Regions
Northwest Coastal.
Confidence
Fair - Due to limited field observations for the entire period
Weather Forecast
Wednesday: Moderate snowfall with 5-15cm of accumulation, strong to extreme southerly winds, and freezing levels remaining at or near valley bottoms. Thursday: Light snow with only a few centimetres of accumulation, moderate southeasterly winds and freezing levels in valley bottoms. Friday: Mostly cloudy with isolated light flurries, continued moderate southeasterly winds, and freezing levels remaining in valley bottoms.
Avalanche Summary
Reports from Monday include more evidence of the natural avalanche cycle from Friday, which included 30-75cm deep storm slab avalanches up to Size 2.0 on all aspects, but was primarily soft wind slab avalanches up to Size 2.0 on steep east through northeast aspects in the alpine.
Snowpack Summary
Up to 65cm of recent storm snow is settling rapidly. Recent compression tests gave easy to moderate sudden planar shears down 20cm and 35cm, as well as moderate resistant shears down 70cm, all on recent storm snow interfaces. A buried surface hoar layer, reported to be 5 to 10mm thick, recently produced sudden snowpack test results down 60-80cm in the Shames backcountry, and may exist in other areas too. Avalanche professionals continue to monitor a crust down near the ground. It seems this problem is confined to alpine areas and likely needs a large trigger and/or a shallow snow pack feature to trigger. Cornices are also really big and forecast strong winds and cooling temperatures could make them especially heavy and brittle.
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.
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.
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.