A widespread natural avalanche cycle is expected to occur throughout the region on Wednesday. Travel in avalanche terrain - including areas threatened by overhead hazard - is not recommended.
Persistent weak layers that have been slowly healing will be progressively tested by loading from forecast snowfall and wind. Shallower storm slab releases carry the risk of triggering one of these deeper weak layers to create a larger, more destructive avalanche.
Forecast snowfall will cover a new layer of surface hoar observed across the region while burying another recent surface hoar layer even more deeply. New snow is expected to become increasingly easy to trigger as its depth increases. Slab problems will become widespread, with wind loaded areas seeing more rapid accumulation and slab formation and sheltered areas holding more pronounced weak layers.
Tuesday night: Cloudy with continuing snowfall bringing 20-30 cm of new snow. Moderate southwest winds.
Wednesday: Cloudy with continuing snowfall bringing approximately 10-25 cm of new snow and 2-day snow totals to 40-70 cm, continuing overnight. Light to moderate southwest winds shifting to west. Alpine high temperatures around -5.
Thursday: A mix of sun and cloud with 3-day snow totals of 55-80 cm and flurries beginning again overnight. Light west winds. Alpine high temperatures around -12.
Friday: Cloudy with continuing snowfall bringing 15-25 cm of new snow, totaling 20-30cm with overnight accumulations, continuing overnight. Moderate south winds, becoming strong at ridgetop. Alpine high temperatures around -9.
A large (size 2-2.5) persistent slab was triggered by a skier in the Selkirks near Revelstoke on Sunday. The avalanche occurred just below a convex roll situated above a rocky cliff band at 2300 metres and is suspected to have failed on one of our mid-December surface hoar layers. Its crown fracture was up to 60 cm deep.
Other reports from the past 3 days included observations of recent natural and skier-triggered wind slabs reaching size 2.5 (large). One wind slab release managed to trigger a very large (size 3) deep persistent slab on a steep, unsupported slope in the high alpine. Its crown fracture was 3 metres deep. All of the above occurred above 2100 metres.
Looking forward, continuing snowfall is expected to maintain very dangerous avalanche conditions, with storm slabs and persistent slabs becoming increasingly likely to trigger.
A variable 10-30 cm of new snow has buried a mix of large surface hoar reported below treeline into the alpine. This interface may present instead as a sun crust on many steep sun-exposed aspects. Snowfall is forecast to continue through Wednesday.
The new snow adds to 10-30 cm of recent storm snow which was either blown into wind slabs In exposed areas at higher elevations or remains low density in more sheltered areas. The interface below this older snow may present as a sun crust on steep sun-exposed aspects, as surface hoar in more sheltered lower elevations, or as a more widespread melt-freeze crust below about 1700 metres. In some places it may begin to behave as a primary storm slab interface below our new snow accumulations.
110 to 180 cm of snow is now resting on a widespread layer of large, feathery, surface hoar from mid-December. Activity on this interface has tapered off, but there is some concern for loading from forecast snowfall to reinvigorate avalanche activity at this depth.
Another weak layer formed in late November is now over 170 cm deep. Concern for this layer is limited to rocky or variable snowpack depth areas in the alpine where it most likely exists as a combination of facets and crust.
