UPDATE 6:40 AM: The air did finally become cold enough for snow on the back edge of the precipitation shield in parts of Henry, Franklin and Bedford counties into the Lynchburg area early this morning, where some places did get a white ground, up to half an inch. One more round of precipitation is headed through the Roanoke and New River valleys over the next several minutes that will likely bring freezing rain, sleet and perhaps even some snow, before this round of wintry mix comes to an end. END UPDATE
UPDATE 2 AM: The “warm nose” — or a layer of milder air aloft — is clearly winning tonight’s battle. In spite of the earlier dry slot, we now have more moisture overhead than most forecast guidance suggested, but with it, much more above-freezing air in the layers above the surface. And so we are seeing rain, freezing rain and sleet, and very little if any snow. Even the surface temperatures have buoyed a little above freezing in many locations — and where they are 32 or below, ice is becoming a problem, again. There is some chance yet that the warm nose could relent or be eroded by cooling, and allow a little snow in the Roanoke and New River valleys. But it’s looking unlikely. Strangely there is some snow at higher elevations in northwest North Carolina, so it’s possbile that cooler layer drops a little more to include some of the higher elevations of Southwest Virginia later tonight. And of course it’s snowing at Ingalls Field atop the mountain above Hot Springs in Bath County. But this complicated and not well-organized system is clearly not behaving as expected just a few hours ago. Do be careful if you drive, there could be icy patches in just about any part of our region. END UPDATE
UPDATE 12:30 AM, 12/29: No sooner had I posted about the dry slot than it began filling in partially and numerous reports of sleet began in the Roanoke Valley. The heaviest band of precipitation in Southwest Virginia has been sliding up the east side of Interstate 81 and will likely bring moderate to heavy mixed precipitation to the Roanoke Valley in the next hour. Roads may slicken rapidly, at least becoming slushy in spots. If the heavier precipitation can cool some layers above the surface sufficiently, snow may begin to fall at some point. If not, the “warm nose” from the south may push aloft and keep us in sleet and freezing rain, which has indeed been the case along the southern tier of Virginia counties this evening. END UPDATE
Ah, there it is, the Southwest Virginia snow fans’ loyal companion, the dry slot, in the circle on this radar image from about 10:15 p.m. stretching from Interstate 77 to Nashville, Tenn. This one was somewhat expected, with northern and southern pieces to this loosely organized storm system. The key to whether this storm system will verify even the low end forecasts on snowfall (1-2 inches) depends on whether that dry slot at least partially fills in. The precipitation behind the arrow in eastern Tennessee has been expanding northward and lifting northeastward the past few hours. Forecast models continue to show about 0.25 liquid eqivalent in most of Southwest Virginia overnight and early Saturday as moisture streams in from the southwest. Beyond that, there is the question of the thermal profile of the atmosphere — the temperature at different levels. There have been several reports of sleet from the southern New River Valley southward, so obviously there is already some cooling going on in the slightly milder layers just above the surface revealed by the early evening weather balloon launch at Blacksburg. Cooling the air all the way from cloud to ground is the key to getting snow to reach the surface.
So where does that leave us? Not hugely different than before, really. A poorly organized storm system is lifting moisture into colder air over us, resulting a mixed bag of precipitation that likely will lean toward snow with time. Light amounts, an inch or two for most from the Roanoke Valley west and north, maybe 4 for some higher elevations that get under somewhat heavier snow for a while, still seem reasonable. But there is at least the question of whether that dry slot will be a permanent fixture in part or all of our region. All we can do — if you’re really motivated to stay up and do so — is watch the radar (linked here) and the outdoor lights and see what happens.