Avalanche Forecast

Saturday night: Cloudy, scattered flurries with 2-5 cm accumulation, moderate southwest wind, alpine temperature -8 C.

Sunday: Mostly cloudy, flurries with 2-5 cm accumulation, moderate southwest wind, alpine temperature -10 C.

Monday: Mostly cloudy, 5-10 cm of snow, light west wind, alpine temperature -11 C. 

Tuesday: Cloudy, 15-20 cm of snow, light southwest wind, alpine temperature -7 C.

Reports of avalanche activity on Friday show several large (size 2-2.5) natural, human, and explosive-triggered avalanches running at tree line and in the alpine on a variety aspects. Much of this activity was concentrated to north and east aspects where active wind-loading played a role. 

Explosive control work and other large triggers have produced large and very large (size 2-3.5) deep persistent slab avalanches on all aspects in alpine terrain. Characteristics common to these avalanches include wide propagation, remote triggers, and full depth avalanches scouring away the snowpack to ground.

30-70 cm of new snow has fallen throughout the past week creating a sensitive storm slab problem. At high elevations, this snow has been redistributed by strong southwest winds, loading lee features near ridges and exacerbating reactivity. The storm snow overlies a weak layer of feathery surface hoar and a hard melt-freeze crust on sun-exposed aspects, also increasing the reactivity of these slabs.

There are notable weak layers buried 70 to 180 cm deep, including two more surface hoar layers from earlier in December. These layers are expected to be progressively gaining strength. 

The base of the snowpack in the Purcells is very weak, much weaker than in an average season. This weakness is widespread across all aspects and elevations, and it consists of crust, facets and depth hoar.

Recent snowfalls in the past two weeks have overloaded these deeply buried weak layers resulting in very large and destructive avalanches. It is possible that easier-to-trigger storm slab avalanches could step down to these deeper, persistent layers or that the weak layers could be human-triggered in areas where the snowpack is thin, rocky, or variable. 

• Continue to make conservative terrain choices while the storm snow settles and stabilizes.
• Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
• Storm slabs in motion may step down to deeper layers resulting in large avalanches.
• Remote triggering is a concern, watch out for adjacent and overhead slopes.