Regions
Kootenay Boundary.
Recent storm slabs continue to be a concern for human triggered avalanches. Storm slabs or cornices in motion may step down to the deeply buried persistent weak layer resulting in large destructive avalanches.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
Some chance of convective flurries overnight with moderate west or northwest winds and freezing levels down to valley bottoms. A mix of sun and cloud on Wednesday with some lingering flurries, moderate westerly winds and daytime freezing levels climbing up to 1500 metres. Mostly sunny on Thursday with light winds and a good overnight freeze; daytime freezing levels up to 1500 metres. Clear on Friday with a good overnight freeze, light winds and daytime freezing levels climbing up to 1700 metres.
Avalanche Summary
On Monday there was an avalanche in the Mt Mackie area southwest of Castlegar that resulted in a fatality. We have limited information about the size and sliding layer at this time. We will release more information when it becomes available. There was also a skier accidental size 2.5 that started as a cornice failure and propagated down to one of the buried crusts; this was near Red Mtn in the Rossland range. There were also a couple of natural avalanches and a skier accidental avalanche size 2.0 in the eastern part of the region near Nelson. The storm snow continues to be a concern for human triggering, and avalanches that start in the storm snow may "step-down" to deeper buried weak layers. Cornices are also a concern for natural avalanches and human triggering.
Snowpack Summary
There is now 25 to 50 cm of storm snow combined with moderate southwest winds which have likely created deep wind slabs on the leeward side of ridgecrests and mid-slope terrain features. In some drainages these storm slabs may be sitting on a thin crust that was initially buried on March 11th. A second curst, (the early March melt/freeze crust) can be found down 50 to 80 cm. A persistent weak layer buried late-February is typically down 60 to 100 cm and remains sensitive to triggering as indicated by recent avalanche activity. This weakness is widespread buried surface hoar on sheltered slopes above 1700 m, and a crust potentially with associated facets on previously sun-exposed slopes (primarily south aspects).
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.