National Weather Service forecasters are skilled enough to provide advance warnings for about 75 percent of tornadoes. But they weren’t skilled enough to warn people in Aumsville on Tuesday. At 11:53 a.m. forecasters issues a severe storm warning, nine minutes after a tornado ripped through the town.

The infrequency of destructive tornadoes in the Northwest can also make them harder to predict. Oregon gets about one tornado per year. Most are small and cause little damage. Kathie Dello, a research scientist with the Oregon Climate Service at Oregon State University said the moderating influence of the Pacific Ocean and the rough topography of the Cascade Mountains limit the potential for strong tornadoes.

Dello said the Aumsville tornado formed when a cold front slammed into the wet and unusually warm air covering the Willamette Valley. “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,” she said. “That cold trough really kicked things off.”

The radar signals don’t say everything about the structure of a thunderstorm. They usually can’t detect the tornado itself. The radar typically spreads too wide to resolve even large tornadoes if they are miles away from a transmitter. Oregon’s radar sites are in Medford, Pendleton and Portland. Forecasters use radar to measure strong winds blowing in opposing directions. That suggests intense circulation and a possible tornado.

Tornadoes don’t always form when conditions are prime, and sometimes they form when the conditions aren’t so favorable. Atmospheric scientists don’t fully understand why. Before issuing warnings, forecasters still depend on reports from spotters in the field and their own judgment based on all the information from radar, satellite images, and other instruments.

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