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Monday, November 17, 2008

Weather columnist Kevin Myatt: Pressure ushers in Arctic chill

Kevin Myatt is The Roanoke Times' weather columnist.

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When California burns, we freeze.

That's not an absolute rule, but it's usually the case for reasons related to the placement of a key weather feature.

When the Santa Ana winds crank up in Southern California, it's the result of strong high pressure over the western U.S. to the northeast of California. The clockwise flow around this high pressure system forces strong winds out of the Desert Southwest, across Southern California's mountains.

These winds, accelerating through valleys and canyons, dry out and heat up as they are forced downhill toward the coast. The result is almost always a spate of rampant wildfires.

Southwest Virginia sometimes sees a similar but weaker downslope effect on strong southwest winds over the Appalachians. The fire-spawning, tree-toppling windstorm of Feb. 10 is probably the closest thing to the Santa Ana that most of us will ever experience here.

The same rotation of high pressure responsible for easterly winds in Southern California brings northwesterly winds out of Canada into the eastern United States, often driving through a strong Arctic cold front.

That, indeed, happened on Saturday. The front triggered some showers followed by blustery winds in Southwest Virginia. Encountering more instability in North Carolina, it also spawned deadly tornadoes.

Saturday's cold front will be reinforced by at least two more fronts this week, producing a windy, cold week more akin to January, with a few snowflakes in the air for dramatic effect.

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