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Friday, July 25, 2008

Weather columnist Kevin Myatt: Summer of severe weather continues

Kevin Myatt is The Roanoke Times' weather columnist.

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@roanoke.com

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Let's all take a deep breath.

There were no severe thunderstorms in Southwest Virginia on Thursday. There also aren't any named tropical systems in the Atlantic basin for the first time since July 3.

Those will happen again soon enough.

This is shaping up to be the summer of the severe storm in Southwest Virginia.

Every few days, we've dealt with wind damage and hail.

June 1. June 3. June 11. June 14. June 22. June 26 to 29. July 6 to 9. July 22 and 23.

Each of those days has produced significant severe weather -- storm winds of 58 mph or greater, or hail at least ¾-inch in diameter -- somewhere in Southwest Virginia.

And it's not all been the typical pulse storms that build on hot, sticky summer afternoons, quickly expending all their energy in one short-lived but mighty self-destructive blast of wind and rain.

Much of this summer's storminess has been more organized, with enough upper-atmospheric wind support to keep storms going awhile.

We've seen many multicell clusters. These occur when there is a moderate amount of atmospheric shear, which refers to winds changing directions and/or velocities with height.

The winds aloft are enough to at least partially separate updrafts and downdrafts, but not enough to fully separate individual storms and give them a good spin. Instead of a single storm blowing itself out, the clustering of cells allows a new intensifying storm to take over when another weakens, so multicell clusters last longer than single pulse storms.

But we have also seen an unusual number of rotating storms this summer, most significantly on June 3, when a compact but rapidly twisting supercell spun out a brief tornado over Roanoke.

More commonly, storms have exhibited some weak rotation for an hour or two near the beginning of their life cycle, before fusing into lines or clusters. It's been enough to propel damaging winds across many miles with a few storms, and also allow long enough updrafts for some large hail.

The next week or two is likely to bring additional chances for severe thunderstorms. A jet stream dipping unusually far south over the eastern United States will bring cold fronts and upper-level disturbances. It will also supply more vigorous upper-level winds than we typically see in July or August, helping organize storms.

The diving jet stream will keep the worst summer heat away, but yet more severe storms may be the price we pay.

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