Philly's part of a mini-tornado alley that stretches from York eastward. I've been around more tornados living up here for 18 years than I did living in Texas, Missouri and Oklahoma. However, most of the tornadoes here are dinkers - a F3 is pretty rare. Usually they are no worse than an F1.
I've heard of at least another south of there in the DC area (particularly since you had two tornadoes go through or near La Plata, MD). Do you have a source for that?