There is something about the President’s Day holiday and snow that just seem to go together. Low pressure will move well north of the Mid-Atlantic region later tonight and early Tuesday, but a second area - supported by strong upper-level dynamics and upward motion - will try to organize farther south along the coastline (see 12Z NAM surface forecast map for early Tuesday). This secondary low pressure system combined with advancing milder air from the southwest will likely generate a "quick thumping" of "warm advection" type snows late tonight and early tomorrow in the DC, Philly, NYC corridor. In fact, the recent trend in most of the computer forecast models has been to increase precipitation totals for this upcoming event from late tonight into early Tuesday.
As a result, snowfall accumulation estimates have been bumped up to 2-4 inches in the DC metro region, and 3-6 inches in the Philly and NYC metro regions (mixing issues on Long Island cut accumulations in that area). As far as the timetable is concerned: in the DC region, snow begins around midnight and ends early in the morning, in Philly, the snow begins after midnight and continues until mid-morning, and in the NYC metro region the snow begins towards morning and continues there until midday. In all locations along the I-95 corridor the heaviest snow will come in a "burst" and there is the chance that the snow mixes with or changes to sleet and/or freezing rain for awhile before ending on Tuesday after the precipitation intensity diminishes. Numerous slick spots are possible for the Tuesday morning commute and - at least in Philly and NYC - the snow could actually be falling at its heaviest rate right around the morning rush hour.
Milder conditions move in for the second half of the week, but winter is far from over as there are signs for much colder air to return later next week.
[NAM precipitation totals for the upcoming storm]