Avalanche Forecast

Thursday night: Clearing overnight, light northwest wind, alpine high temperature -18 C.

Friday: Cloudy, 5-10 cm of snow, moderate southwest wind, alpine high temperature -10 C.

Saturday: Cloudy, scattered flurries with 2-5 cm of snow, light southwest wind, alpine high temperature -8 C.

Sunday: Cloudy, 10-20 cm of snow, moderate southwest wind, alpine high temperature -9 C.

The majority of avalanches reported on Wednesday occurred where the wind played a role in stiffening slabs, particularly below ridge crests. These avalanches were small (size 1-1.5). Forecast snow and wind for Friday are expected to increase the size and sensitivity of this avalanche problem.

In the aftermath of the most recent storm, several large (size 2-2.5) and very large (size 3-3.5) avalanches released from natural, human, and explosive triggers in the storm snow and on deeper weak layers across aspects and elevations. Persistent slab avalanches were breaking 80-200 cm deep on a layer of surface hoar from late December. Be sure to check out this MIN for a helpful example of slopes that are likely to harbor this problem. 

A couple of large deep persistent slab avalanches were reported Thursday. These avalanches released naturally on early season weak layers on alpine slopes. Characteristics common to these avalanches include wide propagation and full depth avalanches scouring away the snowpack to ground. Freshly formed wind slabs on Friday may have the potential to step-down to these deeper layers.

Low density snow will begin to accumulate tomorrow afternoon with 5-10 cm possible. Moderate southwest winds will likely drift this new snow into stiffer slabs on lee terrain features at higher elevations, especially near ridge crests.

The most recent storm delivered 15-35 cm of snow across the region with strong southwest winds. Two mid-pack surface hoar layers from mid to late December are now buried 70-130 cm deep. Snowpack tests and avalanche activity give evidence to this weak layer's potential to produce large, destructive avalanches.

The base of the snowpack in the Purcells is much weaker than in an average season, and there are deeper weak layers down 120 to 200 cm. This weakness is widespread across all aspects and elevations, and it consists of crust, facets and depth hoar. It remains possible to trigger these deep weak layers in areas where the snowpack is thin, rocky, or variable.

• Seek out wind sheltered terrain below treeline where you can avoid wind slabs and find great riding.
• Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
• Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.