Watch for banded snow showers developing at night during current late-season NW Flow Snow event.

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).

Mar27_00Z wrf

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!)


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1 Response to Watch for banded snow showers developing at night during current late-season NW Flow Snow event.

  1. lglee says:

    An interesting aspect of this event worthy of examination is the evolution of the upstream boundary layer. As documented in Miller (2012), sensible heat fluxes at the surface upstream of the mountains can deepen the boundary layer and contribute to an increase in water vapor content through turbulent mixing. Certainly the snow cover affected the near-surface temperature (and lapse rate), but what about moisture? Detailed trajectory analysis will be needed to assist in a determination, but to what extent did the Midwestern and Ohio Valley snow cover influence the moisture available for upslope-enhanced precipitation in the central and southern Appalachians? Was more moisture available through melting and/or sublimation than would have been available from bare ground? What was the moisture content of the soil, and what would the precipitation have been if that had been the source of surface moisture? What role did the nearly ice-free Great Lakes play as a moisture source? Was most of the extensive cloud cover embedded in the northwest flow simply “exhaust” (left-over cold conveyor belt clouds) from the departing cyclone?

    Lots of questions, but I have no answers. Simply food for thought and encouragement for additional posts.

    Miller, Douglas K., 2012: Near-Term Effects of the Lower Atmosphere in Simulated Northwest Flow Snowfall Forced over the Southern Appalachians. Wea. Forecasting, 27, 1198–1216.

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