Regions
Northwest Coastal.
Rider triggered storm slab avalanches are likely. Use a very conservative approach to terrain and watch your exposure from avalanche paths above you.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
The dominating stormy weather pattern has come to an end. Unsettled conditions persist Sunday while the Arctic front descends south and brings cold air and outflow winds for the forecast period. Sunday: Cloudy with snow amounts 4-10 cm. Ridgetop winds will be light from the northeast and freezing levels will be at valley bottom. Alpine temps near -8.Monday: Mix of sun and cloud. Ridgetop winds will be light from the northeast with moderate outflow winds through the inlets.Tuesday: Sunny with alpine temperatures near -15. Possible alpine inversion. Winds light from the northeast.
Avalanche Summary
On Friday, explosive triggered storm slabs were reported from northerly aspects at 1400 m and above up to size 2.5. Some of these ran on the buried surface hoar layer. At treeline and lower elevations numerous loose wet natural avalanches were reported from steep, rocky terrain features. Natural avalanches are still possible and rider triggered avalanches remain likely through Sunday.
Snowpack Summary
Storm snow totals reached 30-65 cm by Saturday, with the potential for much more on lee features due to strong winds. The new snow will load a buried surface hoar layer reported 60-80 cm below the surface in many parts of the region. Recent snowpack tests indicated the layer can be triggered with moderate loads and has the potential to propagate over long distances. This layer will likely become reactive under the stress of this new load. A thick rain crust exists 20-30 cm below the surface hoar layer, with isolated reports of weak facets (sugary snow) forming above the crust. Treeline snow depths are around 120-140 cm in the Terrace and Stewart areas, but substantially less further north. Snow in these thinner areas, such as Ningunsaw, may be faceting and developing weak basal layers.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.