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Avalanche Forecast

Archived

Feb 6th, 2020–Feb 7th, 2020

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

Purcells.

Be on the lookout for areas where recent snow has formed a slab from either wind or settlement. A wind slab release or cornice fall in a shallow rocky start zone could be the trigger for a large deep persistent slab avalanche.

Confidence

High -

Weather Forecast

Thursday night: Becoming cloudy. Light northwest winds.

Friday: Mainly cloudy with light flurries bringing a trace of new snow before increasing overnight. Light northwest winds shifting southwest. Alpine high temperatures around -10.

Saturday: Cloudy with continuing light flurries bringing up to 5 cm of new snow, up to 15 cm with the previous night's accumulations. Light northwest winds shifting northeast. Alpine high temperatures around -9.

Sunday: A mix of sun and cloud. Light west winds, becoming moderate or strong at ridgetop. Alpine high temperatures around -9.

Avalanche Summary

Since the weekend storm, there have been several reports of failing cornices and small wind slabs. See this MIN report for a helpful illustration. Explosive triggers have also released several large avalanches breaking on deeply buried weak layers on a variety of aspects above 2200 m. This activity is a good reminder that this layer remains a problem in the region. Easier-to-trigger wind slab avalanches or cornice fall may have the potential to step-down to this layer.

During the weekend storm, numerous size 2-3.5 slab avalanches released naturally. A few were thought to have stepped down to deeper layers. These avalanches primarily occurred on leeward aspects at treeline and alpine elevations. Below tree line, wet loose avalanches ran naturally during the warm temperatures on Saturday. 

Snowpack Summary

20-40 cm of snow from the early part of the week now overlies older wind-affected snow at high elevations, or over a widespread melt-freeze or rain crust with a variable upper extent of 1800-2000 metres in elevation. The recent snow has mainly shown reactivity where winds have had a chance to redistribute it into new slabs. A few reports from adjacent regions have shown poor bonding with the buried crust where it exists.

A weak layer of surface hoar may still be found about 90 cm deep around Golden and Kimberley, 50 cm deep around Invermere, and 130 cm deep along Kootenay Lake. Recent reports from the Golden area suggest a positive stabilizing trend. At lower elevations this layer has likely been bridged over by the above-mentioned crust.

As usual for the Purcells, the base of the snowpack contains basal facets. It remains possible to trigger these deep weak layers in shallow rocky start zones or from a heavy trigger such as a cornice fall. Given this layer is most prominent in the alpine, it is unlikely to be effectively bridged over by our most recent crust.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Seek out wind sheltered terrain below treeline where you can avoid wind slabs and find great riding.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

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.