Regions
Northwest Coastal.
Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period
Weather Forecast
A series of frontal systems will affect the coast for the next few days. The timing is a bit uncertain but the general pattern is stormy.Thursday: Strong W winds. Alpine temp -4. Moderate snow (locally heavy).Friday: Moderate to strong W winds. Alpine temp -6. Light to moderate snow.Saturday: Light winds. Alpine temp -10. Moderate to heavy snow.
Avalanche Summary
A natural cycle occurred on Tuesday with avalanches to size 2.5 running in steep terrain at treeline and alpine elevations. Explosives testing 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. Recent storm/wind slabs above 1600 m which were triggered by cornice fall or explosives also stepped down to basal facets. Low elevation steep terrain was also shedding numerous small, moist loose avalanches during Tuesday’s warming.
Snowpack Summary
Storm snow instabilities are likely for the next few days. New and buried wind slabs are widespread and exist on a variety of alpine and exposed treeline slopes due to variable winds. A rain crust is now buried by new snow at low elevations. A November facet/crust layer can be found near the base of the snowpack. Very large releases are still occurring on this layer with a heavy trigger, such as storm slabs stepping down, explosives, or cornice fall.
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.