We have entered a low probability/high consequence period. Large and destructive human-triggered persistent slab avalanches remain possible.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
The ridge of high pressure shifts into Alberta allowing a weak cold front to reach the interior Thursday overnight. Thursday should see a mix of sun and cloud up high with a layer of valley fog in most of the valleys. Alpine winds should remain light and freezing levels are expected to reach around 800m. Thursday night and Friday are forecast to receive 5-15mm of precipitation with moderate-to-strong SW alpine winds. Freezing levels are expected to be around 1000m or so. By Friday evening the system should be finished. Saturday is expected to see a mix of sun and cloud, freezing levels around 800m, light alpine winds, and the possibility of light flurries. Another weak system is expected for Sunday.
Avalanche Summary
No new slab avalanches were reported on Tuesday but some loose sluffing from steep sun exposed slopes was reported. On Monday, isolated natural activity was reported in Rogers Pass. On Sunday, several natural size 3 glide slabs were reported around Nakusp. These released on the ground with average depths of 2.5m. These were east aspect between 1200 and 2000m elevation. Also reported was a size 2 which was remotely triggered by a helicopter from 100m away. This occurred on a west aspect at 2200m and released down 1m on the mid-Dec layer. Finally, a snowcat intentionally started a size 2 avalanche by pushing snow over the edge of a ridge. This also released on the mid-Dec layer down 60-100cm and occurred on a NE aspect at 1900m. Natural avalanche activity is not expected on Thursday. Skier triggering a persistent slab avalanche remains the main concern for the foreseeable future.
Snowpack Summary
A breakable melt-freeze crust exists below around 1600-1800m on all aspects and on steep sun-exposed slopes at all elevations. A layer of 10-20mm surface hoar sits on top of this crust on all aspects up to ridgetop. Recent warm temperatures have aided in the settlement of the week old storm snow. Down 80-120cm is the touchy mid-December surface hoar/crust layer which remains sensitive to human triggering. This persistent weak layer remains reactive in snowpack tests and continues to produce large, destructive avalanches. The layer appears to be the most reactive at treeline or just below treeline.
Problems
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.