Potential for a “surprise” NW flow snow event on 23, 24 December 2013?

It is interesting to look at the GFS forecast (1200 UTC init) for overnight Monday (23 December)  into Tuesday morning (24 December) as very little precipitation is predicted for western NC. This might be an interesting case to see if the rainfall occurring Friday – Sunday in the upstream location of the mountains is enough to offer a surprise in the accumulation amounts by Tuesday morning. Of course, the low-level flow direction will be key. It appears that the predicted strongest low-level pressure gradient doesn’t line up with the optimal time of day on Monday (1800 – 2100 UTC) for good banding. However, the gradient overnight Monday and Tuesday might still produce strong enough winds that we could see higher accumulations than currently expected in the models. This all assumes that the over-land sensible and latent heat fluxes are potentially important in some instances of NW flow.

This event is different from the 5-8 December 2010 case studied in the October 2012 WaF NW flow snow article (http://dx.doi.org/10.1175/WAF-D-11-00103.1) in that the cold air mass predicted by the GFS to move into the southern Appalachian Mountain region early next week is not as frigid as was observed early in December 2010. However, the NAM forecast (1200 UTC init) valid 0000 UTC 24 December 2013 shows a colder air mass moving into the region than does the GFS. Another difference between predictions of the upcoming event and that documented in the December 2010 event is that there does not appear to be a significant vorticity maximum upstream of the region that could assist with lift and precipitation generation.

With a relatively warm and saturated ground, combined with a strong low-level pressure gradient, the PBL upstream of the southern Appalachian Mountains could become destabilized and moistened enough that the clouds produced after sunset on Monday might surprise some residents in western North Carolina with a good chance of a white Christmas.