Purcells

Between fresh wind slabs and a reactive persistent weak layer, human triggered avalanches are possible. Be prepared for unusually cold temperatures for this time of year when venturing out.

Friday night: Mostly cloudy, 5-15 cm of snow, moderate east wind, alpine high temperature -22 C, freezing level at valley bottom.

Saturday: Mix of sun and cloud, scattered flurries with up to 5 cm of snow accumulation, moderate east wind, alpine high temperature -22 C, freezing level at valley bottom.

Sunday: Sunny, light northeast wind, alpine high temperature -12 C, freezing level at valley bottom.

Monday: Mostly sunny, light northerly wind, alpine high temperature -10 C, freezing level at 1200 m.

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 past week, human-triggered and explosive-triggered avalanches releasing on the February 22 surface hoar have been consistently reported from the north and west of the region. 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. 

The east of the region is expected to receive the largest snow amounts of up to 15 cm with the current system. The storm at the beginning of the week delivered 20-50 cm of snow with highest amounts in the north and west of the region. This snow may sit on old wind slabs, a weak surface hoar layer and sun crusts on steep solar aspects. Human triggering remains possible 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.

• Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
• Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
• Be aware of the potential for large avalanches due to the presence of buried surface hoar.
• Avoid shallow, rocky areas where the snowpack transitions from thick to thin.