Regions
Banff Yoho Kootenay.
Seek supported features without overhead hazard. Today's skier accidental at Bow Summit confirms that large avalanches can be triggered from shallow areas on large terrain features. See discussion.
Weather Forecast
A cooling trend is expected to start tomorrow along with tapering snowfall amounts. The freezing level is expected to be around the surface. Wind is Westerly and tapering to the light/moderate range towards the end of Monday.
Snowpack Summary
5-10cm of recent storm snow with West wind is forming cohesive slabs in lee areas up to 30cm deep. Discernable layers are difficult to identify below the storm snow, however expect a mix of facets and crusts and in shallow areas depth hoar. In shallow areas snowpack tests results are typically collapses on the ground.
Avalanche Summary
Forecasters received a report of a skier accidental size 2 in the slide path above Peyto Lake, which is accessed from Bow Summit. The party indicated they received numerous whumphs while approaching the slope. They then committed to the slope and the second skier triggered the slide. 15-100cm deep, 30m wide and 300m long. No burials or injuries.
Confidence
Freezing levels are uncertain on Monday
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.