Regions
Banff Yoho Kootenay.
Some instances of cornices acting as the trigger for wind slabs and deeper releases. Warmer temperatures for Sunday. Watch for areas where wind slabs sit over the rotten facet layers that make up the bottom half of the snowpack. SH
Weather Forecast
Alpine temperatures between -12 and -8 C for Sunday and light gusting moderate Westerly winds. Eastern regions could see valley bottom afternoon highs in the -3C range. Some minor snow is currently forecast starting Monday night.
Snowpack Summary
Lots of hard wind slabs at treeline and above from a variety of wind directions over the past while, most notably N winds from the arctic air. This has produced a reverse loading pattern in many locations. These hard slabs in many starting zone areas overly a weak, faceted snowpack with numerous instabilities in the lower half of the snowpack.
Avalanche Summary
There was a cornice triggered size 2.5 off of a SE aspect at 2800m on Cascade Mountain Friday afternoon. It was hard to tell the bed surface but looked to be over a meter deep. Another size 2.5 with explosives on an ESE aspect to ground at Sunshine. Other similar avalanches initiated by large triggers have been noted over the last few days.
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.