Friday, October 31, 2008

Whether the winter weather?

Forecasters: Predicting conditions for this winter is difficult



Snow removal equipment goes to work after a particularly heavy snowstorm in Ketchum last winter. This winter, National Weather Service forecasters predict the Wood River Valley could see huge snowfall amounts. Then again, snowfall could be average or below average. Express file photo

Forecasting snow levels in the central Idaho Rockies is a dicey game each winter. But that's especially true this year, forecasters say.

In the best of years—when strong El Niño or La Niña conditions dominate the Pacific Ocean—forecasters have an easier time sorting out expected temperature and precipitation.

El Niño and La Niña are extreme phases of a naturally occurring climate cycle referred to as El Niño-Southern Oscillation, or ENSO for short. Both terms refer to large-scale changes in ocean surface temperature across the eastern tropical Pacific. A warm pool expands to cover the tropics during El Niño, but during La Niña, the easterly trade winds strengthen and cold upwelling along the equator and the west coast of South America intensifies. During La Niña conditions, ocean surface temperatures along the equator can fall as much as 7 degrees below normal.

Generally, snowfall in Idaho increases under La Niña conditions.

It's what comes between—an ENSO phenomenon forecasters refer to as neutral conditions or "El Neither"—that causes forecasters to scratch their heads. It's those conditions that have developed in the Pacific now, said Mike Huston, the National Weather Service's senior forecaster in Pocatello.

Long-term forecasting under such conditions becomes about as certain as reading tea leaves. Huston said snowfall in the Wood River Valley this winter has about equal chances of being above-average, about average or below average.

"Right now we don't have anything to make a forecast," he said. "It's sad."

Does any of this really matter for snow junkies? Depends on which winter you look at. Anyone who wants a pick-me-up can recall the 2005-06 winter, when neutral ENSO conditions produced 146 inches of snowfall at the Ketchum Ranger Station. It was the seventh snowiest winter since 1971. A whopping 53.9 inches of snow fell in Ketchum in January 2006, which was just shy of the record 56.5 inches that fell in January 1993.

But other winters with neutral conditions have had below-average snowfall.

Far better for more certainty in forecasting would have been strong La Niña or El Niño conditions. Last winter—when 144.7 inches of snow fell at the ranger station—was a moderate La Niña.

In the western United States, La Niña often brings drier-than-normal conditions in the southwest in late summer through the subsequent winter. By contrast, the Northwest is more likely to be wetter than normal in the late fall and early winter with the presence of a well-established La Niña. La Niña winters are typically colder than normal in the Northwest.

The La Niña event of 1998-99 season buried Mt. Baker Ski Area in northern Washington under a world's record 1,140 inches of snow. Ketchum received about 145 inches that winter.

El Niño events typically result in cooler and wetter-than-normal conditions in the southern states and warmer and drier weather in the north.

But there are exceptions.

Of the 10 wettest years on record in Ketchum since 1971, three were El Niños, two were La Niñas, and five were neutral conditions.

Might as well roll your dice.




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