Thousands are still in the dark in Virginia and West Virginia — centered in Floyd County, locally — from Wednesday’s wintry weather system. If you live in the Roanoke Valley and looked at the mountains to the west and southwest today, you could easily see just how ice-encrusted and snow-covered many higher elevations remain — stunningly beautiful to look at, but not much fun for those who live up there and still don’t have power. The Dec. 25-26 storm has left a wide impact over much of the central and eastern United States, as you can read in the article linked here and also note from the raw numbers in NOAA’s storm summary, linked here.
I’ve gotten lots of requests to add a short summary of the blog discussion for those who may be viewing quickly or just don’t want to dig into the longer weather talk. So I’ve added that with this post. I may not include that with every post, but will plan to do so with many posts near weather events with some degree of potential widespread impact.
Summary: Snow will become likely in much of Southwest Virginia by late Friday into early Saturday as a surge of Gulf of Mexico moisture streams into marginally cold air in the lower layers of the atmosphere. Snow will begin as rain in many locations. 1 to 4 inches of snow will be possible, primarily along and west of the Blue Ridge Parkway and along and north of U.S. 460 (Roanoke to Bedford to Lynchburg) with the larger amounts more likely with higher elevation. To the south and east, snow may develop by Saturday morning, but will likely be brief. Substantial questions remain about the volume of moisture, the depth of cold air and exact track of multiple fairly weak low-pressure centers that could alter the forecast in either a somewhat more snowy or much less snowy direction.
Discussion: The potential Friday night-Saturday morning snowfall for Southwest Virginia remains somewhat fuzzy for several reasons. A surface low-pressure system in Louisiana will begin lifting a plume of Gulf moisture northward Friday, ahead of a somewhat unorganized wave of upper-level energy with a few different centers. This moisture will overspread the lower Mississippi and Tennessee river valleys on Friday, before starting to advance into Southwest Virginia by sometime mid to late Friday evening. A seasonably cold air mass remains in place over Southwest Virginia, reinforced by the cold front behind Wednesday’s strong low-pressure system that moved into Ohio Valley then re-formed off the Atlantic coast. This system (forecast map for Saturday morning at left), likewise, will make a similar jump from inland to offshore, with northern and southern pieces of energy eventually firing a rather strong low-pressure system off the Atlantic coast by late Saturday. But that will come too late to have much direct effect on our weather, beyond the expected northwest winds and upslope snow showers on Saturday evening. As the moisture moves over and into the cold air, banked somewhat by high pressure to the north, a mix of rain and snow, perhaps some sleet, is expected to begin Friday evening. As the column of air cools downward with evaporational cooling from the precipitation, snow is expected to reach lower and lower elevations through Friday evening and early Saturday morning, until most locations along and west of the Blue Ridge Parkway and along and north of the U.S. 460 corridor (Roanoke to Lynchburg) have snow falling. It is possible snow will even spread farther south and east by Saturday morning, depending on the depth of the cold air and the effectiveness of cooling processes to lower the freezing level.
Accumulations are dependent on multiple factors, the two biggies being how fast the lower atmosphere cools to support snow and how much moisture arrives in Southwest Virginia. Forecast models have varied quite a bit on both of these factors, but it is a safe bet that accumulation chances will increase with height in elevation, as the higher spots will see snow first and have it collect first (especially with many high ridges remaining icy and snow covered from Wednesday’s system). The system’s arrival in the early morning hours will enhance the cooling, and keep solar radiation collected by the ground from being a negative factor in accumulation, so that may help the snow stick a little better than it would if it were arriving during an afternoon. The moisture question has varied from 0.10 to 0.75 liquid-equivalent amounts on forecast models, which presuming everything fell as snow and everything stuck, would be the difference in 1 and nearly 8 inches of snow. Most models are settling more toward the 0.25-0.50 end of things. Allowing at least a quarter of that to either not immediately stick or fall as rain, that brings us to about 1-4 inches of snow. (It should be noted that the Hydrometeorological Prediction Center is going as low as 0.10 to 0.25 on Friday evening to Saturday evening liquid equivalent, yet also has the region in a slight risk of 4-plus inches of snow, as of early Thursday evening. So even the HPC is expressing uncertainty about the moisture flow for this event.) There remains some chance that a more vigorous storm system or more robust plume of overrunning moisture would push the amounts into low-end winter storm watch/warning range at 3-6 inches, but a widespread 6+ inch snow is highly unlikely in our region with any scenario. Far more likely is the 1-2-inch end in most locations, particularly if moisture flow is weak, and of course there is some chance that many lower elevation areas see mostly rain if the cold air is somewhat overstated. Just because Wednesday’s system went to the cold side of forecasts doesn’t necessarily mean this one will.
By Saturday afternoon, most locations will rise above freezing, and highs in the 40s are possible from Roanoke south and east. A new cold front arrives Saturday night with more strong wind gusts and potential for upslope snow showers.
Next week remains enigmatic. There are some pieces on the board that could form a fairly large southern-stream storm system, and possible Eastern U.S. winter storm, from about New Year’s Eve to Jan. 4 or so. There could be a couple of smaller systems move through during that time, or one pretty large one, or none. Temperatures are expected to remain seasonably cold, but not extremely cold, so we could have 1 or 2 more borderline wintry events next week. Tis the season …