Update, 2 PM: The Storm Prediction Center has upgraded areas along and north of I-78 to a moderate risk for severe weather for Thursday...and are also hinting at the possibility of a long duration line of storms with excessive wind...and perhaps even a derecho event in a worst case scenario. The Philadelphia area is still under a slight risk. The areas in the red shading on the graphic below a generally those that have been upgraded.
Thursday features an approaching cool front from the west and a hot airmass pushing back north into the Philly area. There's a slight risk for severe weather out for the Delaware Valley from Long Beach Island to Wilmington and points north and west from there for tomorrow afternoon and evening as thunderstorms will likely develop in a hot and humid airmass ahead of the front. There is some question with how far east and southeast those thunderstorms get and it's possible Philadelphia misses out (again) on more storms as the best forcing for thunderstorm development resides to the northwest (in the red shaded region above, also illustrated on the "lift" graphic below) across Central Pennsylvania.
Depending on timing and where storms first fire up, scattered thunderstorms could slip into Philadelphia during the evening hours but a better chance for thunder resides to the city's north and west tomorrow afternoon and evening.
There could be an initial round of thunderstorm activity late, late tonight not far from Philadelphia as these storms over the Upper Midwest as of this morning move east and then southeast along the warm front. Thunderstorms were over Minnesota and Wisconsin early this morning...they should ride through the Great Lakes and into Ontario, pushing southeast through Upstate New York this evening. The tail end of this activity could push far enough south to graze the Poconos or Lehigh Valley later tonight or first thing tomorrow morning but a majority of this activity looks to push into New England.