Reviewing the latest models for today, Sunday.  Original forecast that much of the daytime will be dry still holds, but thunderstorms are expected to develop far northern Bucks and Berks counties by about 2 PM.  Additional shower and thunderstorm  development with a push southward towards Philadelphia should occur during the 4-8 PM time frame.  Early evening hours may have severe thunderstorms with heavy rain.

With sunshine expected before then, there will be plenty of thermal energy for the development of moderate to severe thunderstorms.  Check the NWS  for severe storm warnings.