The hot spell that is about to assert itself abruptly in the region, perhaps persisting into next week, has the potential to be dangerous, health officials are warning.
Coming after a cool, damp May that left the region's vegetation about as lush as it ever gets in early June, the first heat wave of the season could be particularly uncomfortable - and perhaps even deadly in some urban neighborhoods.
The National Weather Service has extended its "excessive heat warning" through 8 p.m. Monday.
"Early season heat waves are the most dangerous," said Laurence Kalkstein, a retired University of Delaware professor who is a heat-mortality expert. That is because people are not acclimated to the heat.
The forecast calls for temperatures in the mid- to upper 90s both Sunday and Monday and tomorrow - with a nearly 100 percent chance of extreme sweat for those participating in tomorrow's annual Pro Cycling Tour race. And it will feel like it's 100 in the afternoon - in the shade.
Similar readings are possible through Tuesday evening, when a heat watch is in effect. Duration is critical, because heat-related deaths tend to spike upward after three or four consecutive hot days and warm nights.
"I think we're teetering near the duration threshold," said Tony Gigi, a meteorologist at the National Weather Service office in Mount Holly.
The weather-service warning sets off a variety of programs in Philadelphia, which has been cited by the federal government as a model for heat-wave response. They include a "heatline," run by the Philadelphia Corporation for Aging, where seniors can call for help. It will be open from 8:30 a.m. to midnight today, tomorrow and perhaps Monday.
Project HOME, which looks out for Philadelphia's homeless, will beef up its efforts during the weekend, with at least four teams patrolling the city.
Camden will keep its spray pools open for those who want to cool down, said the Rev. Tony Evans, the city's director of health and human services.
Delaware County's eight senior centers will have extended hours for the duration of the heat wave, said Louis Colbert, director of the Delaware County Office of Services for the Aging.
The National Weather Service said a weak cold front is expected to slip through the area Tuesday
night allowing a drop in temperatures and humidity for Wednesday.
The heat is the result of a strong area of high pressure out over the Atlantic, the Bermuda high, so called because it often is centered near Bermuda. Winds circulate clockwise around highs, so areas to the west of the center - as in Philadelphia - get warm, humid winds from the south.
The high already has baked the Carolinas, where temperatures shot into the upper 90s yesterday. Generally, the Carolinas have been dry, and that's one reason it's been so hot there, said Ken Reeves, forecasting operations director at Accu-Weather Inc., the commercial weather service in State College, Pa. The sun heats up dry ground in a hurry.
The ground is so wet around here that it may impede the heating somewhat, said Gigi. Damp soil and foliage divert some of the sun's heating energy to evaporating the water, and that can shave a few degrees from daytime highs.
The downside to the moisture, said Kalkstein, is that it generates more humidity. That water vapor keeps nights warmer, but trapping heat that otherwise could escape into space.
Warm nights can be deadly in brick rowhouses. Absent overnight cooling, a brick house can warm up in a hurry during the day.
The Philadelphia Health Department is urging people to check on older relatives and friends. Most heat-wave victims are elderly and live alone.
At the Faison Mews senior citizen residence in Camden, Patty DuBose, the site manager, said the staff would check on residents to make sure their air-conditioners are working.
The facility is planning a fund-raising yard sale and bake sale today that was supposed to last until 4 but may end early if it gets too hot.
"I told my seniors if it gets hot by 12 or 1 o'clock, we have to shut it down because we can't have our seniors outside," she said.
"And I don't want to be out there, either."