Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain on Friday
Weather Forecast
Friday: Light to locally moderate precipitation. Alpine temperatures -3. Moderate to strong southwesterly winds switching to northwest in the afternoon. Freezing level 1200mSaturday: Mainly dry with broken skies, winds moderate to strong from the northwest and alpine temperatures -8.Sunday: Light snowfall, alpine temperatures -9, winds light to moderate from the northwest.
Avalanche Summary
A few avalanches to size 1.5 have been reported in the last 2 days. These are isolated to the most recent storm snow. I would expect to see an increase in avalanche activity as wind speeds pick up and new snow amounts accumulate on Friday.
Snowpack Summary
Moderate to strong southwesterly winds have built new wind slabs in alpine and treeline lees. 20-40 cm of new snow sits on a weak layer consisting of surface hoar crystals, stellar crystals and/or crust. Around 60-80 cm below the snow surface, the early December surface hoar, facetted snow, and/or a crust interface sits and has been reportedly more stubborn to rider triggers.The late-November persistent weak layer consists of a sun crust on steep south facing slopes and surface hoar in sheltered areas. It is now buried 70-90cm below the surface. Snowpack tests vary on this interface with some results producing "sudden" shears and others producing no results. This interface is generally considered to be dormant. However, professional operators are keeping a close eye on it.A weak layer of facets sitting on a crust that formed in October, sits near the base of the snowpack. This layer is predominantly found on north-facing alpine slopes with smooth ground cover. Triggering this weakness is unlikely, however if triggered the resulting avalanche would potentially be a large event with high consequences. Recent reports indicate that the average snowpack depth at treeline is 100-150 cm.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.