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Avalanche Forecast

Archived

Feb 5th, 2021–Feb 6th, 2021

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

Sea To Sky.

Watch for reactive wind slabs in lee terrain features at upper elevations. Uncertainty around buried weak layers is best managed through conservative terrain selection.

Confidence

High -

Weather Forecast

Friday night: Mainly cloudy, moderate to strong northwest ridgetop wind, alpine temperature -7, freezing level 500 m.

Saturday: Mainly cloudy, moderate to strong westerly wind, alpine high -7, freezing level 900 m.

Sunday: Mix of sun and cloud, light westerly wind, alpine high temperature -10, freezing level valley bottom.

Monday: Mix of sun and cloud, light northwest wind, alpine high temperature -12, freezing level valley bottom.

Avalanche Summary

Skier triggered wind slabs up to size 1.5 were reported on Thursday. Explosive control work conducted over the last few days have produced size 2 cornice and storm slab avalanches.

Earlier in the week, many of the numerous size 1-2 natural, human and explosives triggered avalanches were reported to have failed on the recently buried persistent weak layer.

Snowpack Summary

Westerly winds are likely redistributing surface snow into wind slabs in leeward terrain features at upper elevations. 50-100 cm of recent snow sits on a persistent weak layer that consists of facets at upper elevations, surface hoar in sheltered areas, a melt-freeze crust below 1900 m, and a sun crust on south-facing slopes. There could be more than 100 cm on this layer in wind loaded areas.

A crust from early December may be found around 200+ cm deep in the snowpack. In shallow rocky areas, it may still be possible to trigger this layer. In deeper snowpack areas, it appears to be unreactive.

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • Be aware of highly variable recent wind loading patterns.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.

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.