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Weather twists make tornadoes

Wednesday, June 03, 1998

By Sharon Voas, Post-Gazette Staff Writer

Tornadoes are rare because so many weather factors must come together at once to produce them.

"Mother Nature really has to be in a bad mood," said Lee Hendricks, a National Weather Service meteorologist.

Obviously, she was in an unusually foul mood yesterday when she swept through this area, leaving roofless homes, sheared trees and toppled power lines in her wake.

One of the factors at work came from northern Canada. A mass of cool, dry air pushed southeast from there through Michigan and Ohio yesterday. It smacked into another factor, an especially warm humid air mass hanging over the mid-Atlantic states. At the same time, there was a third factor: Strong winds rotated in varying directions at different elevations.

As the cold air hit the warm air in midafternoon, it spawned thunderstorms in Ohio, West Virginia and southwest Pennsylvania.

The difference between the temperature and moisture in the two air masses was so much greater than usual that it sent the air twirling up into the atmosphere. The air began rotating near the ground, and the column of swirling air rose upward in a thunderstorm, said Laura Hannon, an Accuweather meteorologist.

The calamitous clash spawned a number of thunderstorms that moved through the state, spinning funnel clouds down indiscriminately here and there.

Normally, tornadoes aren't attracted to the topography of Pennsylvania, partly because the mountains here disrupt their swirling motion. The state averages one to 1.5 tornadoes a year, said Hendricks of the weather service.

"It's been a very busy year for severe weather," Hannon said. "There's no one reason you can give. And no, it's not El Nino."

The jet stream -- a fast-moving river of air that holds great sway over the weather -- is staying farther south than usual, and that causes more severe clashes between air masses, she said.

By this morning, the storms were expected to be over the ocean off the Eastern Seaboard.



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