It’s going to seem kind of strange to think about the potential for a weekend winter storm when you’re wearing short sleeves on Thursday and Friday as highs climb into the 60s to some low 70s. Record highs for December 5 of 70 for Blacksburg and 76 for Roanoke, both from 2001, probably won’t be touched, but it will be oddly springlike nonetheless. Enjoy it — it won’t last into the weekend. A rainy Friday night and a colder, breezy Saturday will lead into a confusing wintry mix for Sunday.
About that weekend winter system: I feel comfortable at this point saying that it will not be all rain and it will not be all snow. There are myriad combinations of precipitation in between those extremes of purity. Wednesday evening’s model runs have trended a bit colder both at the surface and aloft, which could lead to a longer period of snow and/or sleet to begin, if that verifies. It is not unusual for cold air damming situations to move colder on the models as we approach the event. Also, there will be some very dry air just above the surface to start the event, and this will induce evaporational cooling as the first precipitation arrives, which could chill some layers above for initial precipitation to be sleet or even snow. It would all come down to how strong the warm air advection is aloft that could set up a situation where snow high above us melts into rain on the way down and then re-freezes an objects at the surface.
There have been some exclamations about the potential for a “major” or even “historic” ice storm floating around the Internet from some pretty credible weather sources. Certainly, the kind of ice storm that would lead to thousands of power outages is within the realm of possibility for our region, and should be considered seriously as a potential though far from certain outcome. Factors that could limit it would be sleet/snow at the outset using up a lot of the moisture (though it is quite possible to have a couple of inches of snow and a significant ice storm on top of it!), and the fact that there may not be a ton of moisture involved in our region. A lot of model output is around a half an inch, and not every drop of that is going to freeze even if it is in the upper 20s — because some of it runs off before it can freeze. A long light to moderate rain with temperatures mid 20s or lower is optimum for heavy ice accumulation. Heavier rain runs off before much of it can freeze, and rain is slower to freeze at somewhat milder temperatures — though many power-crashing, tree-bending ice storms have occurred with temperatures right smack at 32 degrees. Road surfaces will retain some of the warmth of the next couple of days and will be much slower to freeze than exposed objects. Ironically, sleet and snow at the outset would increase the chance of slick roads while decreasing the chance of power outages.
My best stab at it now is this: An inch (or maybe 2) of sleet/snow to start Sunday for the Roanoke/New River valleys, less to the south, a little more to the north, gradually shifting into light freezing rain or drizzle through Sunday afternoon and evening. A widespread glazing event that will be significant (at least a quarter-inch of ice) for at least some. I think we may have a hard time getting above freezing over much of the area until sometime late Sunday night or early Monday.