On Saturday a large avalanche was skier-triggered on Mt Fernie. Nobody was hurt, but it demonstrates the potentially touchy conditions in wind-affected terrain.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
Saturday night: 5-15cm of new snow / Variable winds / Alpine temperature of -13Sunday: Overcast skies with light flurries / Extreme northeast winds / Alpine temperatures of -20Monday: Mix of sun and cloud / Extreme northeast winds / Alpine temperatures of -22Tuesday: Mix of sun and cloud / Light north winds / Alpine temperatures of -20
Avalanche Summary
On Saturday, a size 2.5 slab avalanche was skier-triggered on an east facing slope on Mt Fernie. The slab failed on a cross-loaded treeline feature and ran full path. Although the exact failure plane is unknown, it points to the potentially touchy conditions on wind-affected features. A few days ago, a large (size 2) human-triggered avalanche was reported near Corbin in the Flathead Range in an area where wind had firmed up the surface snow and where sugary facets were present lower down in the snowpack. Evidence of a natural size 2.5 persistent slab avalanche was also noted on a treeline feature in the same area. When the wind picks up again on Sunday, new accumulations from Saturday night will get shifted into lee and cross-loaded features and we can expect an increase in wind slab avalanche activity.
Snowpack Summary
On Saturday night 5 to 15cm of new snow is expected to fall. On Sunday, northerly winds are expected to howl and the new snow is expected to get shifted into new wind slabs. The new snow will overlie about 50cm of storm snow from last week which sits above old hard wind slabs and spotty surface hoar. The cold weather in early December left several layers of weak surface hoar and facets which are now buried 60-80 cm deep. These layers may be more in reactive in areas where the overlying slab has gained cohesion with wind or settlement. A thick crust rain crust from mid-November is near the bottom of the snowpack and remains well bonded to the surrounding snow.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.