A difficult forecast period for late Sunday night through Monday, as the recent models have significantly changed the track of a developing low pressure system and associated frontal boundary. There is low confidence with the weather forecast for Monday. (Indeed, there has been more sunshine both Saturday and today, so even in the short range, the models have not been capturing this setup very well.)
Previously, the models had the low tracking across PA and we would be in the warm sector on Monday. However, the new track of the developing low will be to our south, allowing significant cold air to mix in at some levels of the atmosphere. While snow is not expected for much of the day in the immediate PHL area, the precipitation type is looking to be more in the sleet and possible freezing rain variety. This mixed precip will likely change to snow before ending. QPF values remain light, so only 1 or 2 inches of snow is expected by late Monday night. However, the real hazard may be the sleet and freezing rain.
I should point out that there is uncertainty about the development of this storm as it reaches the coast late Monday night. Right now, it’s expected not to affect us, but there is low confidence with that scenario.