BOISE COUNTY, Idaho — Idaho Highway 21 will close at 12 p.m. Wednesday between the Grandjean junction and Banner Summit, the Idaho Transportation Department announced Wednesday morning.
ITD said recent rain and warmer temperatures in the mountains northeast of Boise have increased the avalanche hazard.
ITD on Thursday morning checked conditions and determined that the highway should remain closed. ITD plans to check again at 3 p.m. Thursday afternoon.
The section between Grandjean and Banner Summit is a stretch of about 12 miles, located between the towns of Lowman and Stanley. It is nicknamed "Avalanche Alley," in part because about 90% of avalanches that affect highway travel in Idaho occur there. It's not unusual for ITD to close the highway several times each winter for work to mitigate avalanche danger or because of actual avalanches.
The wet, warmer conditions reported by ITD Wednesday morning raise avalanche concerns because significant snowfall, especially wet snowfall, or rain can make the snowpack unstable for several days.
The Sawtooth Avalanche Center early Wednesday morning reported "moderate" avalanche danger at Banner Summit. That's number 2 on a 5-level scale, with 5 being "extreme." The center advises travelers of "heightened avalanche conditions on specific terrain features."
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