Regions
Banff Yoho Kootenay.
The hazard may go to high Thursday if we get more snow than forecast. Given the sensitive nature of the basal weakness, it will be best to limit your exposure to avalanche terrain until we see what this storm produces. SH
Weather Forecast
Snowfall amounts have backed off from previous forecasts. Starting Thursday afternoon, we can still expect 10-20 cm of snow by Friday (lesser amounts in Eastern regions). Winds Thursday will be mod-strong westerlies, with gusts into the extreme range. This may bump the hazard to High in the bulletin region on Thursday and Friday.
Snowpack Summary
W. of the divide there is 40-70 cm of fresh snow over the past 6 days and 10-20 cm on the East. Continued west winds have added to the slab condition in the alpine and at treeline. The treeline snowpack depth is roughly 1 metre deep. The midpack is supportive on skis but is consists mostly of weak facets with a persistent weak layer at the ground.
Avalanche Summary
Avalanches to size 2.5 over the past few days have been reported with mainly the same characteristics, which are wind slabs failing to ground on the basal weaknesses. Two cornice triggered size 2 avalanches were observed on E/SE. facing Healy creek paths. These were new wind slabs which did not step down to ground and were roughly 40-50cm thick.
Confidence
Forecast snowfall amounts are uncertain on Thursday
Problems
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.
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.
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.