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Weathercatch: Pacific Northwest no Tornado Alley, but twisters do occur

One afternoon last month, eight separate twisters dropped down on southern Minnesota, earning that day the name “Tornado Tuesday” in media reports. Fortunately, the twisters touched down in mostly open areas and no one was injured. A month earlier, 24 tornadoes ripped through Oklahoma. One person was killed and a state of emergency was declared in 15 counties.

The Pacific Northwest is pretty much isolated from these kinds of calamities. Beyond the movie “The Wizard of Oz,” most of us have never witnessed a twister. Nonetheless, these fierce manifestations of nature do occur in this region, and occasionally people get hurt. The worst recorded event was on April 5, 1972, when the country’s deadliest tornado that year tore through Vancouver, Washington, killing six people and injuring 300, including 70 schoolchildren.

Though rare, it was a wake-up call that the right conditions can and do align to create tornadoes in this part of the country. An average of two twisters touch down in Washington and Oregon each year, according to the National Oceanic and Atmospheric Administration. Luckily, they tend to be weak and usually don’t strike in urban areas. By comparison, Texas gets an average of 146 tornadoes each year, followed by Kansas with 92 and Oklahoma, 65.

Why the stark disparity? The ingredients that create frequent and powerful twisters assemble far more easily in the nation’s midsection than here in the Northwest.

Tornadoes form when warm, humid air collides with cold, dry air. The geography of the eastern portion of the country allows just this sort of thing to happen: Warm, moist air flowing northward from the Gulf of Mexico interacts with cool air from the Rocky Mountains.

But that’s not the case here, where we’re bound by the Pacific Ocean to the west and the Rockies to the east. A key ingredient that’s lacking is humidity, or water vapor in the atmosphere’s lower level. Air emanating from the Gulf of Mexico may be warm and moist, but not the air moving off the chilly Pacific waters. Because cooler air holds less moisture than warm air, there’s less fuel to kick-start thunderstorms and fewer thunderstorms that can spawn tornadoes.

Nic Loyd is a meteorologist with WSU’s AgWeatherNet. Linda Weiford is a WSU news writer and weather geek. Contact: linda.weiford@wsu.edu or nicholas.loyd@wsu.edu.