Avalanche activity is expected to increase as temperatures rise and the storm snow settles into a cohesive slab. Moderate to strong winds are also promoting wind slab development. Human triggering is likely. Stick to conservative, supported terrain.
Weather Forecast
Another day of fresh trail breaking will bring scattered flurries with 5cm of snow accumulation, an alpine high of -4 with winds ranging from 20-50kph from the SW. The freezing level will rise up to 1500m so watch for tree bombs at lower elevations. 25cm of snow forecasted by Tuesday evening and 10cm on Wednesday.
Snowpack Summary
10cm overnight brings our settled storm snow to ~90cm. Fresh wind slab in alpine and open tree line areas. The December 9th persistent weak layer is buried 60-100cm and producing large 'whumphs' and sudden planar test results. The November 21st persistent weak layer is down 120-160cm. Height of snow at treeline is ~ 2m, and ~1m at Rogers Pass.
Avalanche Summary
Numerous avalanches size 2 to 3 were observed at all elevations and aspects over the weekend. A skier-triggered size 1 avalanche off a steep gully sidewall partially buried one person in NRC Gully on Saturday. Large settlements on the Dec 9 layer are producing 'whumphs' that travel for over 100m in open terrain and seem most prevalent at tree line.
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.