Although cold may cause our persistent slab problem to trend a bit less reactive, it also slows the process that would allow it to heal it into the snowpack. Instead of gambling on which effect is more significant, choose terrain that minimizes your exposure to the problem.
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 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.
Wednesday night: Clear. Light north winds.
Thursday: Sunny. Light northeast winds. Alpine high temperatures around -23.
Friday: Sunny. Light northeast winds. Alpine high temperatures around -20.
Saturday: Becoming cloudy. Light to moderate east winds. Alpine high temperatures around -18.
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 our active persistent weak 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 similar avalanche (size 3.5, step down to December crust) occurred naturally in the same area in the first week of February.
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.
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, even as its likelihood of triggering gradually diminishes.
There are potentially 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.
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.
