UPDATE 10 PM: Precipitation is moving in faster than projected, already along Interstate 81 in Bristol and Abingdon. A mix of rain, freezing rain, sleet and snow will likely develop along the I-77 corridor within the next 1-2 hours and spread into the New River Valley by midnight and the Roanoke Valley shortly after midnight. Click on Radar/Future Cast at right for the latest on its progress. END UPDATE
Winter weather advisory expanded to include all of the region along/east of the I-77 corridor, including the New River and Roanoke valleys, Southside, and north into Alleghany Highlands and southern Shenandoah Valley. Ice accumulations of up to 2/10 inch and sleet/snow accumulations of up to 1 inch expected.
For the second time this week, a low tracking well to our northwest will spread moisture over cold air at the surface. But there is something different this time than Tuesday morning. High pressure, centered over southeast Canada but also bulging southward east of the Appalachians (shown as an “H” on the mid-morning map of the North American Model at left), is better positioned to push in cold, dry air at the surface, even as milder, moist air moves over the top. This is a big part of why the winter weather advisory covers a much wider area than Tuesday morning, with somewhat larger amounts of ice expected, possibly a tenth or two of an inch, and maybe some snow and sleet of less than an inch before it begins. As the moisture moves in after midnight, significant evaporational cooling is expected to bring temperatures rather quickly down to near the freezing mark. Early precipitation may be a mix of rain, freezing rain, sleet and snow, with more sleet and freezing rain as milder air aloft and colder air at the surface becomes established, and later on Friday morning more freezing rain and rain depending on local, subtle differences between locations that wander up to 33-35 degrees and others that hang at or just below the freezing mark. Just because you see no ice on the road, sidewalk or trees at your house doesn’t mean you won’t see some as you drive to somewhere else.
With sunshine and temperatures having reached the upper 40s and low 50s in much of the area, road surfaces will hold in some heat, so won’t most freeze over immediately. As is typical, higher elevations, protected valleys, and shaded spots on roads will develop ice first. If there is substantial sleet or snow on the outset, slushy conditions may develop, even on warmer road surfaces in lower elevations. A general rule of thumb for the early morning hours of Friday would be to expect widespread light icing on roadways- above 2,500 feet in elevation, patchy ice between 1,200 and 2,500 feet, and spotty icing below 1,200 feet, with more road problems if sleet/snow becomes heavy enough to slush up roads. Exposed objects such as trees and power lines will develop icing quickly just about anywhere the air temperature drops to 32, but at this time, amounts are not expected to be significant enough for large-scale power outages or tree damage. High elevations along the Blue Ridge – Floyd County/Bent Mountain come to mind – may have the greatest risk at some 0.25 or greater ice accumulations, generally considered the bottom line for significant tree or power line issues with icing. Roanoke city often floats down to about 33 or 34 degrees and escapes major ice problems in similar previous storms, but there may be just enough cold air wedging and evaporational cooling this time to even put some ice on the trees in lower elevations of the valley. (At left, inset map shows 50+ percent chances of .01 inch of ice in shades of blue, as determined by the Hydromteorological Prediction Center.)
The persistent wedge will only allow temperatures to struggle upward on Friday – not like Tuesday’s vault into the 40s and 50s with only a weak, easily scoured-out wedge of residual cold air in place. It may only crawl into the mid 30s in the Roanoke and New River valleys on Friday, and some particulary iced-up areas may actually not get above freezing. Another round of rain will mostly go southeast of our region late Friday night and early Saturday, but enough will move into Southwest Virginia to pose new ice accumulation potential to any locations that don’t get above freezing or only slightly do, then drift back downward. Locations north and west of Roanoke have the greatest chance of additional ice Friday night and early Saturday, though it appears amounts will again be light.
It looks like there will be two storm systems next week – one about Monday night/Tuesday, the other about Thursday or so – that will again ride the line between rain, ice and snow in our region.