With the region baking toward its hottest July on record, the sixth heat wave of the season is under way, and it could rival last week's for duration.

Officially, the temperature reached 92 at Philadelphia International Airport, with a heat index of 101. While that's certainly muggy enough to make a shirt stick to the skin, it's a relative day at the beach compared with last Friday's index of 119.

And some places could get a cooling, albeit nasty, thunderstorm this evening, and the National Weather Service has issued a severe-storm watch for the entire region until 11 p.m.

At least one more heat-related death was reported today - this one in Chester, Delaware County - bringing the region's summer total to at least 29.

The body of Janie Johnson, 76, was found at her home in the 100 block of Worrell Street, according to the Delaware County medical examiner. As most of the other victims this season, she had a background heart condition.

The heat spell that kicked off today won't rival last week's in intensity, but right now it is forecast to hit 90 or better for at least seven more days - with at least seven more steamy nights.

Thus, it all but certain that this month will finish a few tenths of a degree warmer than July 1994, the reigning warmth king in the 138-year period of record.

Today marked the 19th day of the month and 27th of the season of 90-degree highs. But this time last year, when Philadelphia had its warmest summer on record, so far, that total stood at 36. July 2010 finished right behind 1994.

As Eli Jackson, a meteorologist at the National Weather Service headquarters, outside Washington, pointed out, last year's heat was accompanied by drier air.

That's probably why the Philadelphia heat-related death toll was only five in 2010. This summer, it stands at 25, 22 of those occurring last week. Two deaths were recorded in Montgomery County, and one in Burlington.

This year the intense heat has conspired with high humidity to create dangerous conditions.

Jackson blamed the oppressive air mass on an "extraordinary" ridge of high pressure that had superheated the South. That heat was transported to the north, where the air was filled with water vapor.

It has been a busy season for the Philadelphia Corporation for the Aging, which operates at heat-line when the city and the National Weather Service post heat warnings.

So far, the agency has taken over 1,300 calls from Philadelphia residents during the three activations, according to PCA's Chris Gallagher.

In the benign summer of 2009, by comparison, it received just 131.

This has become the deadliest hot-weather season in Philadelphia since 2008, when heat was blamed for 26 deaths.

Contact Anthony R. Wood at 610-761-8423 or twood@phillynews.com