4/19/2012
Reed Hollinshead
ITD Communication
208-334-8881
reed.hollinshead@itd.idaho.gov
Rising temperatures can lead to increased avalanche danger on Idaho 21
BOISE COUNTY - The warmer weather and first signs of spring - birds chirping, flowers blooming, fresh breezes blowing – are almost universally welcomed. Almost.
For ITD's avalanche forecasters on Idaho 21, for example, it often means things are about to get dicey.
Warm, spring weather causes melting, and the runoff combines with spring rains to create snowpack instability in the notorious "Avalanche Alley" section between the Grandjean Junction turnoff and Banner Summit.
Avalanche Alley, which has already had 50 slides come down on the highway since Nov. 1, is home to about 60 known avalanche chutes and 90 percent of the avalanches in the state that affect highways.
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.
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.
"As spring weather warms and snow melts, it becomes more slushy during the day and refreezes at night," he explained.
"The temperatures help us get a picture of how deeply the warmth is penetrating and can foreshadow avalanches."
Nicholson and his avalanche crew will continue daily forecasting through May 1.