Friday's wind event should have you re-evaluating terrain choices for Saturday. New wind slabs are widespread in exposed areas and sheltered areas harbour dangerous, deeply buried weak layers. The possibility for a wind slab release to step down to a deeper layer is very real.
Widespread new and reactive wind slabs exist at higher elevations as a result of strong easterly winds on Friday. All except due east aspects are well represented in Friday's reports.
60-80 cm of snow sits on a persistent weak layer that was buried near the end of January. This layer has been the most reactive at treeline, where it exists as surface hoar, but attention should also be paid to steep alpine terrain where avalanches have greater potential to step down to deep weak layers from the early season.
Friday: Clear. Light to moderate east winds.
Saturday: Sunny. Moderate to strong east winds. Alpine high temperatures around -16.
Sunday: A mix of sun and cloud. Light variable winds. Alpine high temperatures around -16.
Monday: Cloudy with isolated flurries and a trace to 5 cm of new snow. Light to moderate southwest winds shifting northwest. Alpine high temperatures around -13.
A bout of sustained strong easterly winds prompted widespread wind slab formation and reactivity on Friday. Many 20-50 cm-deep slabs released naturally, with skier triggering and with explosives on Friday, some with remarkably wide propagation. Expect this reactivity to persist through the weekend.
We also have many recent reports of persistent slab avalanches triggered by skiers at elevations around treeline (1800-2000 m) where our active persistent weak layer exists as surface hoar. Check out the MIN for more details - there's a lot to see.
Another explosives-triggered persistent slab in the Bonningtons on Tuesday stepped down to our early December crust layer to produce a 200 cm-deep, size 3.5 (very large!) avalanche. A similar avalanche (size 3.5, step down to December crust) occurred naturally in the same area in the first week of February.
Snow from the past week has seen a great deal of wind effect at upper elevations and may contain a freezing rain crust or surface hoar at lower elevations. It continues to settle over a weak layer buried 60-80 cm deep.
This persistent weak layer may exist as facets or a crust but it has been most reactive where it exists as large surface hoar crystals in sheltered areas treeline and below. Surface hoar has potential to surprise backcountry users with how widely the fracture can travel across slopes. This layer is slow to gain strength and requires careful terrain selection, even as its likelihood of triggering gradually diminishes.
There are several more layers of surface hoar in the mid snowpack, with the most notable one down about 80-120 cm. This layer was buried in early January. These older layers may take precedence over the layer described above in shallower snowpack areas from the Nelson Range through to the Okanagan.
A crust surrounded by weak faceted grains is buried deep within the snowpack. It may be possible to trigger this layer with a large load in a shallow rocky start zone.
