Between fresh wind slabs and a reactive persistent weak layer, human triggered avalanches are likely. Keep in mind it's not always the first rider who triggers the slab and keep terrain choices conservative on Wednesday.
15-20 cm of snow by Wednesday morning combined with strong westerly wind is forming wind slabs in the alpine and open areas at treeline. This problem overlaps with where cornices may be reaching their breaking point. If triggered, wind slabs and cornices could step down to the buried surface hoar layer, forming large avalanches.
A weak layer of surface hoar sits 35-70 cm deep, within the prime range for human-triggering. Over the past several days, large human-triggered avalanches have released on this layer. As more areas approach critical loading from additional snow and wind, human-triggering remains likely.
Weak snow at the bottom of the snowpack continues to pose a deep persistent slab avalanche problem. Treat wind-scoured, rocky, shallow terrain as suspect and avoid big alpine slopes that are threatened by cornices overhead.
Tuesday night: Mostly cloudy, 15-20 cm of snow, strong westerly wind, alpine high temperature -5 C, freezing level 1300 m.
Wednesday: Mostly cloudy, scattered flurries with up to 5 cm of snow accumulation, moderate northwest wind, alpine high temperature -10 C, freezing level 1000 m.
Thursday: Mix of sun and cloud with isolated flurries, light westerly wind, alpine high temperature -10 C, freezing level 1000 m.
Friday: Mostly cloudy, scattered flurries with up to 5 cm of snow accumulation, moderate easterly wind, alpine high temperature -10 C, freezing level at valley bottom.
Over the weekend, a natural storm slab avalanche cycle was observed. These avalanches were small to large (size 1-2.5) and breaking 20-40 cm deep. Shallow slabs in the recent snow have the potential to step down to the buried surface hoar layer, creating very large avalanches.
Over the past week, human-triggered avalanches releasing on the February 22 surface hoar have been consistently reported. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab. These avalanches have been remotely-triggered and have propagated widely across terrain features, warranting conservative terrain margins (see this MIN or this MIN for a helpful example). Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity.
On Thursday, professionals reported large (size 2.5) explosive-triggered avalanches releasing on weak layers near the ground. These avalanches occurred on north and east facing slopes above 2300 m, and they confirm that the deep persistent slab problem remains a concern.
Snowfall amounts from the weekend storm ranged from 20-40 cm. Human triggering remains possible where the wind has transported the snow into deeper drifts. Incremental snowfall and wind forecast for Tuesday night is expected to add to this wind slab problem.
A weak layer of surface hoar from February 22 may be found 35-70 cm deep. It may sit over a crust on solar aspects. With successive storms, more areas have reached critical loading and developed a slab over this layer. Managing this problem requires a patient and diligent mindset, implemented by avoiding suspect slopes and maintaining conservative terrain margins. This persistent weak layer will likely remain a problem until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.
Deep basal facets lurk near the bottom of the snowpack. This layer has been responsible for sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones.
