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Avalanche Forecast

Archived

Nov 29th, 2020–Nov 30th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

South Coast Inland.

Fresh storm slabs are expected to be most reactive in wind loaded lees at upper elevations, and where new snow falls on a melt freeze crust on south aspects or surface hoar in very wind-sheltered areas around treeline.

Confidence

Low - Uncertainty is due to the limited number of field observations.

Weather Forecast

Sunday night: 5-10 cm new snow. Moderate to strong southwest wind, freezing level 1000 to 1300 m.

Monday: 5-10 cm new snow. Moderate southwest wind, freezing level 1100 m.

Tuesday: Clear, light northwest wind, freezing level 700 m.

Wednesday: Clear, light southerly wind, freezing level 1000 m with an above freezing layer 1500 to 3500 m.

Avalanche Summary

Monday we're expecting storm snow avalanches size 1-1.5 especially in lee features where wind is depositing fatter pockets of snow, or on south aspects where the new snow may be falling on a thin melt-freeze crust.

The most recent report of an avalanche on the early-November layer was from November 21 near Duffey Lake. Check out this MIN for a photo.

Our eyes and ears in the mountains are limited at this time of year and may continue this winter due to fewer professional observations. If you see anything (or don't) while out in the field, please consider sharing via the Mountain Information Network (MIN). Photos are especially helpful! Thank you so much for all the great MINs submitted so far!

Snowpack Summary

10-20 cm of new snow falls ontop of wind affected surfaces in the alpine, a thin melt-freeze crust on south aspects, and possibly surface hoar in wind sheltered areas around treeline (observed in the Coquihalla area). Moderate to strong southwest winds will likely load the falling snow into pockets of potentially reactive storm slab in lee features at upper elevations.

The lower snowpack is characterized by a few crusts formed during warm periods over the last month. 

  • In the north, treeline snowpack depth sits around 80 to 120 cm. The crust of note here is sitting near the base of the snowpack, likely surrounded by a weak layer of sugary facets. Snowpack tests and recent avalanche activity suggest that this layer could be triggered by humans and produce large avalanches.

  • In the south of the region, the snow line has been reported around 700 m near Hope and treeline snowpack depth sits around 100-150 cm. The deeper crusts in this area are generally well bonded to surrounding snow.

Terrain and Travel

  • Watch for fresh storm slabs building throughout the day.
  • Avoid freshly wind loaded terrain features.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.