Large naturally triggered persistent slab avalanches failing on a deeply buried crust have recently been reported. Human triggering of this layer is most likely on steep, rocky terrain with a shallow or thin to thick snowpack.
Strong winds may form fresh wind slabs throughout the day on lee features at treeline and above. Wind slabs in motion may step-down to deeper weak layers resulting in large avalanches.
Triggering large avalanches on weaknesses in the lower snowpack remains a possibility. In some areas the concern is weak snow around crusts 50 to 100 cm deep, while in others it's weak snow near the bottom of the snowpack. Human triggering of these layers are most likely in steep, rocky slopes with a shallow or thin to thick snowpack.
SUNDAY NIGHT: Mostly cloudy / Moderate, west ridgetop wind / alpine low temperature -13 / Freezing level valley bottom.
MONDAY: Cloudy with isolated flurries; 0-3 cm. / Strong, west ridgetop wind / alpine high temperature -6 / Freezing level 1400 m.
TUESDAY: Flurries; 3-5 cm. / Extreme, southwest ridgetop wind / alpine high temperature -2 / Freezing level rising to 1900 m.
WEDNESDAY: Snow; 5-10 cm. / Moderate, west ridgetop wind / alpine high temperature -5 / Freezing level 1500 m.
A large (size three) cornice triggered persistent slab avalanche was reported on a large south facing alpine slope in the Crowsnest Pass area on Thursday. This avalanche failed on a weak facets overlying a hard crust deep in the snowpack and is an example of the "low probability; high consequence" scenario that persistent slab problems often create. MIN report HERE.
Recent extreme westerly winds continue to build larger and more fragile cornices which have recently triggered large avalanches when they fail. Fresh wind slabs have also formed in open terrain at treeline and above that may be possible to human trigger. The upper snowpack consists of several layers of old wind slabs, while the lower snowpack consists of decomposing crusts and weak faceted snow. In the Elk Valley, a decomposing surface hoar layer can be found around one of these crusts 50-100 cm below the surface. Avalanche activity on these layers in the lower snowpack have been sporadic, mostly triggered by large loads such as a wind slab in motion or a cornice fall. These deeper weak layers are most likely to be human triggered on steep, rocky slopes with a shallow or thin to thick snowpack.
