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Avalanche Forecast

Archived

Jan 18th, 2020–Jan 19th, 2020

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

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

SATURDAY NIGHT: Cloudy with isolated flurries, trace accumulation, moderate west wind, alpine temperature -10 C, freezing level below valley bottom.

SUNDAY: Mostly cloudy with isolated flurries, accumulation 1 to 3 cm, moderate to strong southwest wind, alpine temperature -5 C, freezing level rising to 800 m.

MONDAY: Mix of sun and cloud, moderate southwest wind, alpine temperature -3 C, freezing level 1000 m.

TUESDAY: Cloudy with snowfall, accumulation 5 cm, light southwest wind, alpine temperature -6 C, freezing level 700 m.

Avalanche Summary

On Thursday and Friday, wind slab avalanches were triggered naturally, by skiers, and explosives. They occurred on north and easterly lee terrain features, generally at and above treeline. They generally averaged 30 to 50 cm deep.

The last deep persistent slabs were reported last Monday and Tuesday. They were large (size 2-3) and were triggered naturally and with explosives in steep alpine terrain. The likelihood of triggering this layer may increase in the coming days as the air temperature rapidly rises.

It has been over one week since any persistent slab avalanches on the December surface hoar layer have been reported, although there could still be lingering concerns about triggering that layer on isolated slopes in steep treeline clearings.

Snowpack Summary

Strong southwest wind may produce fresh wind slabs in lee terrain features, particularly near ridges. Wind slabs have been reported as having the potential to propagate far.

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. The likelihood of triggering buried weak layers may increase as the air temperature rises.

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.