Regions
Kootenay Boundary.
Storm slabs may be easy to trigger, and persistent slabs resulting in very large avalanches continue to be a concern for human triggering.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
Moderate westerly winds overnight with 3-5 cm of new snow and freezing levels down to valley bottoms. The precipitation is expected to be convective and may result in pockets of higher snowfall. Mostly cloudy on Tuesday with a few sunny breaks. Winds becoming moderate northwest during the day and freezing levels rising up to 1500 metres. Less cloud and more sun on Wednesday with light winds and a diurnal freeze (overnight) and thaw (up to about 1500 metres) cycle developing. Mostly sunny on Thursday with light northeast winds.
Avalanche Summary
Explosives control work on Sunday resulted in storm slab avalanches up to size 2.0 on northerly aspects, and skiers controlled avalanches up to size 1.5 on northeast aspects .On Saturday numerous human triggered storm slab avalanches to size 2 were reported on northwest, north, northeast and east facing aspects between 1800 and 2200 m. One deeper persistent slab avalanche was triggered after a snow-cat intentionally pushed a cornice onto a north facing feature at 2200 m resulting in a size 3 avalanche that was 100 cm deep on average. Check out the Mountain Information Network for more details about a human triggered size 2 avalanche on a NW facing slope along the Bonnington Traverse.
Snowpack Summary
Another 5-10 cm overnight has added to the recent storm slabs. There is now 20 to 40 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 40 to 70 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.