Regions
Kootenay Boundary.
Ongoing snowfall will keep avalanche danger elevated over the weekend. The increasing load will test our deeply buried weak layers.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
Saturday: Flurries bringing another 10 cm of new snow, beginning midday. Winds light from the southeast. Freezing level to 1400 metres with alpine temperatures of -1 Sunday: Mainly cloudy with scattered flurries bringing up to 5 cm of new snow. Winds moderate from the south. Freezing level to 1200 metres with alpine temperatures of -3. Monday: Periods of snow bringing approximately 15 cm of new snow. Winds moderate to strong from the south. Freezing level to 1600 metres with alpine temperatures around 0.
Avalanche Summary
Reports from Thursday include results from extensive explosives control in the Bonnington Range, Kootenay Pass, and southeast of Nelson. Results were generally classed as storm slabs, with sizes ranging primarily from 2-2.5 and crown fractures typically 30-60 cm in depth. Two Size 3 results were recorded, one of which is suspected to have stepped all the way down to our November crust, at a 100 cm depth at that location. Several larger results in the Bonningtons are also suspected to have involved our February 3rd interface. Skier controlled storm slabs were also reported to Size 2.5 while a number of wet slab avalanches were recorded at lower elevations.
Snowpack Summary
Upwards of 45 cm of new snow lies at the surface after Thursday's stormy weather. Below the new snow, a widespread sun crust exists on all but high northerly (and some westerly) aspects, where there is still some cold snow to be found. By the looks of Thursday's avalanche activity, this crust appears to have performed as an excellent sliding surface for our new snow.Beneath the crust, approximately 20-30cm of storm snow from late last week lies over our February 3rd interface. Now down approximately 60-100 cm, this interface comprises a variety of surfaces, including scoured surfaces in wind exposed terrain, surface hoar (size 2-3 mm) in sheltered locations, and sun crust on steep solar aspects. Recent snowpack tests and avalanche control suggest that this layer remains reactive in some areas. The Bonnington Range appears to be one of these areas.Areas with a shallower snowpack (less than about 150 cm) have a generally weak snowpack structure with sugary facets near the ground. This includes shallow alpine slopes and most of the Rossland range. In these areas it is possible for storm slab avalanches to step-down to these deeper weak layers, resulting in large avalanches.
Problems
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.
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.