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Heat Dome’ Warning Expands to 26 States as Thunderstorms Loom

'Heat dome' moving east; 100 million face life-threatening temps 3:48

Warnings of blistering temperatures were expanded Friday to include parts of 26 states as a days-long "heat dome" scorched much of the nation, triggering dangerous thunderstorms.

From Austin to Boston, temperatures were forecast to push triple digits, with the heat index — a measure of how hot is feels — reaching 115 degrees in isolated spots. As many as 122 million Americans were under some sort of heat advisory, in an area that stretches 830,000 square miles.

Photo Gallery: How the Nation is Beating the Heat

And it could be even hotter on Saturday.

"Saturday looks to be the peak day for the heat on the East Coast," NBC meteorologist Bill Karins said. "The end of the heat wave appears to be with a cold front on Tuesday."

Related: What is a 'Heat Dome'? Everything You Need to Know

The intense weather is the result of an atmospheric phenomenon called a "heat dome" — a ridge, or high-pressure system, that traps hot air underneath it — creating unusually hot and humid conditions.

The heat dome is currently sitting over 100 million people, prompting preparations nationwide. In Chicago, workers were inspecting train tracks to make sure the warm weather wasn't causing any problems, and in New York City, cooling centers were being set up.

NBC New York reported a heat advisory was in effect for New York City and its surrounding counties from noon to 7 p.m. Friday. Forecasters said it was expected to feel like 100 degrees there.

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An excessive heat watch was in effect for most of New Jersey, including Warren and Morris counties, and all counties south of them, from 11 a.m. Saturday until 6 p.m. Sunday.

On the National Mall in Washington, D.C., temperatures of 100 degrees or above were forecast, along with stifling humidity.

The intense heat was expected to bring strong to severe thunderstorms, with damaging wind gusts and large hail, to major Northeast cities Friday and Friday evening, including the I-95 corridor, before the storms shift back to the Upper Midwest for the weekend, according to the Weather Channel.

Torrential downpours may cause flash flooding in some areas, it said.

In the Dakotas, a few tornadoes are possible Friday, Karins said.

Much of the nation will start to cool off by Monday, with full relief forecast by Thursday, TODAY meteorologist Al Rokers said.

"But until then, it is going to be brutal," he said.