OREGON STATE UNIVERSITY

"Dry" Oregon largely spared the wrath of tornadoes

05/05/1999

CORVALLIS - Oregon may get monstrous winter winds, catastrophic floods and dangerous ice storms, but in all likelihood it will never experience the type of huge tornado that just cut a swath of devastation across central Oklahoma, killing more than 40 people.

It's not that Oregon is immune to tornadoes, because it's not - a few tiny ones usually are recorded every year, a few people have died and a few buildings have been blown over, said George Taylor, the state climatologist at Oregon State University.

But the particular characteristics of the Pacific Northwest maritime climate, Taylor said, largely preclude the type of huge storm events that are common to "tornado alley"- a stretch of the central plains running from Texas northeast to Oklahoma, Kansas, Nebraska, Iowa and Missouri that is far and away the most tornado-prone region on the face of the Earth.

In that area, very warm, moist air comes rolling up from the Gulf of Mexico and often encounters cold, dry air dropping south from Canada.

"When these conditions reach their peak, usually during the months of April and May, they can trigger storms of incredible intensity," Taylor said. "It's almost like an explosion."

The temperature differences are certainly one key to the volatility of tornado alley, Taylor said, and the lack of such a violent contrast in temperatures is one of the reasons that Oregon rarely experiences damaging tornadoes.

But a less appreciated fact - and one that seems to run counter to Oregon's soggy reputation - is that the air of the Pacific Northwest usually holds far less water vapor than the atmosphere of tornado alley. And that water vapor is critical to the formation of large tornadoes.

"It's not unusual to have 100 percent humidity in the Pacific Northwest many times during the year," Taylor said. "But the more relevant issue for the formation of tornadoes is the dew point, which is a measure of the amount of water vapor actually held in the air. Warm air holds more water vapor than cold air, and when our Oregon climate is at its wettest, it's usually cool."

On a rainy winter day in Oregon, there might be a relative humidity of 100 percent and temperature of 36 degrees, yielding a "dew point" of 36.

By contrast, on a brutally hot summer day in Death Valley, Calif., there could be a bone dry relative humidity of 7 percent and temperature of 122 - which yields a dew point of 37. There's actually more water vapor in the air that day in the deathly-dry valley than during an Oregon winter storm.

"It's a complicated mechanism that forms tornadoes, but it requires both contrasting temperatures and a high level of water vapor in the air," Taylor said. "It may not seem like it sometimes, but in Oregon our air is just too dry."

In tornado alley, by contrast, warm humid air comes moves north from the Gulf of Mexico. When it hits the cold, dry air from Canada, it rises, cools off and begins to condense. That process of condensation releases much heat, causing the air to rise further. It then cools more and condenses more. Then it rises and releases more heat.

This deadly spiral during certain types of thunderstorms is what can cause storm clouds ranging up to an incredibly high 70,000 or 80,000 feet, Taylor said, and monster tornadoes.

"The key is the moisture content of the air," Taylor said. "As anyone can attest who has spent a muggy summer day in the Midwest, the warm air there is just full of water. That's why they have the potential for such incredible storms when it hits the cold air coming down the plains from Canada."

Although puny by midwestern standards, the "hot spot" for tornadoes in Oregon seems to be the Willamette Valley, where the most such storms are reported. But that's possibly, or probably a function of where the most people live who are able to observe and report a storm, Taylor said. There actually is more thunderstorm activity, and probably more tornadoes, in sparsely-populated eastern Oregon.

"Often our valley tornadoes don't amount to much more than a large dust devil," Taylor said.

In 1953, a tornado struck downtown Corvallis, appearing about 8 a.m. one morning out of a dark winter cloud. It blew down one building, passed close to a school, crossed the Willamette River and disappeared. In 1994, a small tornado touched down in Albany and blew out a store window.

Bigger storms have also hit. In 1972, a tornado near Portland damaged a number of boats on the Columbia River, crossed into Vancouver, Wash., and caused six deaths and almost $6 million in damage.

And going back all the way to 1888, a fairly powerful tornado swept through three small towns in eastern Oregon's Morrow County, destroying 30 buildings and killing six people.

But all in all, that's not much by tornado-alley standards. And for much of that we can thank that fact that - believe it or not - Oregon's air is just too dry.