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Copyright © 2003 Express Publishing Inc.
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Friday — February 27, 2004


Avalanche danger rises with new snowfall

Large, dangerous slides reported

"We’re just adding more stress to the already
stressed-out snowpack."

JEFF HALLIGAN, Sawtooth National Forest Avalanche Center avalanche forecaster

Express Staff Writer

With an additional foot of heavy snow this week, the backcountry snowpack is increasingly under stress loads that could trigger large and dangerous avalanches.

"We’re just adding more stress to the already stressed-out snowpack" said Sawtooth National Forest Avalanche Center Forecaster Jeff Halligan.

This week’s high avalanche hazard is due, in part, to a buried layer of surface hoar that formed during a dry, cold spell in January. Snowpack evaluation techniques should be used to help determine a slope’s stability before travel commences. Sawtooth National Forest Avalanche Center Forecaster Greg Johnson and Director Janet Kellam demonstrate evaluation techniques. Express photo by Greg Stahl

Last week, the avalanche center received multiple reports of human-triggered avalanches that occurred several days following the most recent storms.

"This is because the snowpack has several weak layers that lie 1 to 3 feet beneath the snow surface," said avalanche center Director Janet Kellam. "The current conditions are similar to the type of snowpack that produced the large, fatal avalanches in Canada last winter."

Halligan, who posted the center’s Thursday avalanche forecast, said the danger is considered "high." That means natural and human caused avalanches are likely, and unstable slabs or wet slabs are likely on a variety of slope aspects, slope angles and at varying elevations.

"We have received large amounts of snow in the higher elevations," Halligan wrote in his forecast. "There is a lot of moisture in the new snow, and it is sitting on some fairly unstable buried layers. There were fairly high winds prior to this storm that were transporting large amounts of snow, forming dense slabs along the ridges that were sensitive to our tests. The new snowfall has loaded these slabs even more with up to 10 inches of moist, heavy snow."

Under current conditions, slopes that appear stable could in fact be dangerous, Kellam said.

"There can be multiple tracks on the slope before they avalanche. It may take a larger trigger, multiple people grouped up or a person crossing over a slightly weaker spot in the slope."

Kellam said slides that occur under current conditions could become "very large and deadly."

"The avalanche danger is not everywhere, but it is widespread and difficult to discern," Kellam said. "The best recourse for people traveling into the backcountry is to stick to low angle slopes and to be conscious of how steep the slopes are next to you or above you."

Slopes steeper than approximately 30 degrees should be considered suspect, she said.

On a brighter note, recent snowfalls are propelling Idaho toward an average precipitation winter and could spell the beginning of the end of four years of drought. Most of the state is boasting snowpacks that are close to 100 percent of average for this time of year.

The Big Wood River basin is 96 percent of the 30-year average and creeping closer to 100 percent with each additional snowflake.

Following December and early January’s heavy snowfalls, a cold, dry spell ensued. It is that dry spell and the corresponding cold weather that helped create this week’s high avalanche danger, when cold weather dew formed a snow feature called surface hoar.

When buried beneath layers of snow, surface hoar becomes a weak layer in the snowpack, and many of last week’s avalanches were reported to have run on the weak layer.

For up-to-date information about avalanche danger, check the avalanche center’s daily forecasts at www.avalanche.org.


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