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.The sun rises behind the skyline of midtown Manhattan, the Chrysler Building and the Empire State Building on a day with an extreme heat warning in New York City, July 2, 2026.Gary Hershorn/ABC NewsWith 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.Extreme heat alerts.ABC NewsNew York City's heat index is forecast to reach 110 degrees, which has only happened three times in the last 80 years. The city has activated its heat emergency plan and is opening hundreds of cooling centers."I'm also asking every employer to plan ahead with their teams, build in flexibility, and make sure that no one is put at risk during their jobs this week," Mayor Zohran Mamdani said.On Friday, the heat index is forecast to hit 108 degrees in New York, 105 in Boston, 112 in Washington, D.C., and 111 in Memphis, Tennessee.Popular ReadsThere will be minimal relief overnight, which makes the heat even more dangerous.Holiday heat.ABC NewsOn the Fourth of July on Saturday, the heat will improve slightly for the Northeast, but will still feel like the triple digits, with the heat index expected to reach 103 degrees in New York and 107 in D.C.Visitors react to the heat while attending the Great American State Fair on the National Mall, July 1, 2026, in Washington.Nathan Howard/AP PhotoBy Sunday and Monday, the strongest heat and humidity will settle over the Southeast. Raleigh, North Carolina, is expected to feel like 107 degrees and Savannah, Georgia, will feel like 105 degrees.Extreme heat is the deadliest weather-related hazard in the U.S. At least 13,000 Americans have died from the heat since 2018, according to data from the Centers for Disease Control and Prevention.Click here for tips on how to stay safe.