Avalanche Forecast

Moderate -

Monday night: Becoming cloudy. Light northwest winds increasing to moderate.Tuesday: A mix of sun and cloud. Light northwest or west winds. Alpine high temperatures around -7.Wednesday: Mainly cloudy with light flurries beginning mainly in the afternoon and continuing overnight. Light southwest winds increasing over the day and overnight. Alpine high temperatures around -5 with a mild temperature inversion.Thursday: Cloudy with continuing flurries bringing an uncertain 5-20 or more cm of new snow, continuing overnight. Light to moderate southwest winds increasing over the day and overnight. Alpine high temperatures reaching about -3 as freezing levels rise to 1500 metres.

In addition to numerous reports of mainly explosives-triggered wind slabs on Sunday, a couple of very large (size 3) deep persistent slabs were reported to have run during the storm. These occurred in alpine terrain and are suspected to have failed on deep weak layers formed early in the season. A convincing pattern of deep persistent slab activity has yet to emerge, but these events highlight lingering hazards in thin or variable snowpack areas in the alpine.Reports from Saturday in the Golden area describe widespread, small (up to size 1) but touchy storm slabs releasing naturally as well as remotely (from a distance) and with skier traffic in steep alpine terrain. Slab depths were generally from 10-20 cm and increasing over the day. Several other small wind slabs were triggered with ski cutting in the Bugaboos area as well as with explosives in the Invermere area.Prior to the storm, avalanche control work on Wednesday and Thursday produced both large and small persistent slab avalanches (size 1 to 2.5) on north facing terrain between 1900 and 2400 m. On Monday and Tuesday control work produced avalanches to size 3 on steep north-facing features between 1900 and 2700 m.

Saturday's storm brought about 20-30 cm of new snow to the region, burying 5 to 15 cm of older low density snow on the surface. These surface layers sit on a layer of wind-affected snow in the alpine, and in some isolated areas above small surface hoar (feathery crystals).A weak layer that formed during the dry spell in early December is now 80 to 110 cm deep. The layer is composed of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south aspects). Another similar weak layer is buried 90 to 150 cm deep. Finally, the base of the snowpack has weak faceted layers at alpine and treeline elevations. The mid-pack and basal weak layers described above have been producing large avalanches as recently as December 25th. Human triggering any of these layers is most likely on slopes that didn't previously avalanche and on slopes that have variable snowpack depth (such as rocky alpine features).