Very dangerous avalanche conditions have developed in the Purcells. The danger is greatest in areas that received more snow from recent storms. A Special Public Avalanche Warning is in effect in this region.
Weather Forecast
Friday: Mainly cloudy with isolated flurries and a trace of new snow, continuing overnight. Light southwest winds. Alpine high temperatures to -6. Saturday: A mix of sun and cloud. Light to moderate west winds. Freezing level rising to 1600 metres with alpine high temperatures to -2. Mild temperature inversion.Sunday: Mainly sunny with valley cloud due to a temperature inversion. Light southwest winds. Freezing level to 1800 metres with alpine temperatures to +2. Cooler at lower elevations.
Avalanche Summary
Reports from Wednesday include observations of several persistent slab releases from Size 1-2.5 that were primarily triggered remotely (from a distance) and failed at the December 15 interface. Several other storm slabs and wind slabs were triggered, naturally, remotely and by skier traffic. Reports have been focused in areas that received more snow from recent storms and show a noticeable increase from recent days.Some of the notable themes that are emerging from recent activity both in the Purcells as well as in neighbouring regions include accidental and remote triggering, 'step down' release types, releases on surprisingly low angle, supported terrain, as well as wide fracture propagations. Looking forward, expect a continuation of heightened human triggering potential that may increase toward the weekend with forecast warming.
Snowpack Summary
Recent storms brought a variable 10-60 cm of new snow to the region. The new snow sits on surface hoar on sheltered slopes as well as sun crust on steep solar aspects. Moderate to strong southwest winds accompanied the snowfall at higher elevations, creating wind slabs in lee features.As the new snow continues to consolidate, it is forming a dangerous slab above several persistent weak layers that exist in the snowpack. The first of these is the surface hoar found at the new snow interface. Recent snowpack tests and avalanche activity show a high degree of reactivity at this layer. Below it, the December 15 surface hoar is now buried a widely variable 40-100 cm deep. This layer has been the failure plane in many recent destructive slab avalanches in neighbouring regions and its reactivity has been most apparent at treeline and below treeline. In the Purcells, both of these weak layers are of greatest concern in areas that received greater snowfall from recent storms.Deeper in the snowpack at depths of about 90 to 150 cm, a rain crust from November is producing variable snowpack test results. This layer may be triggerable in thin spots around variable snowpack areas in the alpine.
Problems
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.
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.