There are many factors that come to play in flying during the winter months. You ahve to not only worry about the weather in PHL but you also need to worry about the weather in the city your plane is coming from and going to. I agree PHL does a good job keeping the airport open and runways open. But there are so many other factors that come into play. There really is no reason to worry because it will just make you crazy. Closer to the day if you see something coming, make alternate plans in your head, like other flights you might get on etc.
PHL is very bad for delays and late planes in general during all times of the year.