Regions
Kootenay Boundary.
A Special Public Avalanche Warning is in effect for this region. The only worthwhile riding is likely on the least trustworthy slopes. Be circumspect on large steep alpine slopes.
Weather Forecast
Saturday: Cloudy with possible light snow. The freezing level lowers to around 1800-2000 m. Winds are generally light from the W-NW. Sunday: Cloudy with light snow and occasional sunny breaks. The freezing level is around 1400-1500 m. Monday: Mainly sunny and dry with light winds and freezing levels around 1500 m.
Avalanche Summary
Reports from Thursday include isolated large explosive triggered persistent slab avalanches failing on the mid-January surface hoar on northeast facing treeline and alpine slopes. These slabs were 80-150 cm thick with some wind-loaded pockets as deep as 2 m. One avalanche was described as 'sluggish', but highly destructive, taking out mature timber on the way down.
Snowpack Summary
Recent warm temperatures have softened the upper snowpack, reawakening deep persistent weaknesses in isolated areas. As temperatures continue to fluctuate and stress the snowpack, expect an increased likelihood of triggering large destructive avalanches in isolated areas (e.g. thin spots) with heavy triggers (e.g. cornice falls, stuck sleds spinning tracks, or groups of people). Approximately 65-100 cm of recent storm snow is bonding well to a crust buried at the beginning of February; however, this bond is much weaker where surface hoar overlies the crust. Below that, recent snowpack tests gave moderate to hard but sudden collapse results on the mid-January surface hoar where it was found down 70-82 cm on a northeast facing treeline slope. This persistent weakness has been found as deep as 110 cm, or deeper on wind-loaded slopes. The mid-December crust/facet/surface hoar weakness may be persisting in the mid to lower snowpack at higher elevations.
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.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.