Summer's 4th heat wave becomes official

Escaping the heat, beach goers play in the surf in Ocean City on July 15, 2013, as longest, hottest heat wave so far this summer settles in. ( TOM GRALISH / Staff Photographer )

Philadelphians know we're in the middle of a heat wave.

And today, the National Weather Service officially recognizes this hot stretch as one, with temperatures soaring past 90 degrees for the third-straight day.

By shortly before noon, the mercury hit 91 degrees at Philadelphia International Airport. And by 1 p.m., the temperature had inched up a few more degrees, to 93.

Forecasters are calling for a high temperature of 97 degrees today in Philadelphia, following Sunday's high of 91 degrees and Monday's high of 95.

Today's heat might not feel quite as humid as Monday, though. The weather service says northerly winds are expected to pick up strength this morning, and move slightly drier air into the region.

The heat wave is slated to continue through the end of the week. Forecasters are expecting the mercury to reach 96 degrees Wednesday, 97 degrees Thursday, 95 degrees Friday and 94 degrees Saturday.

An excessive heat warning is in effect for the region through 8 p.m. Friday. The weather service says to expect heat index values of 100 to 105 degrees, with the hottest times between 1 p.m. and 6 p.m. each day.

Overnight, temperatures will remain in the 70s, and in some areas -- like Center City -- the mercury likely won't drop below 80 degrees, the weather service says.

Those temperatures could threaten the records for the warmest low temperatures. Overnight Monday into this morning, the temperature only dipped to 81 degrees. That will likely beat the record for the maximum low for July 16, which now stands at 77 degrees, as the temperature isn't likely to get cooler than that before midnight.

The average minimum temperature in Philadelphia for July is 74.6 degrees -- the warmest average low on record for any month dating back to 1874, according to the weather service.

The Philadelphia Corporation for Aging will activate its heatline from 8:30 a.m. through midnight today through Friday. Callers can get information about hot-weather precautions, signs of heat stress and heat-related medical issues. The phone number is 215-765-9040.

The heat wave, once official, will be the fourth recorded in Philadelphia this summer.

Contact Emily Babay at 215-854-2153 or Follow @emilybabay on Twitter.

Contact the Breaking News Desk at 215-854-2443; Follow @phillynews on Twitter.