Persistent slab avalanches have surprised riders in the southern Selkirk mountains over the past week.
The most likely triggers of these persistent slabs are strong solar radiation and/or large triggers such as smaller avalanches in motion, cornices, or snowmobiles.
Recently formed wind slabs may remain active to human triggers in isolated locations; especially where they are sitting on a slippery sun crust.
Persistent weak layers of surface hoar and/or crust can be found 40 to 100 cm deep.
The problem is most likely found in the southern part of the Selkirks (e.g. Valhalla, Goat and Kokanee ranges), especially on open convex slopes at treeline elevations.
This problem exists on all aspects, however increased reactivity has been noted on northeast through southeast facing slopes.
Conservative terrain choices and disciplined decision making is required in areas where these persistent slabs remain active.
MONDAY NIGHT: Partly cloudy with isolated flurries; 0-5 cm / Light north winds / Low of -16 / Freezing level surface.
TUESDAY: Mix of sun and cloud / Light northeast wind / High of -7 / Freezing level valley bottom.
WEDNESDAY: Sunny / Light west wind / High of -9 / Freezing level surface.
THURSDAY: Mix of sun and cloud / Strong northwest wind / High of -6 / Freezing level valley bottom.
On Saturday, reactivity in the storm snow was limited to wind loaded areas and convex rolls, producing skier triggered wind slab avalanches to size 2.
Over the past week we have seen the buried weak layers of surface hoar and crust from mid February and late January produce very large persistent slab avalanches up to size 3. Activity has been concentrated in the Valhalla, Goat, and Kokanee Ranges typically on north through southeast facing slopes at treeline elevations.
The most likely triggers of these persistent slabs are strong solar radiation and/or large triggers such as smaller avalanches in motion, cornices, or snowmobiles.
Recent west and northwest winds have formed wind slabs on lee features at treeline and above. These slabs may remain more reactive on south facing slopes where they are sitting on a slippery sun crust. Below the recent snow, there may be a layer of small surface hoar that is likely most pronounced on northerly aspects or sheltered locations.
Lower elevations have seen warm temperatures last week and have a melt-freeze crust on the surface (below 1800 m).
The late February persistent weak layer combination of crust, facets and surface hoar is buried up to 50cm deep, and has been reactive mostly on south facing slopes during periods of sun. There is some uncertainty about whether this interface will continue to be reactive as air temperatures drop.
Two additional persistent weak layers sit 50-100 cm deep, from mid February and late January. They both consist of surface hoar and/or melt-freeze crusts. Recent trends indicate these layers have been more reactive in southern Selkirks around the Valhalla, Kokanee, and Goat Ranges. Several natural and human triggered avalanches have been produced on these layers in the last week. See the Avalanche Summary for full details.
