Regions
Northwest Coastal.
The danger ratings reflect the hazard in the northern part of the region, for the most part, where the persistent March 25th weakness has been most active. Incoming storms and more snow load may wake up this layer in other parts of the region.
Confidence
Fair - Due to the number and quality of field observations
Weather Forecast
Increasing clouds with rain or snow at upper elevations starting today, with moderate to locally heavy snow 20-30 cm. Freezing level should be around 1500m for Friday, then fall to 500m overnight and rise to around 1000m on Saturday during the day.. Winds are forecast moderate to strong throughout the forecast period. Precipitation will be heavier towards the coast, but the inland portion of the Northwest Coast may see as much as 45 cm of snow at upper elevations for the period from Thursday to Monday.
Avalanche Summary
For the past few days, there have been reports of natural, skier-triggered, and remotely triggered slabs up to size 2.5 releasing on the March 25th surface hoar layer. Some of these have been triggered from below or in low-angle terrain. In addition, cornices have been failing and loose wet activity has been noted in the afternoon, both of which have triggered deeper persistent slabs.
Snowpack Summary
The snow surface varies from dry faceted powder and/or surface hoar on alpine shaded slopes, to moist snow or a sun crust on sunny slopes. In some places down-flowing winds have created thin wind slabs in lee features lower down the mountainside. Approximately 40-70 cm of snow sits above the March 25th surface hoar layer. This interface seems much touchier in the northern part of the region, but has been reported throughout the northwest coastal area. The early March facet/crust persistent weakness is now down over a metre and produces hard, sudden planar results in snowpack tests.
Problems
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.
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.