Recent avalanche activity has been extensive. Deep weak layers have reached a tipping point and are producing historic avalanches. A Special Avalanche Warning is in effect in this region. Copy this address to view details: http://bit.ly/2nSOUyX
Weather Forecast
Saturday: A mix of sun and cloud with thicker valley cloud due to a temperature inversion. Light southwest winds, strong at high elevations. Alpine high temperatures of -12. Cooler at lower elevations.Sunday: Mainly sunny with increasing cloud over the day. Light north winds. Alpine high temperatures of -10, cooling over the day as the temperature inversion breaks down. Monday: Mainly sunny with some valley cloud due to another temperature inversion establishing. Light variable winds, increasing overnight. Alpine high temperatures of -10, slightly cooler at lower elevations.
Avalanche Summary
Reports from Thursday included observations of numerous natural storm slab releases, generally from size 1-2.5 but with several outliers reaching size 3.5 to 4.5. These larger examples featured crown fractures up to 500 metres wide with runout distances of up to 3 km. These historic slides destroyed mature timber along their edges as well as on opposing slopes. Several more recent persistent slabs were observed to have run naturally from size 2.5-3. On Tuesday, numerous storm slabs were triggered naturally, by explosives, and skiers. They were small to large (size 1 to 2), on all aspects, with depths from 20 to 40 cm. Persistent slab avalanches were large (size 2 to 3), explosives-controlled, 100 to 200 cm deep, on all aspects, and generally releasing on the late-November crust.Looking forward, it will be essential to maintain elevated caution as danger ratings shift into the CONSIDERABLE range. As the likelihood of triggering avalanches diminishes, the consequences and destructive potential of avalanches will remain dangerously high.
Snowpack Summary
Recent accumulations of 25-70 cm have formed reactive new storm slabs on the surface throughout the region. In some areas the new snow has buried a new layer of surface hoar. About 150-250 cm of storm snow from the past two weeks now overlies an unstable snowpack with four other active weak layers.1) 150 to 250 cm of snow sits on the crust and/or surface hoar layer from mid-January. The crust is widespread, except for possibly high elevation north aspects. The surface hoar was reported up to tree line elevations and possibly higher.2) The early-January weak layer is 170 to 270 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is found at all elevation bands.3) Another weak layer buried mid-December consists of a facet/surface hoar/crust combination, which is buried 200 cm or more below the surface. It is most problematic at and below tree line.4) A crust/facet layer from late November is yet another failure plane responsible for recent very large avalanches.The wide distribution and ongoing reactivity of these layers suggests that choosing simple terrain free of overhead hazard is the best avoidance strategy.
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.