Regions
Kootenay Boundary.
The deep mid-Dec layer remains reactive and there have been several avalanches in the last few days. Use a conservative approach and assess conditions before committing to big terrain features.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Light flurries are possible on Thursday morning. Freezing levels are expected to be around 1000m and alpine winds should remain light. Friday will be unsettled in advance of a frontal system which is expected to arrive in the evening. Friday should be mostly cloudy with freezing levels around 1000m and light alpine winds. Around 5mm is expected Friday night with moderate SW winds. On Saturday, freezing levels are forecast to climb to around 2500m. Another 5-10mm are expected for Saturday.
Avalanche Summary
On Tuesday, one natural avalanche was reported. A cornice broke off and triggered a deep slab avalanche on the mid-Dec layer. Three skier triggered avalanches were also reported and these all released on the mid-Dec surface hoar layer. Depths varied from 30cm to 100cm with typical values around 70cm. On Monday, a natural size 1.5 storm slab was reported. This released down 25-40cm on the mid-Jan surface hoar. Also reported were a few ski cuts and explosive triggered avalanches. These were typically size 1.5-2 and released on the mid-Jan layer. These occurred on north through east aspects between 1900 and 2100m. Natural avalanche activity is generally not expected on Wednesday but remains possible in isolated areas such as steep sun exposed slopes or wind loaded areas. Human-triggering is likely in wind loaded areas and steep terrain features, especially where surface hoar underlies the recent storm snow.
Snowpack Summary
Around 30cm of snow sits on the mid-Jan surface hoar layer. In many places the surface hoar sits above a thin crust that can be found up to about 1900m on north aspects and all the way to ridgeline on south aspects. Strong SW winds during the storm loaded leeward features and the persistent slab may be 50-60cm thick in wind loaded areas (typically north through east aspects). A deeper crust/surface hoar combo buried in mid-December continues to be a concern. This layer can typically be found down 70-100cm. The reactivity of this deep persistent weak layer appears to have become isolated but still has the potential for large, destructive avalanches in some areas. The layer has been the most reactive at and just below treeline.
Problems
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
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.