UPDATE 6:20 PM: Winter weather advisory for counties north/northeast of Roanoke, for snow/sleet accumulation Saturday. Will update with fresh weather-focused post later tonight. END UPDATE
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Saturday appears likely to be the fourth slightly misfiring system of the young season for anyone who would like to see significant snow in the Roanoke and New River valleys and nearby areas. With last weekend’s level of cold-air damming, we would be making snowballs by noon Saturday. With this weekend’s retreating cold-air wedge, there may be a period of sleet mixed with some snow on the front end of Saturday’s rain as Gulf of Mexico moisture overruns it. The Weather Prediction Center starts the better chances of getting at least 1 inch of snow/sleet mix about at Interstate 64 (blue colors at left denote 40 percent chance) with lesser chances to the south (light green over Roanoke/Blacksburg is 10-20 percent). What snow/sleet we get will be primarily caused by evaporational cooling, as very dry layers of air are slowly saturated by falling precipitation. As the initial precipitation falls into the dry air and evaporates, heat from the air is used up in the evapoartion process, and therefore removed from the atmosphere, causing cooling. As more and more layers cool and more and more precipitation reaches the ground, it will likely be sleet at first, as rain falling through the cold layers refreezes. Snow will be possible for a time if all of the layers from cloud to ground are at or below freezing (thin layers slightly above freezing can sometimes be traversed successfully by snowflakes). There is always some chance the evaporational cooling will be more pervasive or extensive than expected and lead to a longer period of wintry precipitation, or that precipitation will be heavier earlier in the event and lead to more snow/sleet falling than expected. What is just about certain, though, is that warmer air will move in aloft, and eventually at the surface, as a fairly weak low moves northeast just west of the Appalachians before transferring energy to a new low-pressure system off the Virginia coast.
Somewhat milder air — maybe some 50s for highs and day or two — will move in next week before more Arctic air arrives by next weekend.