Warnings of blistering temperatures were expanded Friday to include parts of 25 states as a days-long "heat dome" scorched much of the nation, triggering dangerous thunderstorms.
From Nebraska to Ohio and from South to the Gulf Coast, temperatures were forecast to be over 100 degrees, with isolated spots up to 115.
And it could be even hotter on Saturday, the Weather Channel warned.
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"Saturday looks to be the peak day for the Heat on the East Coast," said meteorologist Bill Karins. "The end of the heat wave appears to be with a cold front on Tuesday."
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.
A heat advisory was in effect for New York City and the surrounding counties from noon to 7 p.m. Friday, NBC New York reported.
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.
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.