Regions
Banff Yoho Kootenay.
An intense but short-lived storm last night and an expected dose of snow tonight, will keep windslabs fresh at Treeline and above. Triggering a windslab could have high consequences with the deep persistent slab problem still lingering.
Weather Forecast
Another 5 cm is expected to arrive Sunday night with light to moderate SW winds. Following this freezing levels are expected to drop a bit to 1300m while the light to moderate westerly flow bring trace amounts over the next couple of days .
Snowpack Summary
20-40 cm of snow now sits above crusts to 1800m and all solar slopes. Recent moderate S-SW winds with strong gusts have formed slabs in lee areas. The snowpack is highly variable but in the 150cm average found at TL, a firm upper pack sits over a weak mid-pack of facets. In thin snowpack areas tests results are typically collapses near the ground.
Avalanche Summary
Numerous avalanches to size 2.5 occurred Thursday morning after the passing of the storm. Most appeared to be windslabs in lee or cross-loaded alpine areas, many of these failed on crusts seen on S and W aspects, a few notable examples of deep persistent slabs were seen on east and north aspects.
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.