Avalanche danger lurked later in season - Bonner County Daily Bee: Local News

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Avalanche danger lurked later in season

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Posted: Sunday, February 13, 2011 10:00 am

SANDPOINT — Avalanche danger in the Idaho Panhandle has tended to lurk later in the season over the last decade, according to the U.S. Forest Service.

“The interesting thing, in going through our statistics, that will pop up is that a lot of accidents in this area have happened in February and March,” said Kevin Davis, director of the Idaho Panhandle National Forests Avalanche Center.

Nationally, avalanches over the last 10 years climb aggressively from seven in the month of November to 63 in December. They peak at 78 in January and descend back into the 60s in February and March.

There have been 10 fatal avalanches in the Panhandle in the last 10 years, with four occurring in the month of March and three in February.

“It’s a little different than the national statistics,” said Davis.

Late-winter avalanche issues come from a variety of factors.

It takes more snow to cover trees and terrain features that tend to anchor shallower snowpack, Davis said. As the terrain and trees become covered, layers in the snowpack become continuous across larger areas of the landscape.

As the snow accumulates, high-elevation areas with fewer trees become easier to access, said Davis, adding that avalanche start zones become steeper as more snow loads on lee aspects.

The Panhandle’s bipolar weather is yet another variable.

Davis said weather in February and March tends to fluctuate quite a bit between warm and cold, and periods of high pressure followed by storms.

“The constantly changing weather creates a lot of layers in the snowpack, which leads to more chances for instability,” he said.

Buried surface hoar — cornflake-shaped ice crystals that form on snowpack on cold clear nights — has been found to be the persistent weak layers in five of the last 10 fatalities in the Panhandle.

John “Oly” Olson, another forecaster at the avalanche center, said cold, calm clear nights are more common later in the season.

“It seems like we get more of those in February and March,” said Olson.

Other common denominators in the past 10 deadly slides in the region include group size, slope aspect, slope angle and elevation.

Half of the backcountry groups included five or more people and more than half of the slides occurred on north-facing slopes. Slopes in all 10 incidents were 40 degrees or greater and all the avalanches occurred over 5,000 feet.

In all, 11 people have been killed in Panhandle avalanches. Seven were riding snowmobiles, while two were skiing and two were snowboarding.

Davis suspects the human element is also at play in the late-season avalanche issue. People can become more complacent or comfortable in the backcountry as the season wears on and as their thoughts turn toward other outdoor pursuits.

“The human factors are just as pivotal as the weather and snowpack factors,” said Davis.

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