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Avalanche Forecast

Archived

Jan 19th, 2020–Jan 20th, 2020

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

Regions

Purcells.

Wind slabs may be touchy and a warming trend has the potential of waking up buried weak layers. Travel conservatively during this period of uncertainty.

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

SUNDAY NIGHT: Clearing through the night, moderate southwest wind, alpine temperature -5 C, freezing level rising to 700 m.

MONDAY: Mix of sun and cloud, moderate to strong southwest wind, alpine temperature -3 C, freezing level 1200 m.

TUESDAY: Cloudy with snowfall, accumulation 5 cm, light to moderate southwest wind, alpine temperature -6 C, freezing level 800 m.

WEDNESDAY: Morning snowfall then a mix of sun and cloud, accumulation 2 to 5 cm, light west wind, alpine temperature -8 C, freezing level 800 m.

Avalanche Summary

Numerous large (size 2) wind slabs were triggered by humans and explosives on Saturday. They were generally 10 to 40 cm deep and occurred at treeline and alpine elevations.

A couple deep persistent slabs were triggered by explosives. They were large (size 2) and occurred in the alpine, scrubbing to the ground.

Snowpack Summary

Around 15 cm of recent snow and strong southwest wind has produced wind slabs in lee terrain features, particularly near ridges.

A layer of surface hoar that formed in late December appears to be less reactive than it was a week ago and can be found 70 cm deep around Golden, 30 cm deep around Invermere, 70 cm deep around Kimberley, and 100 cm deep along Kootenay Lake. As usual for the Purcells, the base of the snowpack contains basal facets and it remains possible to trigger these deep weak layers in shallow rocky start zones or from a heavy trigger such as a cornice fall or explosives. The likelihood of triggering buried weak layers may increase as the air temperature rises on Monday.

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • Extra caution for areas experiencing rapidly warming temperatures for the first time.
  • Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.

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.

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.