The government's Storm Prediction Center, in Oklahoma, has placed much of the area under a tornado watch unitl 10 p.m.
In addition to Philadelphia, the counties included are Atlantic, Burlington, Camden, Cape May, Cumberland, Gloucester and Salem Counties, in Jersey, and Delaware, Montgomery, and Chester west of the river.
The storm center puts the chances for a tornado sighting at about 5 percent, but some strong storms have rippled through the area, and more are possible.
Since Friday, close to 5 inches of rain have fallen in Philly.