Grew up in Philly. Big urban watersheds that flash quickly and have killed folks swimming in the creeks or fishing. The spike on discharge in Frankford Creek right now is something you only see during tropical storms.
Yea it definitely was a sneaky day. We get these events where it looks pretty benign and then you get a lone cell or three that would end up verifying a slight risk and a WW. Other times, it's a textbook setup and the entire subforum whiffs.
Sustained convection east of I-81 will probably be sparse through 4pm. Despite the ConvT being reached the shear is minimal so it's going to be hard to sustain the updrafts.