Sam Dean | The Roanoke Times

Tuesday's snow dusted maple leaves only recently fallen on the Roanoke valley floor themselves.

Justin Cook | The Roanoke Times

Much of the region got a taste of winter Tuesday, including snow on the Virginia Tech Drillfield.

Kevin Myatt is The Roanoke Times' weather columnist.

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Upslope snow events typically happen 10 or more times between November and April, but they usually don't dump 1 to 3 inches in the New River Valley and dust the grass and car tops in the Roanoke Valley.

This one had some help upstairs.

Upslope snow showers occur as cold northwest winds glide up the western side of the Appalachians, lifting whatever moisture is available, condensing that moisture in cooler air aloft and squeezing out snow. Usually, only the higher elevations and west-facing slopes get significant snowfall.

On Monday night and early Tuesday, an upper-level disturbance crossed the mountains. This upward swirl of very cold air aloft enhanced the lifting motion and provided instability.

We tend to think of instability as a warm-season phenomenon, when muggy air near the surface lifts into colder aloft, firing thunderstorms. But, when temperatures aloft are extremely low -- many degrees below zero -- cold air in the 20s and 30s at the surface is relatively "warm" and rises into that much colder air aloft.

These more intense snow squalls were better able to survive the trip over the mountains, and as a result, the New River and Roanoke valleys got much more snow than we typically do in an upslope snow situation.

Meanwhile, the favored locations that usually get a few inches in upslope snow got buried. More than a foot fell in the mountains northwest of Lewisburg, W.Va., and Snowshoe Mountain ski resort reported 2 feet.

Another upslope snow event is possible Thursday with a new cold front, but don't expect a repeat. This one wrung out most of the moisture.