Regions
Kootenay Boundary.
A complex avalanche hazard exists in our region. Wind slabs may be strengthening, but the persistent slab problem will linger much longer. Thin snowpack areas are especially concerning.
Confidence
High - The weather pattern is stable
Weather Forecast
Thursday: Mainly cloudy with isolated flurries and a trace of new snow. Winds light from the northwest. Freezing level around 900 metres with alpine temperatures around -8.Friday: Mainly cloudy with isolated flurries and a trace of new snow. Winds light from the west. Freezing level around 800 metres with alpine temperatures around -5. Possible alpine temperature inversion.Saturday: Mainly cloudy. Winds light to moderate from the southwest. Alpine temperature inversion bringing alpine temperatures to near or exceeding 0.
Avalanche Summary
Aside from small loose snow releases from steep terrain, no new avalanches were reported on Tuesday. Reports from Monday include details of two Size 2-3 avalanches releasing naturally on steep terrain over Sunday to Monday morning. Wind loading of a northwest aspect and solar input to a southerly aspect were determined to be the triggers of these separate events. Reports from Sunday include several natural Size 1-1.5 wind slab avalanches that occurred on a northwest aspect after a bout of rapid wind loading over the morning. Preliminary information regarding the backcountry skier fatality that occurred on Saturday is that it occurred on a S/SE facing aspect near 2050m in a cross-loaded feature. We also had a great MIN report of a cornice-triggered Size 2 on Friday near Cabin Peak in the Bonnington range. The initial failure was in the storm slab on a southeast aspect near 2000m but more significantly was observed stepping down to trigger a weak layer deeper in the snowpack. This 'step down' character is keeping this report in our discussion in spite of its age.
Snowpack Summary
A complex and tricky snowpack exists in the Kootenay Boundary region. At the surface, a trace to 5 cm of new snow fell over the region on Tuesday night, giving a thin cover to surface hoar that has been observed growing up to 6 mm. Combined with Sunday's 3-12 cm and 35-55 cm from last week's storms, the storm snow forming our upper snowpack now totals a rather variable 40-70 cm. The initial snowfall last week was accompanied by moderate to strong southerly winds, resulting in touchy storm and wind slabs forming and bonding poorly to the previous (January 17) snow surface that includes facets and large surface hoar on sheltered slopes and/or a sun crust on steep sun-exposed aspects. This varied layer continues to give easy to moderate and occasionally sudden results in snowpack tests (down anywhere from 30 to 60cms depending on location). In some areas of the Rossland range, the mid-November rain crust / facet layer is now found down 60-100 cm and has proven reactive with recent loading and warming. In thin rocky areas, particularly in the Rossland range, recent reports include easy sudden collapse snowpack test results on facets down 120-150 cm near the ground.
Problems
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.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.