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Wednesday, March 12, 2008

Weather columnist Kevin Myatt: High pressure means weather should be mild

Kevin Myatt is The Roanoke Times' weather columnist.

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Two more winter storms veered wide left last week, solidifying this as the winter of the Southeast ridge.

The "Southeast ridge" refers to a persistent area of high pressure that has been located over the Atlantic Ocean off the Southeast U.S. coast. High pressure systems are often referred to as "ridges" while low pressure systems are called "troughs."

This feature has been with us at some level almost constantly since late summer. It frequently occurs with La Nina, the periodic pooling of abnormally cold water in the central Pacific that has been occurring since midsummer, but it has seemed particularly persistent this time around.

The high pressure system, a huge mound of stable or sinking air that squelches most large-scale precipitation, has expanded and contracted and moved around a little bit. It has consistently been strong enough to bump storm systems that might otherwise move up the East Coast more toward the nation's midsection.

This happened once again twice last week, leaving us with showery, gusty rain while the big snows clocked locations from Arkansas and Mississippi to the Great Lakes.

Thanks to 7 inches of snow Friday, Memphis, Tenn., now has a higher seasonal snow total than Roanoke (4.9 inches since December) -- not unprecedented, but certainly unusual, considering Roanoke historically averages about six times more snow per season than Memphis.

The most recent storm systems passed farther east than most have this winter. The center of low pressure on Friday passed almost directly over Roanoke. The area of snow typically occurs to the north and west of the low pressure center.

Instead of the heaviest snow occurring in northern Illinois and Wisconsin, as has occurred much of this winter, places farther east in the Ohio Valley such as Louisville, Ky., and Columbus, Ohio, got hammered with 1-2 feet of snow late last week.

But the high wouldn't budge enough to allow the storms to move any farther east, and as a result, what likely was our last two chances at a significant winter storm this season have gone by the boards.

Winter is fading away now, but the Southeast ridge is not.

In fact, over the next several days, the Southeast ridge is likely to become more prominent, as the jet stream dips across the Western U.S. and flows up and over the ridge in the East.

The probable result will be milder weather with only occasional cold-front passages bringing showers followed by brief, gusty cool shots.

It doesn't look like we're about to spring into the kind of balmy March we had last year, but the next couple of weeks do not look wintry, either. Except for the highest elevations, it will take a late-season fluke to bring snow now.

And the pattern does not look especially wet, either. We didn't reap tons of moisture off the lows last week, as the best rains went to the south and east of us while the heavy snows went north and west.

We've had enough precipitation now and then this winter that we're not in a dire drought situation, but it wouldn't take but a few weeks of very dry weather to get us there again.

So we enter spring having missed what was a winter of ample to excessive snow for almost the entire rest of the country, excluding the far southern tier of states where it rarely or never snows anyway, and our rain prospects overall don't look especially good.

The Southeast ridge has not been friendly to snow lovers, and it could be a nemesis for all of us if something doesn't budge it to allow more rain this spring.

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