14 Desember 2010

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What caused the Aumsville tornado?

A tornado that caused substantial damage in Aumsville late Tuesday morning likely was caused by unstable cold air slamming into the unseasonable warm, moist air mass that has lingered in the Willamette Valley for several days, climate experts say.

Tornadoes in Oregon are unusual, though not exactly rare. Most of them are mild in nature, so the damage caused by Tuesday’s twister caught many people by surprise.

“Oregon’s moderate climate and topography are not conducive to frequent powerful tornadoes,” explained Kathie Dello, a research scientist with the Oregon Climate Service at Oregon State University. “The Pacific Ocean moderates the climate and the Coast Range protects us.”

“Typically, to get a strong tornado, you need a heat source to provide ascent, cold air aloft, moisture and upper-level wind shear all at once,” she added, “and that’s what we got today.”

The National Weather Service lists recent tornadoes in Oregon, including a December 2008, F2 tornado near Newberg described as “the most powerful tornado in Oregon in many years.”

The most recent listings came from fall of 2009, when small (EF0) tornadoes touched down near Lincoln City on Nov. 7, and near Oregon City on Oct. 26.

During the past 20 years, about 20 tornadoes have been logged by the National Weather Service and they were scattered around the state. Several were in the Willamette Valley, but others were along the coast, and still others in eastern Oregon.

These events are the results of atmospheric instability, Dello pointed out, and are not necessarily tied to either climate change or the La NiƱa winter.

“Oregon often switches from a warm front to a cold front, and vice versa,” Dello said, “but usually that transition is much more moderate. We had a lot of warm, moist air that had settled in the valley beneath a cold pool of air aloft, and a very strong low-pressure system that created a wind shear. That cold trough really kicked things off.”