Natural avalanche activity will taper off Sunday, but human triggering will remain likely for the next while.
Increased winds, warmer temps, and new snow have added load to a very weak faceted snowpack. A conservative approach to terrain is recommended through the holidays until things improve.
Strong S-W winds and 5-15 cm of new snow continue to build fresh wind slabs in alpine and treeline terrain. These new wind slabs overly a very weak faceted snowpack. If triggered the slab will likely gouge down to the deep persistent problem and increase in size.
The whole snowpack is thin, weak and facetted, but the biggest facets and depth hoar are at the bottom. New snow, wind, and warm temperatures are increasing the slab over top of this weak base, and we are expecting more avalanche activity on the basal facets over the next week. As this surface slab develops we may also start seeing more avalanche activity below treeline on this problem.
Newly formed wind slabs have been reactive to skier and explosive control in the past couple of days. Local ski hills and touring groups reported several new avalanches up to size 1.5 with failures occasionally stepping down into the basal facets to create larger slides. Wind-triggered sluffing in extreme terrain has also been observed.
5-15 cm of new snow and wind are creating new slabs of varying thickness overtop of a weak faceted snowpack. The Dec 17 layer of facets and surface hoar is down 10-25 cm, and the Nov 16 facet, crust and surface hoar layer is down 30-60 cm, just above the weak basal facets. Both of these layers have been reactive in the past couple of days and avalanches triggered on either of these layers can quickly step down to the basal facets producing larger avalanches.
Total snowpack depths at treeline range from 60-120 cms.
Another 5-10 cm of snow with extreme west winds is expected Saturday night into Sunday. On Sunday an upper ridge develops over the Rockies and the alpine winds will decrease slightly, becoming moderate-strong out of the west. Some flurries are expected Sunday along the divide with a mix of sun and cloud to the east. Alpine temperatures will stay between -5 to -10°C during the day, dropping to -10 to -15°C Sunday night.
