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Avalanche Forecast

Archived

Dec 12th, 2020–Dec 13th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

South Coast Inland.

The wind is forecast to pick up Sunday and at upper elevations there is plenty of soft snow to blow around. Watch for fresh wind slabs forming in lee terrain features.

In areas where wind remains light and snow transport is not observed, danger may be a step lower in the alpine.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Saturday night: Increasing cloud, moderate southwest ridgetop wind, freezing level valley bottom.

Sunday: Sun and cloud, isolated flurries, moderate to strong south ridgetop wind, alpine high -5C, freezing level valley bottom.

Monday: Scattered flurries, moderate southwest ridgetop wind, alpine high -8 C, freezing level 500 m.

Tuesday: 5-10 cm new snow, strong southwest ridgetop wind, alpine high -5 C, freezing level 800 m.

Avalanche Summary

On Sunday, as the wind picks up, we can expect to see natural and human triggered wind slabs in lee features at upper elevations.

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

At upper elevations, winds are forecast to pick up during the day Sunday. The will likely redistribute into wind slabs the 10-30 cm of dry snow that sits over a crust.

  • In the south, 20-30 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

  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Wind slabs are most reactive during their formation.
  • A crust on the surface will help bind the snow together, but may make for tough travel conditions.

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.