Summary bold-faced at bottom
Winter 2012-13 is doing it to us again on Tuesday morning. I’ve lost track of how many of these relatively minor, but potentially traffic-troubling, borderline winter-weather situations we’ve had in Southwest Virginia since the first on the day after Christmas. Here’s the setup: A low pressure system is tracking through the Great Lakes, pulling up a swath of Gulf of Mexico moisture northward ahead of a cold front. This is far and away NOT a pattern that produces significant widespread winter weather events in Southwest Virginia. But, because there remains a thick layer of cold, dry Arctic air banked against the Appalachians, as precipitation falls through that air early Tuesday morning (3 a.m. to 9 a.m. is a generally good timeframe to consider for this, right now, from I-77 eastward), evaporational cooling will quickly pull much of the atmosphere below freezing from cloud to ground (or very nearly so) leading to more snow and sleet rather than rain or freezing rain, at least to start. Yes, the temperatures at the surface got into the 50s Monday, but the precipitation is arriving at the coldest part of the 24-hour cycle, surface dew points remain low, and the layers above the surface are still even colder and drier than what we have at ground level. As precipitation falls, the first part of it will evaporate on the way down, and that process removes heat out of the atmosphere, cooling temperatures. With the low so far removed from our region, the flow of moisture is not overwhelming, so we’re talking light amounts, likely no more than a quarter-inch of liquid, and probably closer to a tenth of an inch for most.
Most of this winter’s systems have produced some kind of oddball surprise, sometimes with localized areas getting a lot more precipitation than expected (Floyd County’s power-crippling ice storm on Dec. 26; 3-5 inches of snow in the Roanoke Valley on Feb. 7-8; 4-8 inches around Appomattox this past Saturday) and sometimes with much less occurring (Dec. 29 projected snow largely busting, Bedford County snow hole on Jan. 17). One surprise that is possible Tuesday morning is if the moisture just dries up and very little of anything falls. Another would be if stays just cold enough for snow and the upper-end liquid amounts are realized, there might be more 2-3-inch snow amounts than expected. What I think is most likely is a lot of sleet and some snow gradually giving over to more rain mix for 3-6 hours on Tuesday morning, with widespread 1/2 to 1 inch amounts from the Roanoke Valley and Blue Ridge westward, and locally up to 2 inches. Whatever happens will be over quickly and temperatures will rise well above freezing (40s, some 50s) by Tuesday afternoon.
Regarding the Thursday night-Friday system — well, it looks sort of similar to this with a weakening moisture field advancing eastward with a low far to the northwest and a middling cold air wedge that could lead to some frozen precipitation at the outset.