UPDATE 1:45 PM: Winter weather advisories continue for Bath, Rockbridge, and Amherst counties northward into Saturday morning, but have been lifted for all counties to south. Temperatures are expected to remain above freezing as additional rain moves in this evening, except for pockets in the advisory counties. END UPDATE
UPDATE 11:15 AM: Winter weather advisories have been lifted from Roanoke and Blacksburg southward. They remain in effect for Craig, Botetourt and Bedford counties northward for some patchy freezing drizzle. END UPDATE
UPDATE 6:45 AM, 2/22: Patchy freezing rain, rain, sleet and snow (mostly east/northeast of Roanoke) continues this morning (radar above from 6:38 a.m. … click here for current radar) in parts of Southwest Virginia, though it appears drier air may punch in from the west and bring a lull to the precipitation in the next few hours.Some patchy drizzle/freezing drizzle or light rain/light freezing rain may re- develop through the day as southeast winds lift moisture up the slope of the higher terrain, but temperatures will slowly warm above freezing where it is not there already, except maybe for a few isolated locations, primarily valleys north and west at Roanoke, that cling near the freezing mark. The early morning period has brought light amounts of snow, sleet and glaze ice to many parts of the region. Beware if traveling of patchy ice on bridges and overpasses and in higher elevations, as temperatures vary from near 30 to the mid 30s. END UPDATE
Precipitation — rain and sleet, mostly, some snow — has been advancing northeastward out of Tennessee into Southwest Virginia on Thursday evening, but is hitting a wall of dry air near the Interstate 77 corridor (radar above from just before midnight … you can link here for current radar). This is evidence of the wedge of cold, dry air in the layer of the atmosphere above the surface. As the precipitation advances into the drier air, it will continue to moisten it, but early precipitation that melts on its way to the surface will help cool those layers through a process known as “evaporational cooling.” Eventually moisture will reach the surface in the New River and Roanoke valleys, likely beginning as sleet and snow (mixed with some rain at the outset in lower elevations). Later in the morning, warmer air building aloft on top of the colder air at the surface will change the precipitation to more of a rain/sleet or freezing rain/sleet mix — depending on whether surface temperatures at your locality are just above freezing or just below — and eventually to a cold rain in many areas before tapering on Friday morning. Snow/sleet amounts of 1/2 to 2 inches plus glazing of a tenth or two can be expected in much of the region, which is why winter weather advisories cover a large swath of western and southern Virginia. Beware if traveling in the morning … conditions may be much different in various places you travel than they are at your front door.
Another round of rain on Friday night and early Saturday will again be flirting with the freezing mark, with some locations especially north and west of Roanoke getting additional icing.