Storm snow and wind are adding to an already complex snowpack with a number of buried weak layers that have produced several recent, large avalanches. Simple terrain with no overhead hazard is a good choice should you venture into the backcountry.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
TONIGHT: Snow. Accumulation 10-20 cm. Ridge wind strong, southwest. Temperature -2. Freezing level lowering to valley bottom.THURSDAY: Mostly cloudy, flurries. Accumulation 5-10 cm. Ridge wind strong, southwest. Temperature -3. Freezing level rising to 1000 m.FRIDAY: Mostly cloudy, light flurries. Accumulation 2-6 cm. Ridge wind strong, southwest. Temperature -4. Freezing level valley bottom.SATURDAY: Cloudy, flurries. Accumulation 2-8 cm. Ridge wind light to moderate, south. Temperature -4. Freezing level valley bottom.
Avalanche Summary
Wednesday numerous explosive and skier triggered storm slab avalanches to Size 2 were reported up to 20 cm deep, with propagations from 20 up to 200 m wide, and running long distances.On Tuesday widespread sloughing and and several storm slabs up to Size 1 were observed in steep terrain. Also a Size 2 was triggered from 50 m away by a snow machine on a northeast aspect at 2050 m. The crown was 120 cm deep and is suspected to have failed on the mid-December weak layer.Reports from Monday included seven deep persistent slabs that were released with small explosives in the alpine in the Fernie area. Sizes ranged from 2.5-3, crown depths averaged about 200 cm and all results indicated the deeply buried late-November crust as the failure plane. These results demonstrate the need for ongoing vigilance with regard to overhead hazards and the ongoing potential for deep, destructive avalanches to occur.
Snowpack Summary
About 40-60 cm of storm snow now covers a layer of surface hoar on sheltered aspects (especially prominent from 1400-1900 m) as well as sun crust on solar aspects that was buried mid-January. Beneath the mid-January interface lie a number of very concerning buried weak layers. A layer of surface hoar from early-January is buried 90-110 cm below the surface. A weak layer buried mid-December (predominantly surface hoar and/or a sun crust) is around 120-160 cm below the surface at treeline and below treeline elevations. A rain crust with sugary facets that developed late-November is near the bottom of the snowpack. All of these layers have produced recent large, destructive avalanches and remain a concern.
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.