The temperature could reach triple figures during the weekend for the first time in six years as what is looking like a prolonged heat wave crests over the region. It got cooking Friday when the official high reached 91 just before 4 p.m. in Philadelphia.

The National Weather Service issued an "excessive heat warning" for heat indexes up to 105 on Sunday, and 107 on Monday.

And while afternoon readings could top 90 for the next six to 10 days during the season's first official heat wave, health officials warn that they are at least as concerned about what will happen after dark — or what won't happen. It's not going to cool off much, and that speaks to a disturbing and dangerous trend.

Temperatures could struggle to get below 75 for the next several nights in the city, according to AccuWeather, and that could be life-threatening for some vulnerable populations, particularly the elderly who live alone in homes without air-conditioning. With no overnight cooling, heat quickly accumulates when the sun comes up.

"What worries me is anything above 75,"  said Dr. Steve Alles, director of disease control for the Philadelphia Health Department. "We tell everyone to go check on our numbers."

Temperatures are forecast to reach the low 90s on Friday and the mid-90s Saturday. The National Weather Service said it might have to issue an "excessive heat warning" for Sunday, when the high could make a run at 100.

"We are not talking about snow anymore," said Dave Dombek, a meteorologist with AccuWeather Inc.

>> READ MORE: How to keep your dogs and cats cool when it's hot

He said that a hot dome of higher pressure in the upper atmosphere will migrate toward the East, "and it's going to be hard to break it down."

In houses lacking air-conditioning, heat is going to accumulate, particularly in the city, where the so-called urban heat island effect can add several degrees to temperatures, relative to the surrounding areas, especially overnight.

Buildings and paved surfaces absorb the sun's energy during the day and are reluctant to give it up when the sun goes down.

Urban nighttime warming has been well-documented around the country, coinciding with worldwide increases in temperatures. In Philadelphia, for example, through last year, the average summertime official low temperature in Philadelphia in the 21st century was 68.3, or 2.6 degrees Fahrenheit above the average for the previous 127 years in the period of record (65.7).

The average summer highs in the last 17 years were 2.1 degrees above those of the earlier period (85.6 vs. 83.5).

As for the days, Dombek said that if showers show up at midweek, they might offer a brief respite in the 90-plus run, but that's not a done deal.

If you're looking for relief, you won't have to go all that far.

The Shore, thanks to that great body of cooler water to the east, the Atlantic Ocean, will be spared. Highs will be in the 80s. The hottest days will be Saturday and Sunday, when the mercury will reach the mid-80s.

And here's something we haven't seen much of in a while: No rain is in the forecast for the weekend.