A warming event will be testing the snowpack this week. The first impact will be felt on sun-exposed slopes on Thursday.
Weather Forecast
Wednesday night: Decreasing cloud. Light northwest winds increasing to strong in the alpine.Thursday: A mix of sun and cloud. Light to moderate southwest winds becoming strong northwest in the alpine. Alpine high temperatures around -9, increasing to around 0 overnight.Friday: A mix of sun and cloud. Strong northwest winds decreasing over the day. Alpine high temperatures around 0 with freezing levels reaching 2000 metres and increasing overnight.Saturday: Mainly sunny. Strong west winds. Alpine high temperatures around +5 as freezing levels rise to a possible 2900 metres.
Avalanche Summary
A size 1.5 natural wind slab avalanche was observed in steep terrain on Sunday. Skier traffic produced a few small (size 1) avalanches in the new snow on Thursday.The most recent reports of larger avalanches are from over a week ago, when warm temperatures stressed the deeper layers in the snowpack and produced a few size 2 avalanches.
Snowpack Summary
Strong winds have redistributed snow into thin winds slab in alpine lees and scoured windward areas. In more sheltered alpine terrain and between 1600-2000 m, 5-15 cm of recent snow overlies a layer of weak surface hoar and crusts (on solar aspects). Large variability in snow depths still exists in the region, ranging from almost no snow to nearly 200 cm in some areas. For average snowpack areas, expect to find weak and sugary faceted snow around 50 to 100 cm deep, which extend to the ground. This weak bottom half of the snowpack has been the culprit for large avalanches in the region over the past few weeks.
Problems
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
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.