Regions
Northwest Coastal.
Recent snowfall and winds have been driving a wind slab problem at higher elevations. The potential for a wind slab release to trigger a deeper persistent slab needs to factor into your terrain selection.
Weather Forecast
Sunday: A mix of sun and cloud with isolated flurries and a trace of new snow. Moderate to strong southwest winds. Freezing level to 400 metres with alpine temperatures of around -8.Monday: A mix of sun and cloud. Light east winds. Freezing level to 1000 metres with alpine temperatures of around -4.Tuesday: Cloudy with sunny periods, flurries beginning in the afternoon. Light southeast winds. Freezing level to 1000 metres with alpine temperatures of around -3.
Avalanche Summary
Reports from Thursday include several observations from the north of the region, where skier traffic triggered a Size 2.5 wind slab as well as a Size 2 persistent slab over the late February interface down 50 cm. The large wind slab release was triggered from a thin spot and had a crown fracture that ranged from 30-100 cm. This should draw attention to the significant effect of recent winds as well as the touchy nature of the thin edges of large wind slabs. The bulk of recently reported activity took place on north to northwest aspects.
Snowpack Summary
A steady supply of new snow over Thursday and Friday has been blown into touchy wind slabs by strong southeast to southwest winds at higher elevations. Below the new snow, the previous week of stormy weather delivered approximately 30-60 cm of recent snow, with much deeper areas where the wind transported this snow. At treeline and above, new snow and wind have been loading and stressing a weak interface from February composed of facets, crust, and surface hoar buried over a metre deep. This layer was active prior to the storm and remains an ongoing concern. A non-supportive surface crust has formed over moist or wet snow at 1100 metres and below.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.