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Avalanche Forecast

Archived

Dec 11th, 2020–Dec 12th, 2020

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

Regions

South Coast Inland.

Watch for small solar triggered loose dry avalanches in steep terrain where direct sun grazes dry snow for the first time.

Confidence

Moderate - Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack.

Weather Forecast

Friday night: Isolated flurries, moderate northerly ridgetop wind, freezing level valley bottom.

Saturday: Sunny, light variable ridgetop wind, alpine high -2C, freezing level 500 m.

Sunday: Sun and cloud, moderate southwest ridgetop wind, alpine high -6C, freezing level 500 m.

Monday: Sun and cloud, light to moderate southwest wind, alpine high -5C, freezing level 600 m.

Avalanche Summary

Since Wednesday, natural avalanche activity has been limited to loose dry sluffs of diminishing size.

A widespread natural cycle occurred during the storm Monday night through Tuesday, storm slabs size 2-3 and loose wet size 1-2.

On Tuesday December 1, a large size 3 natural avalanche was observed on a NE aspect at 2400 m in the Duffy area. It was reported as a cornice failure triggering the slope below and appeared to have slid on the early November crust near the base of the snowpack.

Have you been out and about in the mountains? If so please submit your observations to the Mountain Information Network (MIN). It doesn't have to be technical - photos are especially helpful! Thank you so much for all the great MINs submitted so far! 

Snowpack Summary

  • In the south, 10-20 cm of dry surface snow bonds to the underlying crust formed earlier this week when 25 mm of rain soaked the upper snowpack at all elevations. We've got our eye on a couple of crusts in the upper to mid snowpack that are producing moderate to hard snowpack test results. Treeline snowpack depth sits around 80-130 cm.

  • In the north, around 30 cm of recent snow sits over a thin zipper crust above 2300 m, snow tapering with elevation. Between 1600 and 2000 m, the crust is punchy and unsupportive. In the lower snowpack, a crust/facet layer sits near the ground. The most recent avalanche observed on this layer was on December 1st, while recent snowpack tests in the Cayoosh zone found this layer down 60 cm and unreactive. We have a lot of uncertainty on the reactivity of this layer so we're keeping it on our radar. 

Terrain and Travel

  • A crust on the surface will help bind the snow together, but may make for tough travel conditions.
  • Be alert to conditions that change with elevation.
  • Minimize exposure to sun-exposed slopes when the solar radiation is strong.
  • Small avalanches can have serious consequences in extreme terrain. Carefully evaluate your line for wind slab hazard before you commit to it.

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.