Monday starts off sunny, followed an increase in the clouds. A weak wave of low pressure will impact the region Christmas Eve into Christmas Morning. It will be cold enough to consider other precipitation types besides liquid for the region. Our best guess based on the latest data shows that Philadelphia and the Interstate 95 Corridor is going to be somewhere close to the rain/snow line. My thinking is that north and west of Interstate 95 and 195, you have the best chance for all snow out of this event. South of these major highways, rain could mix with snow or it may even be warm enough for mainly rain. As one approaches the Atlantic City Expressway, rain will be the primary precipitation type, although a few snowflakes could mix in here too. This is a weak system and I don’t expect intense precipitation rates. Therefore, evaporational cooling may be limited in the areas that are on the cusp between rain and snow. In the portions of our region that see all snow, a light snow accumulation is possible. Elsewhere, it will likely just be wet. There is some wiggle room to drag the snow area further south or to move it further north as model consistency with the temperature profile and system track isn’t there yet. Later on Christmas Day, the mix of rain and snow will taper off and some peaks of sun are likely to develop. This will be followed by another increase in clouds Christmas Night.
A larger storm system is expected for Wednesday into Thursday. I am fairly confident in this being a strong storm system with a lot of moisture. This system also is likely to bring strong wind gusts to the region at some point as well. But the details of precipitation types will be dependent on the exact track of the area of low pressure. I believe it is still too far out to nail down the details and the models have been waffling run to run with the track. The 12z model runs would have air warm enough for rain in the city. But just yesterday, Philadelphia had an icy look to it. I am concerned the models may be currently missing the potential for cold air damming, especially for the Lehigh Valley and Poconos. While I could run with the headlines of drenching rain after a brief wintry mix, I think it is best to hold off and use caution while acknowledging the uncertainty. The bottom line is that I want to see a few more model runs before getting fancy with precipitation types.