Friday, February 17, 2012
Weather columnist Kevin Myatt: A blast of snow amid the warmth?
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Too warm to snow?A few examples in the past decade of significant snow occurring at Roanoke the day after mild to warm temperatures.
- Feb. 21, 58 degrees
- Feb. 22, 3 inches
- March 29, 60 degrees
- March 30, 6 inches
- Feb. 26, 52 degrees
- Feb. 27-28, 8 inches
- Feb. 27, 61 degrees
- Feb. 28-March 2, 4 inches
When it's sunny and in the 50s today, snow may be the furthest thing from your mind.
But it could only be a couple of days away, as a situation develops Sunday that provides the best chance we've seen all winter of widespread, significant snowfall across Southwest Virginia in this oddly mild and almost snowless 2011-12 winter.
At the least, it appears likely that most of the region will see rain turn to snow by late Sunday, with at least some slushy accumulations possible.
The season's mild weather is one of the biggest hindrances to any potential snow.
Historically, it's not unusual at all in mid- to late February to see a substantial snowfall follow a few days with highs in the 50s. But this is a winter averaging more than 4 degrees above normal over the past two and a half months, still vying to be among the five warmest winters in the past century in Roanoke.
Cold air shots have been in and out in a few days, so the ground has not gotten very cold. In turn, the lower layer of the atmosphere has also had no time to chill, either.
There is no vast source of Arctic air to accompany this weekend's storm, which otherwise appears likely to be well organized and follow a textbook track for a significant winter storm in Virginia, moving from the Gulf of Mexico to near Cape Hatteras, N.C.
It will have to find cold air from someplace else, and that is straight up.
As the storm strengthens, a strong upper-level low will provide a very cold pool of air high in the atmosphere. The combined action of the surface low to the southeast and falling precipitation will pull that cold air downward, which may be enough to drop temperatures into a range that would support snow reaching the surface. This is called dynamic cooling, a common mechanism that allows snow to fall late in the season.
If the snow is heavy enough for long enough, things could turn quite white. It will take heavy snow, or at least a long period of moderate snow, to overcome the built-up warmth at the surface. Snow falling on the ground will melt at first, but if it falls heavily enough, a slushy layer could form upon which additional flakes can stick.
Tracking where the heaviest snow and coldest temperatures are likely to develop is difficult, depending on several atmospheric features flung across the Northern Hemisphere being tracked by computer models and human forecasters. The ribbon of heaviest snow may shift somewhat north or south of the region. As is typical, higher elevations will have the best chance of seeing accumulation.
We've been following it for days on the Weather Journal blog, and will continue through the weekend.
Even if it does snow enough to stick, it won't last long, with mild temperatures again appearing in the weekend. Nor will it likely be a game-changer for the weather pattern, which still leans to more mild days than cold ones over the next week or two.
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