Avalanche danger is expected to drop a bit for Monday in the wake of the recent storm as a drier and cooler ridge moves into the region.
Confidence
Fair - Due to the number of field observations
Weather Forecast
Flurries overnight combined with moderate Westerly winds and freezing levels around 800 metres. A ridge of High pressure is forecast to move over the province on Monday resulting in drier and cooler conditions. Freezing levels should drop to valley bottoms and there may be some periods of broken skies or scattered cloud. The next Pacific storm should move onto the coast sometime early Tuesday morning. Expect strong Southwest winds combined with 5-10 cm of new snow and freezing levels spiking up to about 1600 metres by the afternoon. The storm should move to the East by Wednesday leaving unsettled weather including flurries, dropping freezing levels, and light Northwest winds.
Avalanche Summary
Ski cutting resulted in storm slab avalanches up to size 2.0 on Saturday, and explosives control resulted in persistent slab avalanches releasing in the storm snow and stepping down to the November 28th crust. I suspect that storm snow avalanches continued to release on Sunday with the additional load of new overnight on Sunday morning.
Snowpack Summary
The new storm slab is between 40-50 cm thick above the mid-December crust/facet/surface hoar layer that extends at least up to treeline. This new storm slab has been transported by strong winds in the alpine and at treeline developing deep pockets of windslab that may not be well bonded to the old settled snow surface. The November 28th rain crust may still be exposed in some areas due to the stripping effect of the strong Southwest winds, or it may be deeply buried down 100-150 cm. The forecast ridge of High pressure should allow for some settling and bonding of the new storm snow, except where it is sitting on a hard crust with weak surface hoar or facets at the interface. The new storm brought some snow to below treeline terrain, but it still looks like only the upper part of this elevation band has enough snow for avalanches.
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.
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.