A fatal avalanche occurred yesterday in a cross-loaded terrain feature (see avalanche discussion for further details). Cautious route-finding is essential in this period of continued CONSIDERABLE hazard.
Weather Forecast
Tuesday will be a mix of sun and cloud with alpine temperatures reaching a high of -2 C. Freezing levels will reach 2000 metres. Ridge-top winds will be from the northwest at 10-25 km/h. Wednesday, Thursday and Friday will bring light accumulations of new snow, cool temperatures and strong west winds.
Avalanche Summary
Several large avalanches have occurred over the past week. Many of these have been triggered by cornice failures, and most have failed on the Jan 6th layer and/or ground.A fatal avalanche occurred on Tent Ridge on Sunday March 20, 2016. A solo snowshoer triggered a size 3.0 on a North aspect and was completely buried. The avalanche initiated near 2400m and ran for over 800m. The fracture ranged from 40 to 150cm deep and involved a wind slab that stepped down to the Jan 6th layer and to ground. The subject who was not wearing a transceiver was recovered by rescue crews on Monday morning.
Snowpack Summary
A few cm's of new snow has fallen in the past 24hrs. This overlies a variety of previous surfaces including wind slab and facets on N and E aspects, and crusts on all solar aspects. There are numerous buried crusts on solar aspects that are giving widely variable results in snowpack tests. Recently formed wind slabs are found primarily in the Alpine in lee and cross-loaded features, and there are a few Treeline features were this condition can be found. These slabs range from 30 to 60cm thick. The persistent weak layer from Jan 6th buried up to 100cm in the snowpack has been active in the past week responsible for several large natural avalanches and one fatal human-triggered avalanche (see avalanche discussion). Cornices have also been failing in the past week, also producing large avalanches on the underlying slopes.
Problems
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
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.
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.