Purcells

Thin new slabs may form over stubborn old ones as the leading edge of the next storm brings light snowfall with increasing winds.

Moderate - Forecast snowfall amounts are uncertain

Tuesday night: Cloudy with possible isolated flurries and trace of new snow. Light northwest winds, increasing to moderate or strong in the alpine.Wednesday: Cloudy with light flurries bringing a trace to 5 cm of new snow, increasing overnight. Light to moderate southwest winds, becoming strong in the alpine. Alpine high temperatures around -6.Thursday: Cloudy with continuing flurries bringing an uncertain 10-30 cm of new snow, increasing overnight. Light to moderate southwest winds increasing overnight. Alpine high temperatures around -4 with freezing levels rising to 1700 metres.Friday: Mainly cloudy with diminishing flurries and another 5 cm by end of day. Strong southwest winds easing over the day. Alpine high temperatures around -6 with freezing levels returning to valley bottom in the evening.

In addition to numerous reports of mainly explosives-triggered wind slabs on Sunday, a few very large (size 2.5 and 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, while winds during and since the storm drove fairly aggressive wind slab development on most aspects at higher elevations. The new snow buried 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. These basal weak layers have been producing large avalanches as recently as December 31st. 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).