The brutal heat wave that has broiled the country this week has saved its worst for last, smothering millions in triple-digit conditions meteorologists called "dangerous."
Temperatures could spike in the high 90s in New York, Boston and Philadelphia on Friday, with the heat index— the "feels like" effect fed by humidity—pushing 110.
"The cities have excessive heat warnings in effect. … Some people don't have air conditioning, so that's going to be an issue especially for the elderly and younger children as well," said Michael Palmer, meteorologist at the Weather Channel. "We could see a few records in spots.
He said of the conditions, "They are dangerous."
Relief is in sight, with cool breezes from the north expected to blast the dome of high pressure which parked itself over the Ohio Valley, causing the near-record highs.
But the break will come at a price. Severe thunderstorms and hail arrive late Friday.
"We'll see a line of storms that will produce some winds that could gust at least 60 mph, we could see some golf ball-size hail in spots," said Palmer.
The worst of the storms, including damaging straight-line winds, hail, and perhaps a tornado, will come in Michigan, eastern Wisconsin, northeast Illinois, far northern Indiana and upstate New York.
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The extreme heat has already claimed the life of a 57-year-old man in Philadelphia, city officials told NBC Philadelphia there. Authorities say the man, who had an air conditioner that was not operating, died from chronic obstructive lung disease and heat exposure. Temperatures reached 98 in Philadelphia on Thursday; the heat index was 106.
Christopher J. Todd, 30, of Rochester, N.Y., also died while hiking in the White Mountains, officials at The New Hampshire Fish and Game Department said Thursday. His friends tried to revive him until the emergency services arrived, but he passed away on the trail.
The department said the cause of death has not yet been determined, but cautioned hikers to stay well hydrated while outdoors.
The heat wave has flummoxed meteorologists, because it has moved backward across America, something that rarely happens.
Normally U.S. weather systems move west to east. The western Atlantic high pressure system behind the hot dry weather started moving east to west last week and by Tuesday was centered over lower Michigan, said Jon Gottschalck, the operations chief at the National Weather Service's prediction branch.
"It's definitely unusual and going the wrong way," Gottschalck said Thursday. "This is pretty rare."
The weather service issued heat advisories and warnings Thursday for parts of 23 states. More than 141 million people live in those areas.
In New York, Gov. Andrew Cuomo urged people to cut back their electricity usage to take pressure off the state's power grid and help prevent outages or brownouts, NBC New York reported.
New York's power grid operator said it expected a record set in 2006 would be broken with a peak in electricity usage.
—By NBC News Contributor Henry Austin.