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Avalanche Forecast

Archived

Nov 27th, 2020–Nov 28th, 2020

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

Regions

South Coast Inland.

Watch for recently formed wind slabs at upper elevations and be aware of the potential for larger avalanches due to a crust that was buried in early November.  

Confidence

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

Weather Forecast

Friday night: 5-10 cm new snow, strong southwest wind, freezing level 1200 m.

Saturday: Clearing, wind easing to light west, freezing level 1000-1200 m.

Sunday: Clear, light southerly wind, freezing level 1000 m with an above freezing layer 1800-2400 m in the south of the region.

Monday: Sun and cloud, moderate southwest wind, freezing level 1800 m, 2000 m in the south.

Avalanche Summary

In the south, size 1-1.5 loose dry avalanches were observed in recent storm snow running on a melt-freeze crust in steep south-facing terrain on Wednesday.

In the north, large (size 2) storm slab avalanches were triggered naturally and by explosives on Wednesday near Duffey Lake. They occurred on northerly aspects in alpine terrain. 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 while out in the field, please consider sharing it with us and fellow recreationists via the Mountain Information Network (MIN). Even just a photo of where you went is helpful. A big thank you to those that have already shared their observations thus far.

Snowpack Summary

Recent new snow and strong southwest wind have produced wind slabs in lee terrain features at higher elevations.

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

  • In the south of the region, where treeline snowpack depth sits around 100-150 cm, the deeper crusts are generally well bonded to surrounding snow. The topmost crust, which was acting as a slick bed surface for storm snow avalanches a few days ago, may be improving with the mild temperatures. 

  • 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.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Be cautious of buried obstacles especially below treeline.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.

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.