Blistering heat and stifling humidity settled in Monday over tens of millions of people from Maine through the Ohio Valley, and forecasters warned that relief was as far off as the weekend -- day or night.
The culprit was a dome of high pressure that effectively turned a quarter of the country into a convection oven. It’s not expected to budge until a cold front shoves it out of the way on Friday.
The heat was expected to be more stifling Monday at the Canadian border than in the Carolinas, and more oppressive along the Great Lakes than along the Rio Grande.
Making matters worse, nighttime lows across much of the Northeast are expected to drop only into the low 80s, compounding the heat for the following day. In the dead of night Monday in Philadelphia, it will feel like 90 degrees.
“If you think you’re going to do a run later in the day or, you know, go into your garden later in the day, you can’t. It’s just too hot,” said Stephanie Abrams, a meteorologist for The Weather Channel.
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Forecasters said the heat would only grow by the day. The forecast high for Philadelphia is 94 on Monday, 95 on Wednesday and 97 on Thursday. For Newark, N.J., the expected high is 95 on Monday, 97 on Thursday and 98 on Friday.
The health commissioner of Philadelphia, where the heat index Monday could reach as high as 103 degrees, activated special summer heat programs, including a hotline for heat emergencies.
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Con Edison, the utility that provides power to New York, said it was prepared for outages and had extra crews on call, although it said a $1.2 billion upgrade after Hurricane Sandy should help keep the juice flowing.
Another worry is a spike in the population of mosquitoes, particularly the kind that carry West Nile Virus.
The season's first West-Nile bearing culex mosquitoes in Connecticut were captured in Norwalk last week, according to the Hartford Courant.
"Unfortunately, the weather conditions we're having, with high heat, high humidity and occasional rain, replenish these sites," state entomologist Ted Andreadis told the Courant. "I think the virus will start to build."
The East wasn’t the only part of the country broiling. The forecast high was 97 for Billings, Mont., 99 for Medford, Ore., and 101 for Boise, Idaho. Those are as much as 10 degrees above average for this time of year.
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Forecasters posted a heat advisory for parts of Massachusetts, Rhode Island, Connecticut, New York, New Jersey and Pennsylvania and warned people to stay out of the sun and check on neighbors and relatives.
They took the more serious step of posting an excessive heat warning — defined as a prolonged period of dangerously high temperatures — for the counties around Philadelphia.
It has already been a hotter July than normal. The average temperature this month has been five to six degrees above normal in Boston and elsewhere and New England, and three to four degrees above normal in New York and Philadelphia.
As far north as Vermont, temperatures were in the 90s. Burlington, Vt., even hit a record high temperature for the day -- 93 degrees, meteorologist Michael Muccilli of the National Weather Service told NBC News. The last time it hit 93 was on July 15, 1955.
People in the area were escaping the heat by going down to Lake Champlain or taking a dip in local rivers and streams, Muccilli said.
Jeff Black of NBC News contributed to this report.