Friday, October 28, 2011

Valley set up for La Niña encore

Weather forecasters’ prediction could bode well for snow lovers

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By KATHERINE WUTZ
Express Staff Writer

A fresh dusting of snow covers the upper reaches of Bald Mountain on Wednesday. The Wood River Valley is set to see another year of La Niña weather, but weather officials say other factors play into the ultimate amount of snowfall. Photo by David N. Seelig

Snow lovers rejoice—tentatively. The Sun Valley region is set to experience another La Niña weather year, but weather officials say the tropical weather pattern does not necessarily mean a record-breaking snow year.

"People want a simple answer," said Rick Dittmann, a meteorologist for the National Weather Service in Pocatello. "They want to hear, 'Oh, La Niña? Now we know, we're going to get a ton of snow.' The planet Earth is a little bit more complicated."

La Niña—Spanish for "the girl"—is a weather system that brings varied but fairly predictable effects across the country. It is associated with cooler Pacific Ocean temperatures and stronger easterly trade winds.

This year, it is expected to contribute to continued drought in Texas, Oklahoma and New Mexico, while dumping more precipitation than usual on the Pacific Northwest.

But Dittman and National Weather Service Science Officer Dean Hazen said La Niña will not be the only weather pattern at play this winter.

A pattern of atmospheric pressure directly over the polar region called the Arctic Oscillation alternates between high and low pressure and impacts storm patterns in much of the northern United States. An oscillation is a shifting weather pattern that swings from high to low pressure.

According to the National Snow and Ice Data Center, the oscillation tends to remain in the positive phase, which means low pressure over the North Pole, keeping temperatures slightly warmer through much of North America.

That oscillation is not as predictable as La Niña, Hazen said. Another pattern known as the Madden-Julian Oscillation can intensify the effects of La Niña on the United States. Again, Hazen said, those oscillations are not always predictable.

"When those happen, you can get increased precip in the Western U.S.," Hazen said. "There are a number of things that come into play, but ultimately, this changes the atmospheric patterns."

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Dittman said the dry spell in the Wood River Valley in January 2011, a La Niña month, was not necessarily typical of the weather pattern, which only makes the Weather Service more aware that its long-term forecasts may be upset by an unexpected oscillation.

"What we normally look for are clues for where our weather patterns might set up," he said. "We look at how many times these other patterns have set up, look at those times and say 'What did we get?'"

The dry spell in January and early February was caused by what is called a "blocking pattern," Hazen said, a stationary ridge of high pressure steering storms away from the valley.

"Once something like that locks in, we need something to knock it out," Dittman said.

Despite the Arctic Oscillation that could cause dramatic short-term swings in temperatures and storm activity, Hazen said he can predict with reasonable accuracy what the general weather patterns will be this winter.

"The system right now appears to favor wetter than normal," he said, but added that his prediction could change at any moment.

"Definitive stuff is pretty tough," he said. "Even when you say likely or favored, you're talking about very small percentages."

In a normal year, there is a 33 percent chance of the winter being wetter than average, drier than average or just average. This year, Hazen said, the odds of the Wood River Valley's having a wetter-than-average winter stands at 50 percent, or 17 percent above normal.

Dittmann and Hazen were careful to keep predictions vague and open to change, but Dittmann had carefully qualified words of encouragement for powder junkies.

"The odds are, looking at La Niña and the other oscillations, there's a better-than-average chance that it will be a more active winter."

Katherine Wutz: kwutz@mtexpress.com

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