South Columbia

Very dangerous snowpack conditions are widespread. Deeply buried weak layers are at a tipping point and producing historic avalanches. A Special Avalanche Warning is in effect in this region. Copy this address to view details: http://bit.ly/2nSOUyX

High -

Saturday: A mix of sun and cloud with thicker valley cloud due to a temperature inversion. Light northwest winds, strong at high elevations. Alpine high temperatures of -10, slightly cooler at lower elevations.Sunday: Mainly sunny with cloud increasing over the day. Light southwest winds. Alpine high temperatures to -9, cooling as the temperature inversion breaks down overnight.Monday: A mix of sun and cloud with thicker valley cloud due to another temperature inversion establishing. Light north winds, increasing overnight. Alpine high temperatures of -10, cooler at lower elevations.

Reports from Thursday included observations of numerous natural and explosives-triggered storm slab releases, generally from size 1.5-2.5 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 2.5-4. Several of these are suspected to have run on the mid-January weak layer.Many avalanches were reported in the region on Tuesday. Storm and wind slabs were 20 to 50 cm deep, but as deep as 100 cm in wind-loaded features, on all aspects, at all elevations, and triggered naturally, by explosives, and skiers. Persistent slab avalanches often released on the mid-December layer, between 100 and 250 cm deep.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.

Recent accumulations of 30-40 cm have formed reactive new storm slabs on the surface throughout the region. 130-240 cm storm snow totals from the past two weeks have now consolidated into a deep slab over four active weak layers:1) 130 to 240 cm of storm snow sits on the crust and/or surface hoar layer from mid-January. The crust is widespread, with the exception of high elevation north aspects. The surface hoar was reported up to tree line and possibly higher. This layer is our primary weak layer of concern.2) The early-January persistent weak layer is 160 to 260 cm deep. 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 170 to 280 cm deep. 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.