Avalanche Forecast

Issued: Dec 3rd, 2020 4:00PM

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

Avalanche Canada astclair, Avalanche Canada

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New snow and wind are expected to build increasingly reactive wind slabs. Weak layers near the bottom of the snowpack remain possible to trigger.

Summary

Confidence

Low - Uncertainties in both the snowpack structure and the weather forecast limit our confidence.

Weather Forecast

Thursday night: Mostly cloudy, up to 5 cm of snow, moderate southwest wind, freezing level around 700 m.

Friday: Overcast, 5-15 cm of snow, moderate southwest wind with strong gusts, freezing level rising to 1200 m

Saturday: Overcast, 10-20 cm of snow above 1500 m, moderate southwest wind with strong gusts. 

Sunday: Mostly cloudy, 5-10 cm of snow above 1500 m, moderate southwest winds.

Avalanche Summary

A series of notable avalanches have been reported in the northern half of the region. These were both explosive-triggered and natural releases that resulted in very large deep persistent slab avalanches (size 3-3.5), failing on weak snow near the ground. The extent of this snowpack structure in the region is not well-known. 

If you're out in the field, please consider sharing a photo or description via the Mountain Information Network (MIN). 

Snowpack Summary

Up to 20 cm of new snow is forecast to accumulate by late Friday in favored areas. Moderate to strong southwest winds may redistribute the new snow into reactive slabs on lee features. The new snow is accumulating above a melt freeze crust and hard, wind-affected snow and may not bond well. 

Reports suggest the bottom half of the snowpack consists of weak snow interrupted by a crust from early November. Further north in the region, we're learning that weak snow developing near the ground has produced very large deep persistent slab avalanches. Although we lack data on the distribution of this problem, the bottom line is that easy-to-trigger wind slabs have the potential to step-down to deeper layers and trigger destructive avalanches. 

Snowpack depths are variable across the region, ranging from 60 cm in eastern parts of the region up to over 100 cm in western parts of the region. Lower elevation terrain may still be below the threshold for avalanches. 

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

An icon showing Wind Slabs

Up to 20 cm of new snow is forecast to accumulate by late Friday in favored areas. Moderate to strong southwest winds may redistribute the new snow into reactive slabs on lee features. 

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

Very large avalanches over the past few days suggest that a buried crust formed in early November and weak snow at the ground are reactive persistent and deep persistent weak layers. We have limited information about the distribution of this snowpack structure in the region. Keep in mind that easy-to-trigger avalanches in the new storm snow have the potential to step-down to these deeper layers, producing large, destructive avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 3

Valid until: Dec 4th, 2020 4:00PM

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