A year ago this weekend. on Saturday, March 24, 2012, parts of Franklin County and the Smith Mountain Lake area gained a white coating up to 6 inches deep … but it wasn’t snow. An unusually well organized supercell thunderstorm for this region tracked from Patrick County — where baseball-sized hail was reported — northeast toward the Ferrum and Rocky Mount areas and then Smith Mountain Lake. You can click here for a reminder of the event as covered on the Weather Journal blog. Numerous tornado warnings were issued on the storm based on Doppler radar indications of tight circulation but there was no ground evidence of a tornado touchdown was ever found. Low-level wind shear was the only parameter that was missing for a tornado.
We’re not talking about tornadoes or severe thunderstorms locally on the same weekend this year — that’s staying near the Gulf Coast. Instead the warmest March weather on record setting us up for severe weather, a March with a legitimate shot at being one of Blacksburg’s five coldest since 1954 and Roanoke’s 10 coldest since 1912 has set us up for an early spring wintry precipitation event on Sunday. Like most of the wintry events when it actually was winter, this one is a murky borderline event between snow, sleet and rain, as the boundary between warm-air advection aloft and cold-air wedging on the east side of the mountains is likely to fall right across our region. Generally, a slushy mix is expected in the Roanoke and New River valleys with minor accumulations (2 inches or less), with more snow and more accumulation the farther north you go, maximized in higher elevations, and less southward. There is some risk, though, that moisture will rush in more intensely than forecast in a slightly colder atmosphere, and, if that happens, snow could come down hard for a few hours and quickly pile up a few (2-4) to even several (4+) inches. A few forecast models have shown this, particularly the North American Model, which has depicted it for several runs. The Weather Prediction Center this morning has a moderate risk of 4+ inches of snow nudging Roanoke and Blacksburg, while also barely having a coin’s flip chance of getting just 1 inch. Spring snow events are known for being mercurial like this, often forecasting busts in either direction. Just a little warmer than modeled in the bottom mile of the atmosphere and it could easily be just cold rain with maybe some mixed sleet.
Most models show two waves of precipitation with the complex and somewhat loosely organized storm system featuring two low-pressure centers, one moving west of the Appalachians and the other developing in the western Atlantic. One precipitation wave arrives during the mid-morning or so and the other on Sunday night into early Monday morning. The first wave has the best chance of being heavy snow, but also more chance of being rain or mix. The second wave would probably be snow, but not as intense. Midday to afternoon snow rates often have to be quite heavy to accumulate much because of the higher sun angle leading to more absorption of solar radiation than in midwinter, even with cloud cover. It is possible some places will see some accumulation in the first wave, have it mostly melt off afterward, then get a little more Sunday night and early Monday.
Bottom line: Expect to see at least some snow in the Roanoke and New River valleys northward on Sunday. Major disruptions are not expected at this time, but do keep an eye on radar and forecasts for changes that could develop rapidly during the day Sunday. Roads can quickly become slushy even in spring, even at 33-34 degrees, if wet snow and/or sleet falls heavily enough. Because of the cold weather, we have not had nearly the same greening of trees we saw in 2003 before the March 30 snowstorm, and therefore would not expect the same level of tree damage if heavier snow occurs, but January 17 showed us that wet snow can still do quite a bit of tree damage and knock out power to thousands even without that. If we start getting above 3 or 4 inches, that would be a concern.