In March, the sun's wattage turns the daytime blacktop into a hot plate. If snow makes it through the atmosphere, usually it quickly melts and commutes to the next phase of the water cycle.
While the snow usually doesn't stick around very long, it has been known to survive landings and accumulate robustly in March and April.
The average snowfall for the two months is around 3.5 inches in Philadelphia, but back in 1915, close to 28 inches fell after March 1, including a mega-storm of 19.4 inches on April 4-5, worthy of the winter of 2009-10.
In the period of record, dating to the winter of 1884-85, 11 storms that have occurred after March 1 left an official 6 inches or more in Philadelphia, including one just last year, which you may have forgotten.