SW VIRGINIA IN HIGH RISK OF 4-PLUS INCHES OF SNOW, PLUS SLIGHT-MODERATE RIKS OF 8-PLUS INCHES AND NEW RIVER VALLEY/MOUNTAIN EMPIRE/I-77 CORRIDOR IN SLIGHT RISK OF 12-PLUS INCHES, through 7 p.m. Sunday, per Hydrometeorological Prediction Center. Also, continued SLIGHT RISK OF 4-PLUS after 7 p.m. Sunday
As hard as it may be to believe after wearing short sleeves and even running my car air conditioner today, I think we’re headed for what I call a “biggish” snow event. It likely won’t be the “big’un” where everyone gets a foot or more, like Dec. 18-19, 2009. But it does appear that a wide swath of Southwest Virginia will be in line for 5-8 inches of snow on Sunday and Sunday evening, with some local amounts possibly reaching a foot depending on elevation and/or where heavy bands set up within the larger precipitation area. I even think the Roanoke Valley is going to get in on accumulations due to the intensity of the heavier snow overcoming surface warmth, though some amounts may be more in the 3-5 inch range at the lowest elevations below 1,000 feet or so.
Timing, to me, is often a harder call than amounts. It appears most of the significant precipitation holds off til after sunrise, though some showers of something may sneak into the southern parts of our area in the pre-dawn hours. Through the day Sunday, there will be periods of rain, sleet and snow, with the all-snow level dropping through the day from highest elevations at sunrise to encompass just about everyone in Southwest Virginia by late morning. There may be a lull or even brief stoppage in precipitation during the afternoon between the first moisture slung northward by the strengthening low and the later snow band with the upper-level low that forms the “comma head” or deformation zone behind the low when it hits the coast. It’s in that deformation zone that the greatest chance for heavy snow lies, and most forecast models bring that right through our area late Sunday afternoon into Sunday night. The heaviest snow still could wiggle north or south some, but it would be difficult to miss us entirely at this point. (At left, HPC map showing probability of getting at least 1 inch of snow by 7 p.m. Sunday — orange is 80 percent, red is 90 percent, dark red is 95-plus percent.)
It hit 64 in Roanoke this afternoon, the second-warmest day in February so far (it was 67 on Feb. 1), 14 degrees above normal, and only 8 degrees off a record high for Feb. 18. How does it shave off half that in the next 12-18 hours?
Well, sub-freezing air is really not hundreds of miles away, like you might think. It’s only a mile and a half away — straight up. Air motion within the storm system will cause lower temperatures to occur throughout the atmosphere, even reaching near the surface in time. That’s called “dynamic cooling.”
Also a factor is that the air is very dry, with dew points running in the 20s this afternoon even as highs were upper 50s to mid 60s. As precipitation starts falling into that dry air, the atmosphere “spends” heat evaporating those droplets and flakes, taking heat out of the air, dropping the temperature. That’s called “evaporational cooling.” Sometimes it’s enough by itself to cool the atmosphere down. This time, it’s only a contributing factor.
Then, there will be some cold air sliding down the east-side of the mountains on northeast winds rotating around high pressure near the Great Lakes. This is called cold-air damming, or a wedge. It is pretty weak in this case, but enough to help a little with the cooling process.
Finally, as the low deepens near the East Coast, north and northwest winds will pull additional cold air along the surface. This is cold air advection. It is not a major factor in getting us from 64 to near 32 this time, but it may help move the needle from near or slightly above freezing to a little below on Sunday night.
There are still ways this storm can go awry, but as time goes along and there do not appear to be major alterations, those ways become fewer in number and less likely. The No. 1 way the snow could come in less than expected is if the surface warmth holds on greater than expected against all four of the factors above. I don’t think it would be a matter of not having snow, but it could delay the onset of snow and reduce accumulations. It does appear this will be a well-organized storm system though, and for an atmospheric machine like that, 30 degrees is not that big a deal to overcome.