State officials: Highway 21 avalanche danger is about to “get dicey”
Birds are chirping, fresh breezes are blowing, and grasses mired in brown for months are starting to feature tinges of green. The signs of an approaching spring are universally welcomed. Well, not quite universally.
For the Idaho Transporation Department’s avalanche forecasters on Idaho 21, it often means things are about to get dicey. Such was the case this past weekend as crews had to close down the notorious "Avalanche Alley" section between the Grandjean Junction turnoff and Banner Summit due to an escalating avalanche risk, fueled by rapidly warming weather.
Warm, spring weather can cause the snowpack to melt, and the runoff combines with spring rains to create instability. Rain was the main reason for the initial closure Thursday late afternoon. The risk of avalanche rose from “Considerable” to “High,” as the weekend progressed and temperatures warmed.
“‘Avalanche Alley’ is home to about 60 known avalanche chutes and 90 percent of the avalanches in the state that affect highways. The section saw 50 slides during last season’s avalanche season. No slides reached the roadway this weekend, but several occurred on the mountainside or fell just short of reaching the highway’s fog line,” said ITD spokesman Reed Hollinshead.
“Winter in the Avalanche Alley region unofficially runs from Dec. 1 through April 30 each year. Spring snows are not uncommon, however, so this timeframe can be just an estimate,” he added.
Although most people are tired of winter weather and long for the return of balmy days, rapid heating can create big problems on the mountainside. Colder temperatures can actually help stabilize the existing snowpack.
Bill Nicholson, ITD's head avalanche forecaster, noted that while the amount of snow on the ground is significant, the more important indicator of avalanche danger in warm-weather conditions is the temperature of the snow at different depths.
“The arrival of spring and warm temperatures can stress the snowpack just as heavy snow loads do," said Nicholson. "Careful study of the existing snowpack and incoming weather changes help to give us an idea of the avalanche hazard each day.”
Nicholson and his avalanche crew will continue daily forecasting through the month, and longer if needed, said Hollinshead.
(photo: courtesy Idaho Transportation Department)