La Niña to hold sway this winter

Using the tropical Pacific Ocean as her cauldron, Mother Nature appears to be brewing a particularly strong La Niña for this coming winter. As a result, the Pacific Northwest may be directly in the crosshairs of some very active weather.

This was the unanimous opinion of four weather prognosticators who spoke to a standing-room-only crowd at the Oregon Museum of Science and Industry in Portland on October 16. It was the annual winter-weather meeting, put on by the Oregon Chapter of the American Meteorological Society, and over 350 folks were lured there by the prospect of hearing what experts in the field of meteorology and climatology had to say about what might lie ahead for us this winter.

Any attempt to predict what a winter season will be like in our part of the world begins by examining sea-surface temperatures and atmospheric pressure patterns in the equatorial Pacific Ocean months before. When temperatures are sufficiently above normal and trade winds are weak in this region, El Niño conditions are said to exist. Conversely, colder temperatures and strong trade winds define a La Niña. For us here in the Pacific Northwest, El Niños produce warm and dry winters, while La Niñas bring cool and wet conditions.

Wintry weather came early to the High Desert last year. On October 4 up to seven inches of snow fell in Bend. An inch or so was measured in Sisters. Then, in the second week of December, bitterly cold air invaded from extreme northwestern Canada, dropping temperatures to near -20 degrees Fahrenheit in Sisters. But as the season progressed the weather got significantly warmer and drier, thanks to a moderately strong El Niño.

By winter's end, central Cascade snowpacks were only about two-thirds of normal. However, a cool and wet spring resulted in additional snow in the mountains, reducing the snowfall deficit to about 20 percent below normal.

The agreement among the forecasters this year was remarkable. The La Niña signal is already so strong that the probability of experiencing a wetter-than-normal winter throughout the Pacific Northwest is very high. Temperatures will also be cooler than normal, except for areas east of the Cascades, where they will be near-to-slightly above normal.

Tyree Wilde, meteorologist for the National Weather Service in Portland, explains that skies will be cloudier this winter because of the frequent storms that are expected to move through the region.

"When skies are cloudy at night not as much surface heat escapes into space, resulting in warmer temperatures," he says. Wilde sees a greater than normal snowpack in the central Cascades.

George Taylor of Applied Climate Services says the most active weather should occur between November and February. He also thinks the odds of flooding in western sections of Washington and Oregon are higher than usual. For Taylor, it's not just the intensity of the upcoming La Niña that's noteworthy, but also the rate at which the change occurred. We went from a moderately strong El Niño last winter to an impressively potent La Niña signal now.

"The only other time things changed that fast was the winter of 1998-99," Taylor said.

Pete Parsons, meteorologist for the Oregon Department of Forestry, thinks a significant amount of snow will pile up in mountains by Thanksgiving. He also believes that during December or January the region is susceptible to an arctic air episode, a large coastal windstorm, and/or flooding in western Oregon or Washington.

The upshot of all these forecasts is that, unlike last season, the wind flow aloft is expected to bring in a number of Pacific storms. This should mean a snowier than normal season here in the High Desert.