Wednesday, February 13, 2008
Weather columnist Kevin Myatt: Fairly common conditions became nasty this time
Kevin Myatt is The Roanoke Times' weather columnist.
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Sometimes, our most dangerous weather isn't the unexpected or unusual, but the expected and usual that just get turned up a notch.
High winds were expected Sunday. A strong Arctic cold front was moving through, and our region was caught between low pressure to the northeast and high pressure to the west.
The arrival of a punch of cold air, the pressure gradient between a high and a low, and the effects of our mountainous terrain always trigger gusty winds in our region.
We typically get at least half a dozen similar cold fronts each winter, even in a mild one, probably two or three of which cause enough wind to warrant a high wind warning. By Saturday, the National Weather Service had already issued high wind warnings for Sunday.
But some things happened this time to crank the dial up a level or two. Instead of an afternoon of 25-35 mph winds with a few gusts of 50-60 mph, which is usually what happens in these situations, we ended up with several hours of 30-50 mph winds with frequent gusts of 60-74 mph.
Instead of some widely scattered power outages, mostly in isolated rural areas, we got tens of thousands of power outages affecting urban areas just as much as rural ones. We also saw structural damage to buildings, which usually doesn't happen, other than a random tree falling on an outbuilding. We saw a lot more fires, too, owing to months and years of rainfall deficit.
It will take quite a bit of research to know everything that can be known about why Sunday turned from just another windy winter day into something historic, Stephen Keighton of the National Weather Service in Blacksburg said.
Keighton is the science officer at the Blacksburg office, which means he ends up doing a lot of research into local weather events and patterns. Sunday's windstorm was easily the worst he said he had seen in a decade at Blacksburg, in its severity, longevity and area coverage.
The most obvious answer to why it got so bad is the strong streak of winds that moved over the region at the 5,000-7,000 foot level and how that pocket of 50-80 mph winds got pulled toward the surface.
With cold, high pressure building overhead, the atmosphere was set up where the general motion was for air to move downward toward the ground, which also pushed those fast winds downward.
Sunshine warming air near the surface also contributed, allowing the colder air to sink into it. The rising warm air and sinking cold air eventually mixed the first few thousand feet of air into one layer, and that mixed layer took on the fast-moving characteristics of the stronger winds.
Keighton had already done research into how westerly winds crossing the mountains break up into waves, and how these waves can end up crashing in damaging gusts. So, undoubtedly, our terrain played a factor, too.
But Keighton said there may be some subtle factors not yet known that made Sunday's winds so ferocious.
He and his colleagues will be studying the situation for months to come, hoping to garner any bits of information that might be helpful the next time a high wind event is forecast.
