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Avalanche Forecast

Archived

Dec 20th, 2019–Dec 21st, 2019

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

Regions

Sea To Sky.

A substantial amount of recent snow has accumulated, which is loading a buried weak layer. Conservative decision-making is essential to safely manage the avalanche problems.

Confidence

High -

Weather Forecast

FRIDAY NIGHT: Cloudy with light snowfall, accumulation 2 to 5 cm, moderate southwest wind, alpine temperature -6 C, freezing level 1000 m.

SATURDAY: Mostly cloudy with isolated flurries, accumulation 2 to 5 cm, light to moderate southwest wind, alpine temperature -7 C, freezing level 1000 m.

SUNDAY: Cloudy with snowfall and afternoon clearing, accumulation 10 to 20 cm, moderate to strong southwest wind, alpine temperature -7 C, freezing level 1000 m.

MONDAY: Mix of sun and cloud, light west wind, alpine temperature -8 C, freezing level below valley bottom.

Avalanche Summary

Many large slab avalanches were triggered naturally and by explosives on Friday. The avalanches failed both in the recent storm snow as well as the mid-November weak layer described in the Snowpack Summary. Some of the avalanches scrubbed to the ground.

Human-triggered avalanches are expected to be likely to occur on Saturday, with the potential to fail in both the recent storm snow as well as the mid-November weak layer. Conservative decision-making is essential given this weak snowpack.

Snowpack Summary

Around 50 to 80 cm of recent snow fell with very strong southwest wind. All of this snow overlies a weak layer of surface hoar as well as sugary faceted grains and hard melt-freeze crust buried mid-November, which is a recipe for large and destructive avalanches. These weak layers may be found around 50 to 100 cm deep.

This weak and touchy snowpack is highly atypical for the region and may persist for some time. Conservative terrain travel along with periods of avoidance of avalanche terrain will be imperative to manage your risk until the weak snow gains strength. Read this informative Mountain Conditions Report for more information on this persistent avalanche problem.

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Make conservative terrain choices and avoid overhead hazard.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.