Avalanche Forecast

Moderate -

Wednesday night: Mainly cloudy with isolated flurries and a trace of new snow. Light to moderate northwest winds.Thursday: A mix of sun and cloud with possible lingering valley cloud. Light west winds increasing to strong southwest in late afternoon. Alpine high temperatures around -1, with a possible above freezing layer between 1400-1900 metres.Friday: Cloudy with scattered rain/snow mix bringing a trace of snow to high elevations. Moderate southwest winds increasing to strong in the afternoon and evening. Alpine high temperatures around -1 to 0 with freezing levels ranging from 1800 to 2000 metres.Saturday: Cloudy with scattered flurries bringing up to 5 cm of new snow to higher elevations. Rain showers below about 1200 metres, transitioning to snow over the day. Alpine high temperatures around -2, decreasing over the day as freezing levels drop from about 1600 to 1300 metres.

A large (size 2.5) storm slab avalanche was triggered remotely (from 20 metres away) by a skier in the Howson Range on Tuesday. This occurred on a southwest aspect at 1700 metres and the slab had a depth of about 30 cm. It is reasonable to assume that the failure plane in this slide was some variation on the surface hoar and crust combination described in our snowpack discussion. This weak layer is one of our primary concerns in the region and can be expected to produce human-triggered avalanches in areas where the overlying new snow has settled into a slab.

New snow accumulations from the past few days are at around 30 cm. A thin crust may be found within the recent snow from a variable mist layer on Sunday. At alpine and high treeline elevations, the recent snow sits on variable surface of wind-affected and faceted snow, and possibly over a weak layer of surface hoar in more protected areas. The new snow sits on a rain crust/surface hoar combination below about 1500 m and may become increasingly reactive over this weak interface as forecast warming promotes slab formation in the new snow.Under the new snow interface, the mid snowpack is generally strong. Although faceted (sugary snow) is present in this part of the snowpack, the overall dense structure here overcomes the inherent weakness of these faceted grains.The bottom 30 to 50 cm of the snowpack consists of a weak layering of facets and crusts, particularly in areas where the snowpack is thin. It may be possible for large slab avalanches to scrub down to ground in these thin snowpack areas, resulting in even larger avalanches.