Mehdi Taamallah / AFP - Getty Images Women sun themselves on Washington Square Park in New York City on Tuesday as a heat wave descended on the city.

The stifling heat wave in the East stretched New York’s emergency crews, triggered power outage and made for potentially dangerous overnight temperatures -- and the worst may be to come.

Temperatures Tuesday hit the mid-90s for another day across the Eastern U.S., and when combined with high humidity it felt a lot more like 100-plus degrees. But the spiking mercury wasn't limited to the Atlantic coast: in all, 43 out of 50 states were near record highs.

New York emergency crews were dealing with a 20 percent increase in emergency calls because of the heat, NBCNewYork.com reported.

High temps have been scorching the East, taking hold in cities where asphalt and buildings hold on to the heat. Adding insult to injury, Maryland's Prince George's County, a water main break will interrupt service for days. NBC's Stephanie Gosk reports.

An 18-year-old city council intern who fainted during an outdoor press conference on trash management had to wait 30 minutes to receive treatment. Fire department officials blamed the delay on the uptick in heat-related 911 calls.

Other New Yorkers reported scattered power outages as air-conditioners were cranked up. Con Edison said it had put crews on standby for the duration of the heat wave.

In Philadelphia, high temperatures spiked into the 90s for a third straight day to make the area's steamy stretch an official heat wave according to the National Weather Service.

NBC10's Chief Meteorologist Glenn Schwartz points out that overnight lows, which were in the 80s, pose the most danger since older brick buildings without air conditioning aren't able to cool and dangerous heat continues to build up, which could lead to heat-related illness, and possibly death.

Philadelphia health officials set up a “heatline” for elderly residents to receive help during the scorcher.

Still, with almost the entire country covered in red on the weather map — the Pacific Northwest and Mountain states were baking, too — forecasters warn that the East wouldn’t get a break until Friday or Saturday.

“Every day gets a little worse," Tony Gigi at the National Weather Service office in Mount Holly, N.J., told Philadelphia Inquirer. "It gets worse on Friday. Saturday looks miserable."

Weather Channel meteorologist Jim Cantore explains the heat gripping the Northeast, Ohio Valley and Great Lakes will not last into the weekend as cooler air begins to march in.

Still, with almost the entire country covered in red on the weather map — the Pacific Northwest and Mountain states were baking, too — forecasters warn that the East wouldn’t get a break until Friday or Saturday.

“Records may be broken each day,” said Kevin Roth, lead meteorologist for The Weather Channel. He said humidity and high temperatures would make it feel as hot as 105 degrees in the Northeast.

In Maryland, where the heat index was expected to reach as high as 105 every day this week, a failing water main meant that people in Prince George's County, Md., will have to go several days without water.

With a heat index near 105 degrees, a heat advisory was issued for Washington, D.C., the Virginia cities of Arlington, Alexandria and Falls Church from noon until 7 p.m. Tuesday, NBC Washington reported.

Gary Gumm, the Chief Engineer for the Washington Suburban Sanitary Commission, urged people to stock up on water as the supply could be out for anything from two to five days.

"We're going to try to leave it in service for as long as we can so that our customers have the ability to prepare by stocking up with water," he told NBC Washington.

He added that county emergency services will be working with hospitals, nursing homes and others to ensure they have access to water.

John Brecher / NBC News A man pours water on his head to cool off during hot weather in Manhattan on Monday.

New England's power grid operator told The Associated Press that demand for electricity could close in on a seven-year-old record as people tried to cope. ISO-New England says the record for power use was 28,130 megawatts on Aug. 2, 2006. ISO said Monday that demand for 27,900 megawatts is possible on Thursday.

Roth blamed the high temperatures on a dome of high pressure sitting in the Ohio Valley.

“There’s a storm in the southern plains, and that’s normally where we would see this big high at this time of year,” he said. “That storm that originated in the Northeast has gone down there and behind it this ridge popped up. For now there’s light winds and nothing really to move it along at the moment, but later this week we will get a cold front move in to sweep that hot air out.”

Relief is on the way, he added, but for much of the country it won’t be until at least Saturday when temperatures begin to drop.

Farther to the north and across the interior Northeast, temperatures in the upper 80s and low 90s were expected in Boston and Burlington, Vt., on Tuesday.

Chicago, Detroit and Ohio Valley were likely to see temperatures in the upper 80s and low 90s, according to weather.com.

Meanwhile, Roth warned that "numerous showers and thunderstorms" were likely across Oklahoma and Texas.

"Heavy rain may produce another one to three inches and some flash flooding through Wednesday," he added.

Henry Austin is an NBC News contributor. The Associated Press contributed to this report.

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