Regions
Northwest Inland.
Confidence
Fair - Timing of incoming weather is uncertain for the entire period
Weather Forecast
Friday: Strong W to SW winds. Alpine temp -5. Light snow starting late in the day.Saturday: Strong to gale SW winds. Alpine temp -6. Moderate to heavy snow. Sunday: Light SE winds. Alpine temp -13. Light snow.
Avalanche Summary
On Wednesday, a snowmobile triggered a hard wind slab at the Sinclair snowmobiling area. It failed on the November crust/facet layer 30 cm above the ground. Evidence of a recent natural cycle was also reported. This deep crust/facet layer has been the failure plane for a number of large avalanches further to the west and was suspected to have failed at the Hankin-Evelyn area last weekend. Unfortunately, this weakness is unlikely to heal very quickly, so further deep avalanches are possible, especially as snow and wind-loading continue to stress the snowpack over the next few days.
Snowpack Summary
Storm snow instabilities are likely to continue developing over the next few days as variable new snow layers build up. Spotty surface hoar is also buried in the upper snowpack, particularly in sheltered treeline areas. New and buried wind slabs are widespread and exist on a variety of alpine and exposed treeline slopes due to variable winds. Of key concern is a November facet/crust layer which can be found near the base of the snowpack. This layer, which is widespread, can be triggered from thin-spot trigger points, or with a heavy load, such as storm slabs stepping down, cornice fall or a snowmobile. It has the potential for very large, destructive avalanches and demands respect. A rain crust is buried in the upper snowpack at low elevations. In general the snowpack is highly variable in depth due to windy conditions this season.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
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.