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Avalanche Forecast

Archived

Nov 30th, 2020–Dec 1st, 2020

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

Avalanche danger will rise to CONSIDERABLE in the afternoon as a pocket of warm air develops at upper elevations. Use extra caution where recent snow is experiencing sun and rapidly warming temperatures for the first time.

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

Monday night: Clearing, northwest wind easing to light, freezing level valley bottom.

Tuesday: Sunny, light northwest wind, freezing level 700-1000 m with an above freezing layer developing 1800 - 3000 m late in the day.

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

Thursday: Sunny, moderate southerly wind, freezing level 1000 m.

Avalanche Summary

Tuesday we may see some size 1-2 storm slab avalanches due to rapidly warming temperatures and sun in the afternoon. They may be especially reactive where recent snow sits over a crust or surface hoar. Loose wet avalanches are also be possible. We think it will take a more prolonged period of warming to tickle the persistent slab, but don't forget that storm snow avalanches are large loads that can act as triggers for deeper layers.

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 recent snow has been wind loaded into lee features in exposed areas at upper elevations, and may sit over a thin melt-freeze crust on south aspects and possibly surface hoar in wind sheltered areas around treeline.

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

  • Extra caution for areas experiencing rapidly warming temperatures for the first time.
  • Avoid steep slopes when air temperatures are warm, or solar radiation is strong.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.
  • The more the snowpack warms-up and weakens, the more conservative you`ll want to be with your terrain selection.

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.