Heat dome threatens record July Fourth temperatures

A sprawling heat dome is putting more than 100 million people under extreme heat warnings as the July Fourth weekend approaches, with forecasters warning that parts of the Midwest, Ohio Valley, Mid-Atlantic and Northeast could see record-breaking temperatures and dangerous heat indices as high as 115 degrees.

The National Weather Service estimated Tuesday morning that extreme heat warnings affected more than 102 million people nationwide, with another 43 million under extreme heat watches. The affected area stretches across Illinois, Indiana, Ohio, Missouri, Iowa, Wisconsin, Michigan, Kentucky, parts of Minnesota, Kansas, Tennessee and Pennsylvania, along with New Jersey, southern New England, downstate New York, including New York City, and potentially the Washington, D.C.-Baltimore area.

Forecasters said the heat wave is being driven by a heat dome, a strong area of high pressure that traps hot air like a lid. The combination of high temperatures and humidity can make conditions especially dangerous because sweat does not evaporate as effectively, limiting the body’s ability to cool itself.

“We’re looking at temperatures generally 100 to 105” for Thursday and Friday, when the heat wave peaks, National Weather Service meteorologist Joe Wegman told NPR. He said the Mid-Atlantic could see temperatures 10 to 15 degrees above normal. “On the East Coast, for sure. We’re expecting record-breaking temperatures,” he said.

CBS News reported that more than 300 temperature records could be set by Saturday, including more than 100 record highs and more than 200 record warm overnight lows. CBS meteorologist Nikki Nolan said some locations could break records that have stood for more than a century, with temperatures running 20 to 30 degrees above average.

Humidity is expected to push heat indices to 100 to 115 degrees from the Midwest to the East Coast. The National Weather Service said warm overnight lows, generally staying in the 70s, will provide limited relief and increase the risk of heat-related illness, especially for vulnerable people and those without adequate cooling.

“These are extremely dangerous conditions,” New York City Mayor Zohran Mamdani warned. For the first time, the city is deploying vans staffed with nurses and paramedics to distribute water, electrolytes and sunscreen, perform wellness checks, transport people to cooling centers and make in-home visits to vulnerable residents, CBS News New York reported. Cooling centers are also opening across 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 the temperature could reach 101 degrees and feel like 114, heat patrols began handing out cold bottled water to homeless residents during peak hours.

Washington, D.C., is under an extreme heat watch for Thursday and Friday, with forecast highs of 103 degrees. A high of 101 is expected on July Fourth, which would surpass the city’s record for that date: 100 degrees, set in 1919. Mayor Muriel Bowser urged residents and visitors to plan ahead, stay in the shade and limit time outside as the city prepares all-day festivities and expanded fireworks for the nation’s 250th birthday.

“This is your friendly reminder that the sun is not playing around,” the National Park Service said on social media, urging people outdoors to hydrate, rest in the shade, wear hats and sunscreen, “know your limits, and look out for one another.”

The Associated Press reported that the heat has already forced cancellations of summer camps and other outdoor activities in the Midwest. Communities have opened cooling centers and urged residents to slow down and drink water.

Wegman said older adults, people with respiratory problems and pets face heightened risk, but added that “it’s the kind of heat that affects everyone of all ages and health conditions.” Doctors say warning signs of heat illness include fatigue, light-headedness, muscle cramps and upset stomach. “For heat stroke, the differential factor is that altered mental status, that confusion,” Dr. Catrina Cropano of 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.”