What is almost certain to become the warmest July on record in Philadelphia already has been blamed for at least 25 deaths in the region, including six announced Wednesday.

With the sixth heat wave due to start Thursday, this summer has become the deadliest hot-weather season in Philadelphia since 2008, when the death toll reached 26. And it has the potential to become the deadliest since 2002, when 40 heat-related fatalities were tallied.

The National Weather Service has issued an "excessive heat watch" for Friday and Saturday, with heat-index numbers heading back toward triple digits.

The death toll from last week's heat wave alone could climb higher as more bodies are found, and one expert said the standing numbers might be underreported by half or more. The deaths "are really grossly underestimated," said Laurence Kalkstein, research professor at the University of Miami, who helped the city develop its lauded heat-alert program.

Logistically, he said, the Philadelphia Medical Examiner's Office simply can't get to all the victims.

Of the most recently announced deaths, all six were males, ranging in age from 44 to 81, and all had background heart conditions, said city Health Department spokesman Jeff Moran.

The city did not disclose precisely where the bodies were found - only that three were discovered in the Northeast, one in Northwest Philadelphia, one in North Philadelphia, and one in South Philadelphia. Four were found Monday and two Tuesday.

In addition to the 22 deaths in Philadelphia from last week's hot spell, two were reported in Montgomery County, and one in Burlington County. Three other deaths were reported before the most recent heat wave.

The true total in the city may be more than double the reported tally, said Kalkstein. The final figures will not be available for perhaps two years, until daily mortality statistics are finalized.

On Friday, the official high temperature in Philadelphia reached 103; it felt like 119.

Temperatures are heading back into the high 90s on Friday and Saturday, although the discomfort levels aren't expected to match last week's.

However, the fresh pulse of heat is likely to propel July 2011 into the record books.

Based on the forecasts, the July average temperature would finish at 82.4 - 0.3 degrees above the current champ, 1994, at 82.1. Last July is No. 2, at 81.7. Temperatures were quite similar to this July's, but the air masses this year have been swollen with water vapor that has made a mockery of sweat.

"The record temperatures are one thing," said Eli Jackson, a meteorologist with the National Weather Service headquarters outside Washington. "But what has made this summer remarkable is the humidity."

If it's any consolation, the heat hasn't been picking on just us. Said Jackson, "Nationwide, it's been extremely widespread."

Contact staff writer Anthony R. Wood

at 610-761-8423 or twood@phillynews.com.