Avalanche Forecast

Issued: Dec 7th, 2012 9:13AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Storm Slabs, Persistent Slabs and Deep Persistent Slabs.

Avalanche Canada triley, Avalanche Canada

Summary

Confidence

Fair - Due to variable snowpack conditions on Saturday

Weather Forecast

Overnight and Saturday: A ridge of High pressure is forecast to move into the interior regions during the night or early morning. Expect moderate Northwesterly winds as the ridge moves over the region and the alpine temperatures drop into the minus teens. There may be a few more cms of snow before the moisture is pushed to the east by the ridge. Broken skies with sunny periods in the afternoon.Sunday: Generally clear and cold in the morning, becoming high overcast in the afternoon. Some snow should begin to fall in the evening in the North of the region, and all areas should see some snow by morning. Moderate Northwesterly winds are expected to continue.Monday: A warm front with moderate precipitation and strong Westerly winds is forecast to move into the region 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

Storm Slabs

An icon showing Storm Slabs
Expect heavy sluffing in steep terrain, and some soft storm slabs on unsupported terrain features. Some new wind slabs may have formed on North thru East aspects at higher elevations.
Be cautious as you transition into wind affected terrain.>Be cautious of sluffing in steep terrain.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 4

Persistent Slabs

An icon showing Persistent Slabs
A variable surface hoar layer may be found buried down 50-80 cms. This layer was buried in most parts of the region on Nov.28th. This may be found mostly at treeline between 1700-2100 metres.
Avoid convexities or areas with a thin or variable snowpack.>Avoid open slopes and convex rolls at treeline where buried surface hoar may be preserved.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 6

Deep Persistent Slabs

An icon showing Deep Persistent Slabs
An old facet/crust combination deep in the snowpack may wake up with heavy triggers, smaller avalanches stepping down, or triggering from thin-spots. Although unlikely, deep persistent slab avalanches can be massive.
Be aware of the potential for full depth avalanches due to deeply buried weak layers.>

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

4 - 7

Valid until: Dec 8th, 2012 2:00PM