What is expected to be an impressive late season NW Flow Snow event to ring in the Spring season has begun today (Monday March 25th, and may last well into Wednesday or Wednesday night). One interesting aspect of these events is often the transition from a cellular nature of the snow shower activity during the day time hours (with a deeper mixed layer), to a more parallel-to-flow-banded structure at night. This is especially the case when steep but shallow lapse rates remain, and the wind directions are nearly uniform through the unstable layer.
This afternoon, the current radar mosaic (seen below) shows this cellular structure, which can still bring accumulations to favored western slope areas, but tends not to as much in the downslope areas even though brief transitory heavy snow showers can occur (as they have been off and on all day here in Blacksburg).
While tonight the unstable boundary layer does become shallower, model forecast profiles upstream suggest the wind directions will not necessarily be from the same direction in the boundary layer, but by tomorrow (Tuesday) evening (which is coincident with a strong upper vort max coming across the Appalachians, the winds are more favorable and the lapse rates in the boundary layer may even be a little steeper (see BUKIT image below from Bluefield, WV at 00Z March 27th).
The SPC 4km WRF-NMM simulated reflectivity also hints at this banded structure developing by Tuesday evening (below), and when this happens heavier snowfall rates can occur over a given area, even into more downslope areas deeper into the Appalachian mountains).
So in addition to the fairly impressive snowfall expected on favored western slopes throughout the during of this event (6-10” in a few spots from NW NC to parts of SE WV), some other locations could see a quick inch or so of snow accumulation outside the traditional favored areas, especially overnight tonight and Tuesday evening. The forecast snowfall image below is valid from this evening through Tuesday evening, but a few locations may see accumulating snow lasting into Wednesday night or Thursday.
Can’t wait to find out (then bring on Spring please!)