A heat dome is expected to build over the eastern two-thirds of the country, with New York City , Philadelphia, Baltimore and Washington, D.C., expected to see record highs into the 100s, according to CBS News meteorologist Nikki Nolan.
Extreme heat warnings are in effect for major cities, including Nashville, St. Louis, Cincinnati, Kansas City, Des Moines, Chicago, and Detroit. Extreme heat watches are also in effect for much of the Northeast, with temperatures over 100 degrees expected through Saturday.
Southern cities, including Dallas, Little Rock and Memphis, will also see dangerously high temperatures. More than 250 record-warm overnight lows are also expected through Saturday night.
High heat is forecast last into next weekend across the Great Plains, the Southeast and the Mid-Atlantic.
CBS News/Nikki Nolan
The heat wave — defined as high temperatures reaching or exceeding 90 degrees for three consecutive days or more — is expanding across the central and eastern U.S.
Meanwhile, in the West, the Four Corners area remains under dangerous fire weather conditions. Three firefighters have died and two sustained burn injuries when they were overcome by flames from fast-moving wildfires in hot, windy conditions near the Colorado-Utah border.
The National Fire Information Center described the condition as “extremely critical fire weather” that is rare for late June, with “high potential for new large fires.”
At the same time, severe thunderstorms continue across the northern Plains and Upper Midwest, with flooding concerns stretching from the Rockies into the Mid-Atlantic.
As temperatures climb, high humidity will lead to the heat index reaching 100 to 110 degrees F (40 C to 43 C), and as high as 115 F (46 C), according to the weather service. Nightly lows in the 70s F (21 to 26 C) won’t provide much relief.
The heat index, which factors in humidity and is included on many weather forecasts, provides a sense of how hot it really feels — and what’s dangerous for prolonged exposure or strenuous activity.
CBS News/Nikki Nolan 
Parts of the U.S., especially Phoenix, Las Vegas, central Texas, and much of the Southwest, were already experiencing temperatures around 100 F.
The first widespread heat wave of the summer will put millions of Americans under a heat dome, or areas of very strong high pressure that trap heat over a large area. The stagnant weather pattern results in weak winds and increased humidity. The phenomenon can last for days to weeks, making the heat feel insufferable and dangerous because the human body can’t cool off as effectively if sweat stays on the skin.
During extreme temperatures, people are advised to limit outdoor activity, stay hydrated and ensure access to air-conditioning and other cooling areas, the weather service said.
The Centers for Disease Control and Prevention says heat can impact anyone. Signs of overheating include muscle cramping, dizziness, unusually heavy sweating, shortness of breath, headaches, weakness and nausea.
The heat comes after Europe has been sizzling for days, with Germany seeing record-high temperatures over the weekend. Switzerland, the Czech Republic, Denmark and France have also been facing high temperatures. In France, dozens of drowning deaths were recorded this month as swimmers turned to the water for relief from the heat.
“Of all climate-related impacts and hazards, heat is the biggest killer,” Ine Vandecasteele, an urban adaptation expert with the European Environment Agency, told CBS News.
Vandecasteele said simply installing more air conditioners can offer vulnerable people like those in hospitals and elderly people immediate relief, but she said that isn’t a long-term solution.
“In the longer term, what happens is, installing more air conditioning actually emits more heat into our environment, so it will actually increase the speed of warming,” she said.
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