Regions
Banff Yoho Kootenay.
Many cornice failures Saturday. Recent avalanches and the uncertainty around the deep persistent weak layers require a conservative approach to terrain features.
Weather Forecast
Valley bottom temperatures will be around 0C Sunday, with alpine temperatures staying in -8 to -10C range with moderate W alpine winds and only trace amounts of new snow. There will be an increase in solar input and warming temperatures this week (12C by Wed. in Banff) which may again increase the avalanche danger, especially in the front ranges.
Snowpack Summary
50-60cm of storm snow settling with warmer temps has been reactive this past week. Moist snow on solar aspects was observed Saturday below treeline due to warmer temps and a bit of sun. Wind slabs have formed in alpine lee areas. Below this storm snow a more settled slab sits over the weaker facets and depth hoar near the base which is widespread.
Avalanche Summary
Large storm/wind slabs have been reactive this last week up to size 3, triggered by explosives or running naturally. Some avalanches have stepped to the basal layers, especially East of the divide. Less natural activity noted on Saturday of these slabs. The big story are widespread cornice failures up to 300m wide reported in many areas.
Confidence
Freezing levels are uncertain
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.
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.