Snow or a mix of snow and sleet will arrive in the DC metro region during the mid-to-late morning hours and then advance into Philly by early-to-mid afternoon and to NYC metro area during the late afternoon hours. As the low pressure system pushes northward towards the Delmarva Peninsula later today, counter-clockwise winds will push in slightly milder ocean air to the upper part of the atmosphere over the I-95 corridor. This push of milder air aloft will lead to a changeover of the snow to mixed precipitation in the DC-to-Philly corridor which will limit overall total snow accumulation amounts. In fact, sleet will become a big player in much of this region during this storm with a pelting of ice pellets on the table. Freezing rain is also going to be part of the mix in the DC-to-Philly corridor and as surface temperatures likely stay at or below freezing in most areas, there can be an ice build-up on untreated surfaces. In the NYC metro region, sleet can mix in early tonight, but snow is likely to be the main precipitation type for much of this winter storm.
As the low pressure system pushes to the east later tonight, colder air will wrap around and any mixed precipitation in the I-95 corridor is likely to change back to all snow and this late night snow can be heavy enough for additional accumulations; especially, in areas to the north of the PA/MD border. In areas well to the north and west of I-95, any mixed precipitation will be short in duration and snowfall accumulations can be considerably higher compared to the immediate metro areas. In fact, there can be a sharp gradient of snowfall amounts over a distance of just 25-50 miles or so from the metro regions to the far northern and western suburbs.