Avalanche Forecast

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Monday night: Cloudy, 5-10 cm of snow, light west wind, alpine temperature -9 C. 

Tuesday: Cloudy, 5-15 cm of snow, light to moderate southwest wind, alpine high temperature -4 C., temperatures warming throughout the day with a high of -3 C in the late afternoon.

Wednesday: Cloudy, 10-25 cm of snow overnight and throughout the day with higher snow totals in the northwest part of the region, light southwest wind, alpine high temperature -5 C.

Thursday: Mix of sun and cloud, light northwest wind, alpine high temperature -10 C.

Numerous natural and human-triggered wind slabs were reported on leeward features on Saturday. These avalanches ranged from 10-50 cm deep and were small to large (size 1-2.5). Check out this MIN report from Reudi's on Saturday for a helpful illustration of this problem.

Reports over the weekend showed large (size 2-2.5) avalanches releasing on layers from mid and late December on a variety of aspects and elevations from natural, human, and explosive triggers. Several of these avalanches were triggered remotely.

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

The most recent snow has been redistributed by strong southwest winds in exposed areas, loading lee features with stiffer, more reactive slabs. 

A total of 50-90 cm of snow has fallen over the past week burying two mid-pack persistent weak layers that are now 50-120 cm deep from mid to late December. 

The base of the snowpack in the Purcells is much weaker than in an average season, and there are deeper weak layers down 70 to 180 cm. 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 wind 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. 

• Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
• Be alert to conditions that change with elevation and wind exposure.
• Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.