Avalanche Forecast

Issued: Dec 28th, 2011 8:35AM

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

Avalanche Canada pmarshall, Avalanche Canada

Summary

Confidence

Poor - Timing of incoming weather is uncertain

Weather Forecast

Wednesday night and Thursday: Another system arrives bringing 15-20cm of snow, tapering off on Thursday afternoon. The freezing level drops from 1000m to valley bottom overnight. Winds are strong from the SW. Friday and Saturday: Mainly cloudy with light snowfall on Friday (around 5cm) and possible sunny breaks on Saturday. Freezing level at valley bottom. Winds are light to moderate from the SW.

Avalanche Summary

Observations from the Microwave area (S. of Dennis Lake) on Dec. 26: Several easily triggered soft slabs on steep features up to treeline. No alpine observations due to cranking winds and blowing snow. I would expect some natural avalanche activity at higher elevations, primarily in wind loaded terrain.

Snowpack Summary

Strong winds have been the biggest story recently. Exposed windward slopes have been scoured and wind slabs have formed in lee terrain and may be susceptible to human triggering. There is now around 55-70cms of snow sitting on a surface hoar/crust/facet combo which is the result of the early December dry spell. This crust is widespread and exists in most start zones up to 2000m. Facets can reportedly be found above or below the crust. Prior to being buried, the surface hoar was destroyed by high winds in the alpine, but still coexists with the crust at treeline and below. What is the tipping point of this weak interface? In areas where this interface has reached its threshold, natural and human triggered avalanches up to size 2.5 have occurred. If they have yet to happen, I suspect they are gaining some strength but would still react to a rider trigger, especially in steeper, unsupported terrain features. In short, the upper snowpack is variable and deserves caution. It will also see increased load with forecast wind and snow. Beneath this the midpack is well settled and strong.

Problems

Wind Slabs

An icon showing Wind Slabs
Wind slabs have developed on lee and cross-loaded terrain in the alpine and at treeline.

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

Elevations: Alpine, Treeline.

Likelihood

Very Likely

Expected Size

1 - 5

Storm Slabs

An icon showing Storm Slabs
Increasing in size and likelihood with each successive weather system.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Persistent Slabs

An icon showing Persistent Slabs
Persistent slabs have now formed at all elevations where the mid December buried surface hoar/crust/facet layers exist. Loading by new snow and wind could be enough to wake these layers up.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 5

Valid until: Dec 29th, 2011 8:00AM

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