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.

Dangerously high temperatures, coupled with humidity, could result in heat indices of 100 to 115 degrees from the Midwest to the East Coast, forecasters said. 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. 

“These are extremely dangerous conditions,” New York City Mayor Zohran Mamdani warned the city of over 8 million residents.

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.

“My recommendation to all New Yorkers is to stay inside and stay cool,” Mamdani said, “and if you happen to be getting married at Madison Square Garden, you will be staying inside and you will be staying cool, and I think it’s a good example to set for the city at large.”

In Nashville, where temperatures could reach a high of 101 degrees and feel like 114, heat patrols began handing out cold bottled water to the city’s homeless population during peak hours.

With more than 100 record highs and more than 200 record warm overnight lows expected through Saturday evening, some spots could shatter records that are more than a century old, according to CBS News meteorologist Nikki Nolan. Temperatures will be 20 to 30 degrees above average.

In Washington, D.C., the National Weather Service issued an extreme heat watch for Thursday and Friday, with forecast highs of 103. A high of 101 is expected on Saturday, July 4, which would top the capital’s highest recorded temperature for that date, 100 degrees, set in 1919. 

D.C. Mayor Muriel Bowser urged residents and visitors to plan ahead, stay in the shade and limit time outdoors as D.C. plans to mark the nation’s 250th birthday with all-day festivities and more fireworks than previous years.

“This is your friendly reminder that the sun is not playing around,” the National Park Service said in a post on social media. “As always, we encourage everyone heading outdoors to stay hydrated, take breaks in the shade, wear a hat and sunscreen, sploot responsibly, know your limits, and look out for one another.”

Health hazards of a heat dome

The sweltering conditions are part of what’s known as a heat dome, a large area of trapped heat under very strong high pressure.  The combination of heat and high humidity feels insufferable and dangerous because the human body can’t cool off as effectively when sweat stays on the skin.  

Heat is the leading cause of weather-related deaths, and is especially hazardous for the elderly. During last week’s heat wave in Europe, more than 1,300 additional deaths were reported.

Warning signs to look for to prevent heat-related health emergencies include fatigue, light-headedness, muscle cramps and upset stomach. If anyone exposed to the sun is experiencing such symptoms, doctors say they could be suffering from heat exhaustion.

“For heat stroke, the differential factor is that altered mental status, that confusion,” Dr. Catrina Cropano, the medical director of the emergency department at New York City’s Mount Sinai West, told CBS News. “If you are experiencing any symptoms of heat stroke, you really need to make sure you come into the emergency department as quickly as possible.”