Spring conditions heighten risks of avalanches
Spring can bring chirping birds, fresh breezes and greener lawns and gardens. Unfortunately it also can bring dangerous spring avalanches. Last week was a prime example.
The stretch of Idaho 21 between Grandjean Junction and Banner Summit was closed from Tuesday evening through Friday morning (March 12-15), opened for about five hours and then closed again until Saturday morning. The highway also closed a few weeks earlier for another spring avalanche threat.
It’s not just this year. It happens every spring, fueled by rapidly warming weather.
Warm, spring weather can cause the snowpack to melt, and the runoff combines with spring rains to create slope instability.
Avalanche Alley is home to about 60 avalanche chutes and represents 90 percent of the avalanches that affect highways in Idaho. That section received 50 slides during last year’s avalanche season. No slides reached the roadway this weekend, but several occurred on the mountainside or fell just short of the highway’s fog line.
Winter in the Avalanche Alley region unofficially runs from Dec. 1 through April 30 each year. Spring snows are not uncommon, however, so the timeframe is just an estimate.
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 actually can 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," Nicholson said. "Careful study of the existing snowpack and incoming weather helps 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.
Published 3-22-13