The triggers of this unusual late season snowstorm, the 4th in March for the Northeast Corridor is entrainment of arctic air in eastern Canada and the Northeast U.S. into a strengthening upper low pressure trough moving into the Mid-Atlantic region. The upper trough slowly meanders northeastward the next 2 days creating a long duration storm
Fortunately, the upper trough is lagging to the west initially causing a marine influence on precipitation type in the Mid-Atlantic region today therefore precipitation is mostly rain or mixed precipitation slowly transitioning toward snow across Pennsylvania.
The southern edge of the arctic air reached the Pennsylvania/Maryland border this morning but that boundary has temporarily slowed southern progress. Gradually, the cold boundary slips farther south into Virginia late tonight and tomorrow as the amplifying upper trough shifts toward the coast. The combination of colder surface air and the cold trough aloft causes a wide area of snow tomorrow morning in the Mid-Atlantic region with increasing strength mid-to-late day. The snowfall extends to southern New England tomorrow afternoon.
Heaviest snow today is across elevated areas of northeast West Virginia and gradually south-central Pennsylvania. 3-4 in. of snow by 8PM are likely in this zone. Overnight the heavier snow stretches from western Virginia to central Pennsylvania across western Maryland where another 3-4 in. could occur.
Tomorrow the heavy snow core shifts east. One foot of snow occurs tomorrow, mostly during the afternoon centered on Philadelphia. The heavy snow core intensifies tomorrow night and stretches from Trenton to NYC to Boston where another 6+ in. will occur. Heavier snow shifts to eastern Massachusetts Thursday morning before ending.
The forecast amounts indicated are conservative based on settling due to time of year and high water content. However, the axis of heaviest snow indicated in the forecast chart could easily be a little higher.
High wind and coastal flooding is an issue for the Delaware and New Jersey coast tomorrow shifting to Massachusetts late Wednesday night and Thursday.