 holiday

A dangerous heat wave is bringing prolonged extreme heat to more than two dozen states across the Midwest, the South and the East Coast.

The heat wave began in the Midwest on Monday and has since spread east, where it's set to last into the Fourth of July weekend.

With holiday travel in full effect, Amtrak canceled over a dozen trains in the Northeast Thursday due to the heat.

On Thursday, the heat index -- what temperature it feels like with humidity -- is expected to hit a scorching 111 degrees in Philadelphia and Washington, D.C., 106 in Boston, 102 in Chicago, 108 in Detroit, and 110 in Nashville, Tennessee.

New York City's heat index reached 106 degrees on Thursday. The actual temperature in Central Park hit 100 degrees, marking the first triple-digit day there since 2012.

The city has activated its heat emergency plan and is opening hundreds of cooling centers.