It's going to be a hot July Fourth for much of the country

A National Park Service employee cools off while working near the North Portico of the White House during a heatwave on Tuesday in Washington. Julia Demaree Nikhinson/AP hide caption

A heat wave centered on the Midwest and parts of the East will break temperature records in some places in the coming days as the thermometer pegs out as high as 105 degrees in some places moving toward the July Fourth weekend, forecasters say.

Weather

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As of Tuesday morning, the National Weather Service (NWS) estimates that the extreme heat warnings affect more than 102 million people nationwide, with another 43 million under extreme heat watches. Geographically, the area includes a broad swath of the Midwest, Ohio Valley and Northeast, including Illinois, Indiana, Ohio, Missouri, Iowa, Wisconsin, Michigan, Kentucky, parts of Minnesota, Kansas, Tennessee and Pennsylvania. New Jersey, southern New England, downstate New York (including New York City), and potentially the Washington, D.C.–Baltimore area are also affected, according to the NWS.

The extreme temperatures are being caused by a heat dome – an area of high pressure that acts like a lid, trapping hot air.

"We're looking at temperatures generally 100 to 105" for Thursday and Friday, as the heat wave peaks, according to NWS meteorologist Joe Wegman. He says the mid-Atlantic will experience temperatures 10 degrees to 15 degrees above normal.

"On the East Coast, for sure. We're expecting record-breaking temperatures," he says.

The temperatures will be especially threatening for older people, those with respiratory problems and pets, but "it's the kind of heat that affects everyone of all ages and health conditions," Wegman says.

He says on the actual Fourth of July, Saturday, temperatures will come down slightly.

To stay safe, people should stay indoors whenever possible, keep well-hydrated, and take frequent breaks from any exertion.

Humidity will produce heat indices of 105 to 115 degrees in parts of the Midwest and Mississippi Valley through Thursday and for the Mid-Atlantic and Ohio in the following days, "with the combination of warm overnight lows, generally remaining in the 70s, providing limited relief and prolonged daytime, there is an increased risk of heat-related illnesses, especially for vulnerable populations and those without adequate cooling," according to the NWS.

The heat wave has already forced summer camps and other outdoor activities in the Midwest to be canceled, according to The Associated Press . Monday, as tens of millions of people across the Midwest endured a heat wave that is expected to spread eastward this week, communities opened cooling centers and urged people to take it easy and stay hydrated.

Climate change, caused primarily by burning fossil fuels, is making heat waves hotter and more prolonged than in the past, with the average number of heat waves in the U.S. doubling since the 1980s.

"What we're seeing ... is exactly the kind of thing that we can expect going forward," says Ashley Ward, director of the Heat Policy Innovation Hub at Duke University's Nicholas Institute.

"The baseline is warmer," Ward says. "So, when a heat dome or a high-pressure system sets up, it's now starting from a hotter floor. And so that makes the extreme heat more likely."