You can call the airport in the morning, but be there on time regardless and just bring some patience with you. There are a lot of people that have been trying to get to their destination for a couple of days and keep getting socked by weather and other delays. You might want to keep an eye on the monitor at the airport for other planes heading PHL or at least that direction, so if you do have a problem with your flight (which you very likely will not), you can offer the agent alternative flights that are going to depart shortly.

Out of SFO, there are sometimes early delays due to fog but stead rain in and of itself should not affect your flight. G'Luck and enjoy your trip.

Mods - this might do better in T-Buzz or USAirways forum.