Purcells

A shift in wind direction to east will load previous windward slopes at all elevations. Between fresh and old wind slabs, reactive storm slabs and a reactive persistent weak layer, human triggered avalanches are likely. 

Thursday night: Cloudy with clear periods, scattered flurries with up to 5 cm of snow accumulation, light northwest wind, alpine high temperature -14 C, freezing level at valley bottom. 

Friday: Cloudy, 5-15 cm of snow, moderate east wind, alpine high temperature -16 C, freezing level at valley bottom.

Saturday: Mostly cloudy, scattered flurries with up to 5 cm of snow accumulation, moderate northeast wind, alpine high temperature -22 C, freezing level at valley bottom.

Sunday: Sunny, light northeast wind, alpine high temperature -15 C, freezing level at valley bottom.

Several storm and wind slab avalanches of size small to large (size 1-2.5) were reported during and after the recent storm on Tuesday and Wednesday breaking 20-40 cm deep. Most avalanches released on the previous snow surface which may consist of old wind slab, surface hoar or a sun crust on steep solar aspects. Shallow slabs in the recent snow have the potential to step down to the buried surface hoar layer from February 22, creating very large avalanches. 

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. 

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. 

Over the past week, professionals reported large (size 2.5 to 3.5) natural and 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. 

Between 20- 50 cm of recent storm snow sit on old wind slabs, a weak surface hoar layer and sun crusts on steep solar aspects. The new snow is not expected to bond well with these previous surfaces. Human triggering remains likely especially where the wind has transported the snow into deeper drifts. 

A weak layer of surface hoar from February 22 may be found 50-100 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.

• Be aware of the potential for large avalanches due to the presence of buried surface hoar.
• Use conservative route selection and resist venturing out into complex terrain.
• Stay off recently wind loaded slopes until they have had a chance to stabilize.
• Avoid shallow, rocky areas where the snowpack transitions from thick to thin.