The National Weather Service has forecast potentially record-breaking hot temperatures just as the season officially begins Wednesday, the summer solstice and longest day of the year.
Readings are expected to be in the mid- to high-90s Wednesday and Thursday in cities including Philadelphia, New York and Boston. Humidity could make it feel even hotter.
Health officials warned residents to drink water, stay out of the sun and in air conditioning, and to check on elderly neighbors and pets. For those without air conditioning, "cooling centers" have been set up in public buildings in dozens of cities.
Philadelphians may see the mercury soar to 95 and 99 degrees mid-week after enjoying relatively mild June temperatures.
"You're talking about almost 15 degrees above normal," said Kristin Kline, a weather service meteorologist in Mount Holly, N.J.
Normally, the high for Philadelphia at this time of year is about 84 degrees -- closer to Wednesday's predicted low of 80 degrees. The city's highs in the next couple of days will approach, if not break, decades-old records of 98 degrees, set in 1931, and 99, set in 1923.
The wilting heat also will hit Boston-area residents hard. Temperatures of up to 97 degrees are forecast for Wednesday, followed by 99 on Thursday, the weather service said. Current record highs for these dates are 98 and 95 degrees, respectively.
New York City's 1.1 million public school students are still in session for another week, and just 64 percent of classrooms are air-conditioned. Temperatures in the city are expected to hit 94 degrees on Wednesday and 98 degrees on Thursday, about 20 degrees hotter than it was in Central Park on Tuesday.
Students were being advised to wear light clothing and drink plenty of water, and schools have been told to limit outdoor playtime, city Education Department spokeswoman Marge Feinberg said.
In Brooklyn, street vendor James Martin said his family's sixth-floor apartment in Coney Island has no air conditioning and can get really hot. But "we open the front door and all the windows, and we get a nice breeze," he said.
On Tuesday, though, he planned to buy a fan -- "maybe two" -- on his way home from work.
Records might also be broken in central Connecticut. A Home Depot in West Hartford had stacks of air conditioners and fans ready to go, and city resident Sarah Savo was ready to buy one.
"Now that I work at home, I just can't work without air conditioning," said Savo, who will also be caring for her 3-year-old daughter. "Our home is very hot."
Forecasts for upstate New York on Wednesday and Thursday called for temperatures to hit the 90s from Niagara Falls to the Vermont border, with highs topping out in the mid-90s in some places.
The cities of Buffalo and Rochester, N.Y., opened several spray parks on Tuesday to help residents cool off as hot, muggy weather settled in. Buffalo, which will only be in the mid- to high-80s on Wednesday and Thursday, doesn't normally open its 11 splash pads until July 1.
Philadelphia began a staggered schedule of opening its swimming pools on Monday, a couple of days after schools let out for the year. Nearly two dozen of the city's 70 pools will be open by Wednesday, with another seven opening Thursday.
"We're very lucky that the pools opened yesterday," James Garrow, a spokesman for the Philadelphia health department, said Tuesday.
He added that the city will activate its heat hotline at noon Wednesday and will work with personal care homes, senior centers, libraries and recreation centers to make sure air conditioners are running.
Officials will be setting up 114 cooling centers at facilities across the city, said Garrow.
The Southeastern Pennsylvania Transportation Authority, the state's largest transit agency, is keeping a close eye on the heat as well, spokeswoman Jerri Williams said.
SEPTA, which operates trains, trolleys and buses in Philadelphia and its suburbs, planned to have extra maintenance workers to help deal with heat-related switch failures, problems with track expansion and any overhead wire issues on suburban train lines.
In Rhode Island, all regular public buses and trolleys will be free on Wednesday due to anticipated air quality issues.
Moderate relief from the high mercury should come this weekend.