Intense sun may make recent snow and cornices more sensitive to failure. Adjust travel to give steep, sun-exposed slopes and cornices a wide berth. A buried weak layer warrants a conservative mindset and terrain use strategy.
Snowfall with wind has formed 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. Periods of intense sun may make these problems more susceptible to failure.
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 and remotely-triggered avalanches have released on this layer. As more areas approach critical loading with settlement in the recent snow, human-triggered large avalanche 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.
Sunday night: Mostly clear, light west winds, alpine temperature -14 C.
Monday: Mostly clear, light west wind, alpine high temperature -5 C, freezing level 1000 m.
Tuesday: Increasing cloud, isolated flurries with trace accumulations, moderate southwest winds with strong gusts, alpine high temperature -4 C, freezing level 1400 m.
Wednesday: Mostly cloudy, scattered flurries with up to 5 cm of snow, decreasing west winds, alpine high temperature -4 C, freezing level 1400 m.
On Saturday, several human-triggered and explosive-triggered storm slab avalanches were reported. These avalanches were small to large (size 1-2) and breaking 25-40 cm deep. Shallow storm slabs have the potential to step down to the buried surface hoar layer, creating very large avalanches. Use added caution on sun-exposed slopes during intense solar radiation, as rapidly warming snow and weakening cornices could trigger loose or slab avalanches.
Over the past week, human-triggered and remotely-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 propagated widely across terrain features, warranting conservative terrain margins (see 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. Slab formation and human triggering will likely be specific to where the wind has transported the snow into deeper drifts. Intense solar radiation may rapidly destabilize recent snow on steep, sun-exposed slopes and weaken cornices.
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. Riders will be capable of triggering this layer for some time 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.
