The most intense - and potentially dangerous - heat wave of the season is likely to sear its way into the record books this week.
But it is unlikely that any records will be set during the sequence of steamy afternoons in Philadelphia.
More likely, new standards will be set for overnight warmth, as temperatures struggle to get much below 80. Health experts warn that hot nights are particularly hazardous for the elderly living alone in rowhouse neighborhoods.
"It's going to be brutal," said Gary Szatkowski, meteorologist in charge of the National Weather Service office in Mount Holly, which has issued a heat warning in effect through 8 p.m. Friday. "People are going to remember this for a long time."
Temperatures throughout the week are forecast to reach the mid- and upper 90s during the day, with heat indexes of 100 or better. A dash of drier air could mitigate the discomfort Tuesday, but the heat index is due to approach 105 Wednesday and Thursday.
The air also will be densely filled with water vapor, and that means warm nights; water vapor retards cooling after nightfall.
The forecast overnight lows through Friday are expected be near 80, threatening records each day for all-time high minimum temperatures.
With little cooling occurring overnight, the brick rowhouses with heat-absorbing dark roofs could start the days with inside temperatures of 90 or better, said Chris Gallagher, director of the Philadelphia Corp. for Aging's Heatline - 215-765-9040 - which will be operating from 8:30 a.m. until midnight through Friday.
"I try to communicate to people that 95 is hot enough if you're outside," he said, "but in some of these homes, it can be 110 to 115 in there."
Historically, most of the heat-related fatalities in Philadelphia have involved elderly people who lived in rowhouses in low-income neighborhoods.
By this late in the summer, warmth has built up inside the houses, making them all the quicker to flare with heat after the sun comes up.
The city generally is the regional hot spot, with its heat-absorbing paved surfaces and buildings conspiring to create an "urban heat island."
What's more, nights have become warmer in recent years at a rate exceeding the warming of the planet. The average summertime overnight low in Philadelphia for the 10 years that ended in 2012 was 68.1, or 2.2 degrees higher than the 140-year average.
In Center City, it was even hotter. The 10-year average low at the Franklin Institute was 70.
Fortunately, the National Oceanic and Atmospheric Administration and the Centers for Disease Control and Prevention have held up Philadelphia as a model for heat-wave response.
In 1993, 118 heat-related deaths were reported in the city, but death tolls haven't approached that level since.
Szatkowski said this one, however, presents serious hazards. "It's going to be a big challenge to get everyone through this," he said.