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Avalanche Forecast

Archived

Dec 13th, 2020–Dec 14th, 2020

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

Watch for wind loaded pockets of snow especially around ridge crests and roll-overs 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

Sunday night: Trace of new snow, light northwest ridgetop wind, freezing level valley bottom.

Monday: Localized flurries, around 8cm near Hope, light southwest ridgetop wind, alpine high -6 C, freezing level 500 m.

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

Wednesday: Up to 5 cm new snow, strong southwest ridgetop wind, alpine high -4 C, freezing level 800 m.

Avalanche Summary

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

On Saturday, small loose wet avalanches were observed on solar aspects around treeline.

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 north, moderate southwest wind is redistributing up to 35 cm of dry alpine snow into wind slabs sitting over a thin zipper crust above 2300 m. As you go down in elevation, the surface snow gets thinner and the crust gets thicker. 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. 

  • In the south, a surface melt-freeze crust exists on solar aspects. In the alpine, moderate southwest wind is redistributing around 20 cm of dry snow into wind slabs over the rain crust. This crust is 10-20 cm thick, reaches all the way to mountaintop and seems to be bonding well to overlying snow. Below, the snowpack is moist and dense.

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.