Regions
Kootenay Boundary.
A developing storm slab overlies very weak crystals. With more snow on the way, conservative terrain selection will be critical.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain on Saturday
Weather Forecast
An intense frontal system will move across the province on Saturday night bringing moderate to locally heavy snowfall, extreme southwest winds and freezing levels to about 1300m. Scattered flurries are expected throughout Sunday. A dry ridge is expected to form on Monday bringing a mix of sun and cloud, moderate northwest winds and freezing levels at valley bottom. The weather pattern will shift again on Tuesday evening as another pacific system moves across the southern half of the province bringing moderate snowfall and rising freezing levels.
Avalanche Summary
On Wednesday, explosive testing produced small (size 1.5) avalanches on S to SE aspects at 2200 m in steep rocky terrain that were suspected to step down to the early November crust. Looking forward, natural storm slab activity is expected with weather forecast for Saturday night
Snowpack Summary
20-30 cm new snow has buried a prominent surface hoar layer above a thick rain crust that extends as high as 2400 m. As more snow falls, temperatures rise and winds increase on Saturday night, this developing storm slab will gain depth and cohesion adding to its destructive potential. The crust/surface hoar interface is expected to produce a very good sliding layer, and avalanches that run on this layer could travel far as a result. High elevation north aspect slopes do not have the rain crust, but on these slopes, facets or buried surface hoar from earlier in the month may be lurking under dense storm slabs. Recent snowpack tests at 2050 m in a NW aspect in Kootenay Pass produced moderate sudden results down 35 cm under the late-November crust and down 79 cm on facets above the early-November crust. There is a possibility that additional load from lots of new snow, or a large trigger such as a cornice release could still trigger a deeper slab on shady aspects at high elevations.
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.