Wednesday, February 22, 2012
Weather columnist Kevin Myatt: Cooling caused shift from wet to dry snow
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High of 52, low of 33 — would you be expecting to see 5-plus inches of snow if you saw those weather statistics listed?
Yet, that was essentially what happened in Roanoke on Sunday. The official low ended up at 32, the freezing mark — but that was recorded literally in the last moment of the day, at 11:59 p.m.
After falling from a high of 52 just after midnight, the surface air temperature at Roanoke hung at 33 degrees for hours as snow poured down, never reaching the freezing mark until the snow had almost dwindled. This is not unusual in late winter and spring.
The result was that accumulated snow continued to melt slowly from underneath as it was falling, partly from the air temperature not reaching freezing and partly because of the warm ground underneath baked in mid-60s sunshine just the day before.
In order to accumulate, snow simply fell at a faster rate than it could melt. This is how heavy snow overcomes the oft-repeated saying about snow not sticking because the ground is too warm.
Roanoke officially ended up with 5.5 inches of snow, measured at WDBJ (Channel 7) television studios near the airport, where all other official Roanoke data is derived.
Aside from reports of 1 to 3 inches in parts of Southside, that was near the lower end of our region's reported snowfall totals, and as much as 2 or 3 inches less than the accumulation at slightly higher elevations just a few miles away elsewhere in the Roanoke Valley.
You may have noticed a change in the snow texture between what fell during the day Sunday and what fell in the early evening.
The early snow, when at its heaviest, consisted of large conglomerated blobs sometimes as large as a quarter. That came from many snowflakes slightly melting into each other in temperatures near or very slightly above the freezing mark.
But later in the day, as very cold air from an upper-level low passed overhead, the snow became individual, small flakes we often call "dry" snow. Its texture on the ground was more powdery than the clumpy "wet snow" we had seen earlier.
Unfortunately for thousands without power, the drier snow kept accumulating on the base of wet snow that had already caked trees and power lines. Accumulations of 5 to 9 inches, or even twice as much, composed entirely of the more powdery snow would have caused very few power outages.
Perhaps it was appropriate that we had a snow event more like early spring than midwinter, as the weather both before and after it is more like spring than winter. High temperatures near 70 degrees are possible by Thursday.
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