Recent MIN reports from the region tell the tale of a widespread and tricky persistent slab problem. Cold temperatures are adding another layer of consequence to triggering it.
Recent wind from southwest to northwest has blown recent snow into wind slabs in lee features at upper elevations.
In wind sheltered areas, low density storm snow may be prone to sluffing especially with skier traffic.
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.
Another layer of surface hoar that was buried in mid-January may still be possible to trigger, and it is now down about 80-100 cm.
Tuesday night: Increasing cloud with isolated flurries and a trace of new snow. Light to moderate west winds.
Wednesday: Becoming mainly sunny. Light northwest winds. Alpine high temperatures around -20.
Thursday: Sunny. Light northeast winds. Alpine high temperatures around -24.
Friday: Mainly sunny. Light northeast winds. Alpine high temperatures around -20.
We are still receiving reports of persistent slab avalanches, triggered by skiers and explosives, generally large (size 2-3) with wide propagation, at elevations around treeline (1800-2000 m) where the persistent layer exists as surface hoar. Check out this great MIN describing a remotely triggered persistent slab avalanche in Kootenay Pass on Sunday. Most other MINs from the region from the past few days detail other persistent slab releases.
An 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 natural storm slab cycle up to size 2 was observed Saturday. Explosive and skier controlled storm slabs to size 1.5 were produced through the weekend.
Last week, an isolated size 3.5 (very large) avalanche was reported to have failed on a crust that was buried in early December. A smaller avalanche stepped down to this deeper layer, resulting in a very large and destructive avalanche.
Snow from the past week has seen some wind effect at upper elevations and may contain a freezing rain crust or surface hoar at lower elevations. It continues to settle over a reactive 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.
There are potentially several more layers of surface hoar in the upper snowpack, with the most notable one down about 80-120 cm. This layer was buried in early January.
A crust surrounded by weak faceted grains is buried deep within the snowpack. It may be possible to trigger this layers with a large load.
