Regions
Northwest Coastal.
Avalanche danger may spike to HIGH in the alpine if stormy weather arrives sooner than forecast on Friday.
Confidence
Poor - Timing of incoming weather is uncertain for the entire period
Weather Forecast
Friday: Strong W to SW winds. Alpine temp -6. Light snow starting late in the day.Saturday: Strong to gale SW winds. Alpine temp -6. Moderate to heavy snow. Sunday: Light SE winds. Alpine temp -12. Light snow.The timing of frontal systems affecting the region is uncertain, especially on Sunday.
Avalanche Summary
On Wednesday, explosives triggered a size 3 slab which failed at the base of the snowpack on glacial ice. Several storm/wind slabs to size 2 were also triggered by explosives. A widespread natural cycle occurred on Monday/Tuesday with avalanches to size 2.5 running in steep terrain at treeline and alpine elevations. Explosives testing on Tuesday produced several very large (size 3-4) slabs on northerly aspects near Stewart. These failed on basal facets and/or glacial ice and were up to 4 m deep and 250-300 m wide. Storm/wind slabs also stepped down to basal facets. Low elevation steep terrain also was shedding numerous small, moist loose avalanches during Monday/Tuesday’s warming.
Snowpack Summary
Storm snow instabilities are likely to continue developing over the next few days as variable new snow layers build up. Spotty surface hoar is also buried in the upper snowpack, particularly in sheltered treeline areas. New and buried wind slabs are widespread and exist on a variety of alpine and exposed treeline slopes due to variable winds. Of key concern is a November facet/crust layer which can be found near the base of the snowpack. This layer, which is widespread, can be triggered from thin-spot trigger points, or with a heavy load, such as storm slabs stepping down, cornice fall or a snowmobile. It has the potential for very large, destructive avalanches and demands respect. A rain crust is buried in the upper snowpack at low elevations. In general the snowpack is highly variable in depth due to windy conditions this season.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.