Avalanche Forecast

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Tuesday night: Cloudy, 10-20 cm of snow, moderate to strong southwest wind, alpine temperature -5 C.

Wednesday: Cloudy, 5-10 cm of snow, moderate west wind, alpine temperature -6 C.

Thursday: Mostly cloudy, isolated flurries with trace accumulations, light northwest wind, alpine high temperature -10 C.

Friday: Cloudy, scattered flurries with 2-5 cm of snow, moderate southwest wind, alpine high temperature -12 C.

Numerous human-triggered have been reported releasing on surface hoar layers formed in late December on a variety of aspects and elevations. Many of these avalanches were large (size 2-2.5), breaking 50-80 cm deep. Several of them have been remote-triggered, like this MIN report from Saturday.

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 from November on all aspects in alpine terrain. One of these avalanches reportedly released naturally on Monday. Characteristics common to these avalanches include wide propagation and full depth avalanches scouring away the snowpack to ground.

As the new snow settles, storm slab avalanches are likely to be triggered and have the potential to step down to these deeper layers, forming very large and destructive avalanches.

20 to 35 cm of snow is forecast to accumulate by Wednesday afternoon with moderate to strong southwest wind. This will form a new storm slab problem that will need to be managed. Expect areas where the snow is being drifted by wind to be more reactive. 

Two mid-pack surface hoar layers from mid to late December are now buried 60-120 cm deep. Snowpack tests and avalanche activity give evidence to this weak layer's propagation potential (check out this MIN from Sunday).

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. Recent snowfalls 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. 

• Be careful to keep storm day fever from luring you out into bigger terrain features.
• Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
• If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.