This July 4th could be hottest ever in parts of U.S. Maps show the forecast expected to break records.

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Updated on: July 3, 2026 / 10:22 PM EDT

/ CBS News

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This July Fourth could be the hottest on record for millions of Americans as a massive heat wave traps more than half of the United States under a heat dome through the holiday weekend. The heat wave sweeping the eastern U.S. was expected to peak Friday and Saturday, forecasters said, as dangerously high temperatures continued to ramp up from the Midwest to the East Coast. Between daily high temperatures and warm overnight lows — which won't be low enough to offer much relief — more than 300 records are expected to be set by Saturday.More than 185 million people were under heat alerts Friday, including residents of multiple major metropolitan areas, like Boston, New York City, Philadelphia and Washington, D.C., according to the National Weather Service, which has issued a mix of "extreme heat" warnings and heat advisories for dozens of states. The weather service said peak heat indexes, which account for temperature and humidity, could soar to 115 degrees in some places.High temperatures were already impacting holiday celebrations on Friday, as safety concerns prompted some event organizers to postpone or cancel plans. Amtrak also canceled some Northeast trains because of the potential effects of excessive heat on its tracks.Heat wave forecast mapsZoom in on any location on the map below, or search for your city or town by name, to see the forecast high temperatures, how hot it will feel, and the overnight lows for each date."These are extremely dangerous conditions," New York City Mayor Zohran Mamdani warned the city of over 8 million residents. On Thursday, the city tied the record high temperature of 100 degrees set on the same date in 1966. It could hit triple digits again Friday, with the heat index even higher.An extreme heat warning is in effect in the city until 9 p.m. ET on Saturday.For the first time, New York City is deploying vans staffed with nurses and paramedics who will hand out water, electrolytes and sunscreen, CBS News New York reported. They will also perform wellness checks, transport people to cooling centers and make in-home visits to vulnerable residents. The city is also opening cooling centers throughout the five boroughs.