Day to day weather changes can be as drastic as the season to season changes, so you will face virtually any kind of weather at any time of year. Some years, summer lasts until late October, and other years, the first snow comes before Thanksgiving. We're kind of like Mother Nature's little freak experiment. The snowfall is lighter than the rest of the Northeast. When it snowed two feet last year, they closed school for two days until they could get plows in to clear the streets. Rainstorms are much more frequent than snowstorms. Philadelphia is also known for the impressive electrical storms that occur during all seasons. You will need clothes for all weather and layers for all but the stickiest summer humidity. Your layers will be handy in the fall when temperatures are erratic and again in the spring.
The dark of winter requires all the warm gear you have, but since all buildings are heated during that time, lighter clothes underneath your sweaters and winter coat will make you more comfortable during class. Waterproof boots will get you through the rain and muddy snow that covers campus for weeks after the white stuff has melted.