Confidence
Fair - Due to variable snowpack conditions
Weather Forecast
Saturday night and Sunday: Ridge of high pressure continues to dominate for the whole period. No precipitation, mild temperatures, scattered clouds and valley clouds and moderate Westerly winds becoming light on Sunday.Monday: Similar conditions with warmer temperatures and freezing level rising to 1100 m. Tuesday: Dry conditions persist, light S winds and freezing level around 1000 m.
Avalanche Summary
Several recent natural slab avalanches up to size 1.5 were reported which would have released in the new storm snow. A glide crack released to the ground producing a size 2.5 was also reported on a SW aspect.
Snowpack Summary
15-20 cm of new snow fell with strong NW switching to SW winds creating windslabs on lee side in alpine and at treeline. The new windslabs were producing some sudden planar tests results and were reactive to skier traffic. The forecasted mild temperatures and reduced wind transport should give them a chance to improve the bond with the underlying surface. Cornice growth has also been observed. The storm snow sits above surface hoar (found especially below treeline in sheltered areas down around 40 cm), a sun crust on steep S to SW -facing slopes and facets. The distribution of these weaknesses is patchy, but where they exist, it may still be possible to trigger a large avalanche with the weight of a person or snowmobile. Indeed, snow tests showed sudden planar results in the easy to moderate range. A strong mid-pack overlies a weak facet/crust layer near the base of the snowpack, which is now considered inactive.
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.