UPDATE 11:50 PM, 12/5: Rainfall amounts (maybe snow chances?) on the uptick this week as cold front slows down
UPDATE 11:50 AM, 12/5: Some model guidance this morning suggests the possibility that the low-pressure system Wednesday and early Thursday may track far enough eastward to pull cold air into the region and throw back enough moisture for a shot at some snow. We’re discussing it on the blog from Comment 27 (10:34 a.m.) onward below. More on this later. END UPDATE
UPDATE 10:30 PM, 12/4: No major changes to below. Rainfall projections around 1.25 inches on Sunday night for most of Southwest Virginia. Some showers could move into the area as early as overnight Monday or very early Tuesday morning, but it appears bulk of rain will be Tuesday night and Wednesday. END UPDATE
One recent early December tradition in Southwest Virginia weather will not be renewed this time around. In 6 of the last 9 winters and 5 of the last 9 at Roanoke, there has been measurable snow on Dec. 5. In both 2002 and 2003, it was the second day of winter storms that started on Dec. 4 leaving widespread 6-12 inch snows (capped by sleet and glaze ice) on the region. Lighter snow in the 1 to 3 inch range fell on Dec. 5, 2005, and lighter still with about a half-inch at both Blacksburg and Roanoke in 2007. Dec. 5, 2009, brought an elevation-dependent wet snow that left nearly 4 inches at Blacksburg but only a trace of snow at Roanoke Regional Airport, more in outlying areas and higher elevations of the Roanoke Valley. And a year ago today on Dec. 5, 2009, an Alberta clipper brought 2.6 inches to Blacksburg. 1.4 inches to Roanoke and widespread 1-3 inch amounts with locally up to 4 inches. With dry weather and highs in the 50s on Sunday (Dec. 4), and some highs maybe clipping 60 on Monday, we won’t be seeing snow on Dec. 5 this time around.
The run-up to this coming week’s rain system looks a lot like about the last two or three that have come down the pike, with the heaviest amounts forecasted in the Tennessee and Ohio river valleys west of us, but the projected heavier amounts slowly encroaching into western Virginia during successive forecasts. The Hydrometeorological Prediction Center’s 5-day rainfall projection map on Saturday night has begun putting Southwest Virginia in the purple colors for rainfall amounts topping 1.5 inches (2 inches along/west of Blue Ridge). Two factors are driving up the projected rainfall totals. The first is indications on all the major forecast models of the cold front slowing down, even stalling, as it advances eastward. A couple of days ago this was being thought of as a Monday night-Tuesday rain; now it looks to be a Tuesday night-Wednesday rain, if not stretching into Thursday a bit. The second is the potential for multiple waves of low-pressure to ride up that front, including one that could come roughly up the Appalachian Mountains. Any waves of low-pressure that far east would not only contribute to the front stalling, but could pull additional moisture out of the Gulf of Mexico and/or Atlantic against the front and the mountains. The exact rainfall amounts, of course, are subject to some up-and-down movement in predictions, just like they were last week. Once the front goes by, colder weather will move in late in the week, and there’ll probably be some snow showers blowing over the mountains about Thursday or so, especially if there is a low to pull in even more cold northwest winds behind it. It’s doubtful the cold air will be able to catch up with the moisture enough for more widespread snow. Next weekend looks pretty cold for the region.