Avalanche Forecast

Archived

Dec 6th, 2012–Dec 7th, 2012

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

South Columbia.

There have been a couple of large avalanches failing deep in the snowpack. See Forecast Details for more info.

Confidence

Fair - Due to variable snowpack conditions on Friday

Weather Forecast

Overnight and Friday: A weak trough of low pressure is moving in from the coast. Expect light winds and light precipitation overnight to bring about 5mm to most parts of the region. Western parts of the region may see up to 15 mm that should fall as snow above 800 metres.Saturday: Light westerly winds are expected to clock to the Northwest and increase to moderate as an upper ridge builds over the Eastern Pacific. Alpine temperatures should dip down to about -12.0 with broken skies becoming clear overnight.Sunday: Clear and cold in the morning, temperatures down to -16.0 in the alpine. This should be short lived as the next Pacific warm front approaches from the coast.

Avalanche Summary

Professionals working in the Southern Selkirks experienced a very large settlement that propagated a fracture about one kilometre (1000 metres) on Wednesday. The fracture was reported to have traveled through some forest and across several pieces of avalanche terrain without releasing an avalanche. The loud whumph was the result of a failure on the deeply buried early November rain crust. After this experience, some explosive control from a helicopter resulted in a size 3.0 avalanche releasing on the buried crust that was approximately 200 cms below the surface. There was also natural and explosives controlled avalanches on the November 28th surface hoar layer that is buried down 40-70 cms.

Snowpack Summary

There are wind slabs of varying thickness and stiffness in the alpine and at treeline that were formed earlier in the week from very strong winds and warm temperatures during the storm. There is a surface hoar layer (November 28th) that is buried between 40-70 cms as the slab above has become more consolidated. This layer appears to be mostly between 1600-2000 metres in elevation, and may be very reactive in areas that experienced valley fog last week when it formed. The big unknown is the deep persistent weak layer (DPWL) of early November rain crust. This layer formed in early November when a warm rain event extended well up into the alpine and than froze into an ice layer. This layer is found where there was enough snow on the ground at the time of this event to become saturated and then freeze. Large scree slopes, areas where snow was still on the ground from last winter, and some smooth glaciers are the most suspect locations.

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.

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.

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.