Expect change to be slow due to the cold temperatures. Wind slabs in motion may step down to deeply buried weak layers resulting in large avalanches.
Weather Forecast
Some thin high cloud overnight with moderate easterly winds. Expect lows around -20. Clear and cold on Tuesday with light northerly winds and alpine highs around -15. Mostly clear on Wednesday with moderate-strong northeast winds. Cloud developing on Thursday with moderate westerly winds and a trace of snow.
Avalanche Summary
On Sunday, pockets of wind slab released naturally and from explosives control up to size 1.5 on northeast aspects in the alpine. The most interesting avalanche recently was a size 2.5 slab avalanche 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.
Snowpack Summary
Up to 10 cm of snow was added to the snowpack Saturday night and Sunday. That means around 60cm of storm snow from last week 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 buried 60-80 cm deep. These layers are more reactive in areas where the overlying slab gained cohesion with wind or settlement. A thick 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.