Avalanche danger rises with new
Large, dangerous slides
"We’re just adding more stress
to the already
— JEFF HALLIGAN, Sawtooth
National Forest Avalanche Center avalanche forecaster
By GREG STAHL
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
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