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Avalanche Forecast

Archived

Nov 26th, 2020–Nov 27th, 2020

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.
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 fresh, reactive 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

Thursday night: Up to 5 cm new snow, strong southwest wind, freezing level 1200-1500 m.

Friday: 5-10 cm new snow, strong southwest wind, freezing level 1600 m.

Saturday: 5-15 cm new snow overnight then clearing, moderate west wind, freezing level 500 m.

Sunday: Sun and cloud, light southerly wind, freezing level 900 m.

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.

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

New snow is falling amid strong to extreme southwesterly wind, likely producing wind slabs in lee terrain features at higher elevations.

Expect to find a treeline snowpack depth of around 100 to 150 cm in the south of the region and 80 to 120 cm in the north of the region.

A melt-freeze crusts from mid-November is generally found 50 cm below the snow surface. Many reports suggest the upper snowpack is well-bonded to this crust. Spotty layers of weak surface hoar crystals may be found in the middle of the snowpack (50 to 100 cm deep), particularly in sheltered terrain features. No avalanches have been observed on these layers to date.

In the north of the region, another melt-freeze crust may be found near the base of the snowpack, which could have a weak layer of sugary faceted grains around it. Snowpack tests and a recent avalanche activity suggest that this layer could be triggered by humans and produce large avalanches.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs 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.