Check out the critical thicknesses from the SPC mesoanalysis site. The 1000-850mb 1300m thickness is a great approximation of the R/S line in this event. Some elevation helps as well. It’s crashing southeast thanks to cold advection in the boundary layer (and some dry air advection too) which is dropping temps along with some evaporational cooling.
At the same time a solid CCB is setting up overhead. Expect snow to begin shortly in many areas that are still raining. Many metro NYC airports (N&W) are transitioning to snow now.
Forecast is on track.