Regions
Northwest Coastal.
The avalanche danger will increase over the weekend with the progression of the incoming storms systems. Wind slabs are the primary concern for Saturday and a more widespread storm slab problem may develop by Sunday.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
The ridge of high pressure that has been producing cold, dry conditions is breaking down and allowing active weather systems into the region starting Saturday. Around 5 cm of snowfall is expected on Saturday with strong northwest winds and treeline temperatures around -10C. Heavy snowfall is forecast to start Saturday night and up to 40 cm is possible by Sunday afternoon. Alpine winds should remain strong on Sunday but are forecast to shift to the southwest. Freezing levels are forecast to rise to around 800 m on Sunday. Another period of heavy snowfall with strong winds is currently forecast for Sunday overnight and into Monday.
Avalanche Summary
No recent avalanches have been reported in this region. Ongoing strong winds have formed hard wind slabs in exposed terrain which may still be reactive to human triggering in isolated areas. Touchy new winds slabs are expected to form on Saturday with the forecast for light snowfall and strong alpine winds. As the storm ramps up Saturday night and Sunday, a widespread storm slab problem is expected to develop. The surface hoar layer from mid-November may still be reactive in isolated areas and is creating a low probability, high consequence problem. As new load accumulates over the weekend, this layer is expected to increase in reactivity where is still exists.
Snowpack Summary
Clear skies and strong outflow winds have created a variety of snow surfaces including scouring and hard wind slabs in exposed areas. Faceting of the upper snowpack and surface hoar up to 12 mm has in reported in sheltered areas. The new snow is expected to bond poorly to this interface. A layer of surface hoar which was buried mid-November is down 60-120 cm in many parts of the region. This layer has been dormant recently but may increase in reactivity as the storm adds new load to the snowpack. Treeline snow depths are around 140-200 cm in the Terrace and Stewart areas, but substantially less further north. Concerns in the north are basal weaknesses that may exist in the shallower snowpack areas especially on smooth alpine features like glaciers.
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.