OREGON STATE UNIVERSITY

La Nina intensifying, setting stage for mid-winter valley snow

01/13/2000

CORVALLIS - That lovely week of sunshine in late December was something of a fluke, but other than that, the Pacific Northwest is following its La Nina script almost to the letter - for what may soon shape up as one of its harshest and snowiest winters in recent history.

In fact, the La Nina characteristics in the Pacific Ocean are actually intensifying even beyond what was evident last fall, said Oregon State University's George Taylor, the state climatologist.

That's a prescription for a mammoth mountain snowpack, heavier than usual precipitation, more cold weather, some heavy snow in the valleys and a better-than-average chance for spring flooding.

"The conditions that are developing this winter are about as good as it gets to produce a big snow across the whole Willamette Valley," Taylor said. "We usually get a big dump on the valley floor about half the time, and haven't had one now for three years. I think we're due."

Last summer, based on historical analysis of the types of winters the Pacific Northwest gets during La Nina years, Taylor predicted a fairly mild start to the winter except for some good rainstorms in late October and November. That's exactly what happened, and it included one "pineapple express" surge of tropical moisture that dropped 11 inches of rain on Tillamook County in a single day.

"The late December sunshine was a glitch in the forecast," Taylor said. "Before that our predictions had been right on the money. But a strong ridge of high pressure gave us many sunny days in December and even kept out the valley fog, and that was really unusual. At the start of January we got back on track."

Last August, Taylor said that early January would bring "an onslaught of winter storms that will continue for months." There's no cause now to change that prediction, he said.

Except it could be even worse than anticipated.

"This La Nina appears to be the strongest one since 1988-89 and among the top five of the century," Taylor said. "Intense La Nina events like this give us our wettest, coldest winters and the biggest snowstorms. During moderate La Ninas, more of the low-level precipitation falls as rain."

La Nina events, Taylor said, are characterized by a tongue of cool water, about five to seven degrees colder than normal, that extend thousands of miles westward across the equator in the Pacific Ocean. They affect seasonal weather all over the world. And on top of that, Taylor believes the Pacific Northwest has entered a 20-year period of cooler, wetter weather that will compound the La Nina effects.

What's highly possible, Taylor said, is that in late January or early February, the Pacific Northwest will get a good dose of cold arctic air that comes down from the north, loops out over the ocean and then comes back onshore to produce a big six- or eight-inch snowstorm in the valleys.

After a somewhat slow start to the winter, the mountain snows are now making up for lost time, Taylor said, receiving a foot or more a day of snow recently at higher elevations. Santiam Pass was closed briefly by snow avalanches, traffic is snarled everywhere and as of Jan. 11 the Willamette Valley drainage area had 123 percent of its average snowpack.

On a statewide level, Taylor said, precipitation so far is about average, a little higher in the west and lower in the east. Precipitation ranges from 57 percent of average in Lake County in eastern Oregon to 128 percent of normal in the lower Columbia River and Mount Hood areas.

But from now on, the winter should worsen.

"With this intense La Nina, we're now going to get hit pretty hard for a while," Taylor said. "And it certainly sets up the potential for some spring flooding."

The most serious floods, he said, take a combination of wet winters, saturated soils, a moderate snowpack at lower elevations and then an intense, warmer storm straight out of the tropics that causes much of the lower-level snow to melt quickly. That's exactly what happened during the 1996 floods, which were considered about a 25-year flood event. But the current conditions will produce higher chances of that happening this year - not 25-1, Taylor said, but about 10-1.

"Sometimes the La Nina events give us a lot of snow and rain, but they also give us cool, wet springs that tend to melt the snow more gradually," Taylor said. "So if we don't get a pineapple express we may be able to get through the spring without major flooding."

In the meantime, it might be prudent for the kids to get their sleds ready and cross country ski enthusiasts to wax them up. A real old-fashioned valley snowstorm may be on the way - soon.