UPDATE 7:40 PM: The first wave of precipitation — mostly if not entirely rain — is moving northward through southern Virginia and will likely overtake the Roanoke and New River valleys within the next hour or two. Some sleet may be possible in higher elevations where temperatures cool sufficiently. Latest National Weather Service radar linked here. END UPDATE
First off — a winter storm warning has been issued for most counties in Southwest Virginia, a larger area than was covered by the winter storm watch earlier (yes, Franklin County has been added, too). The various forecast products will be updated in the next hour or so by the National Weather Service in Blacksburg, linked here.
It would be too warm to snow if stayed 50, obviously. But the question bounced around quite a bit today is whether today’s highs, above 50 at Roanoke and several other sites, preclude snow happening on Wednesday.
The cold air that is expected to produce snow on Wednesday is coming from an upper-level low, a spinning pool of extremely cold air currently over Arkansas and Louisiana. This upper low is expected to take a path over or just south of the region late Wednesday. As it does so, the combination of that cold air being pulled downward into the strengthening surface low on the coast, and the cold air being pulled down by falling precipitation, is expected to pull temperatures to near freezing, enabling more snow to fall and eventually accumulate.
This is not really an unusual situation. A very similar setup occurred on Feb. 11, 2006. Roanoke had a high of 50 on Feb. 10 and was in the low 40s at midnight as the clock rolled into Feb. 11. But an approaching upper low, feeding into a developing coastal low, was enough, along with a morning rain/sleet/snow mix changing to snow, to pull temperatures to the freezing mark the rest of the day. Roanoke got 5 inches in that storm; Blacksburg got 8 inches, and 4-10 inches was common across the region.
Today’s highs have added some warmth to the lowest layer of the air that will need some time to cool. Overnight precipitation will begin doing that — some of it is already showing up on radar, mostly evaporating before hitting the surface. Temperatures will get cold enough to support frozen precipitation first at the high elevations — one reason I’ve given higher elevations a slightly higher snow expectation than lower areas — and then working lower. By the time the main precipitation begins Wednesday with the upper low, it won’t take long for temperatures to drop into the low 30s at most locations.
So a high in the low 50s today doesn’t preclude snow on Wednesday — if all those dynamic processes work as expected. The warmth today may have slightly warmed the ground, allowing for some early melting, but the ground is rather cold from 2 months of below-normal temperatures so it will hold the snow better than it would were we in the 50s after weeks of mild weather. Pavement may hold enough heat for early snow to melt, but if it falls hard enough with temperatures in the low 30s, slush will start developing, and snow can begin accumulating on streets too.
I’m still expecting 5-9 inches west of I-81, 3-7 inches east of I-81 to Lynchburg and Stuart, and 1-3 inches east of there through Martinsville and Southside. Rain and light mixed precipitation should begin overnight, slowly changing to snow from the northwest and from higher elevations downward, with snow beginning in earnest around noon Wednesday and continuing through the afternoon and early evening.