Bulletin d’avalanche

Purcells

Jan 14 – 15, 2021Avalanche Canada
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Archivé · expiré 6 years ago

Danger d’avalanche

Sat
Sun
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

Ease into terrain cautiously on Thursday. Start small and tune in to signs of instability like shooting cracks and recent avalanches. Bear in mind that sunshine can have a powerful effect on stability - and on your desire to push into aggressive terrain. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    Weak layers of surface hoar exist 65-140 cm deep in sheltered treeline terrain (and below). These layers are highly variable in form and distribution. A widespread buried crust also exists near the base of the snowpack. Although these layers have not been associated with recent avalanche activity, they are being tested by a new load of new snow and potential currently exists for avalanches in surface layers to step down to a deeper layer to create larger more destructive avalanches.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely - Very Likely
    Taille
    1–2

    Up to 40 cm of new snow, coupled with strong winds have loaded the avalanche terrain in the region with new slab problems. In exposed areas, thick and reactive wind slabs should be expected in leeward features. In sheltered areas, storm slabs are more likely to exist uniformly across all aspects and be particularly touchy where they overlie surface hoar or crust.

    Cornices have also been growing. A cornice fall is a large load that can trigger avalanches on the slope below.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

Wednesday night: Diminishing cloud with isolated flurries. Moderate to strong northwest winds.

Thursday: Mainly sunny. Light to moderate northwest winds. Alpine high temperatures around -8.

Friday: A mix of sun and cloud. Light to moderate southwest wind shifting northwest. Alpine high temperatures around -7.

Saturday: A mix of sun and cloud. Light west winds. Alpine high temperatures around -7.

Avalanche Summary

Preliminary observations from the Golden area on Wednesday showed explosives control yielding size 2 (large) storm slabs about 40 cm deep, but with a variability in depth that shows good evidence of wind redistribution. Further observations are expected to validate expectations of a busy day for avalanches in the region.

Looking forward, expect Thursday to be another tricky day to be in the mountains, with new and reactive wind slabs formed in wind-exposed areas and even more widespread storm slabs primed for human triggering in more sheltered areas.

Snowpack Summary

15-40 cm of new snow fell in the region during Tuesday's storm. It buried wind affected surfaces at upper elevations, as well as surface hoar in wind sheltered areas and a thin crust on solar aspects. 

The new snow will take time to form a bond with these previous surfaces, particularly in spots harbouring surface hoar or crust. In exposed areas, elevated winds are almost certainly redistributing new snow into reactive wind slabs and adding to cornices. 

A couple of persistent weak layers exist in the upper to mid snowpack:

  • The upper layer, found 40-60 cm down, is a spotty layer of surface hoar at treeline and below. 
  • The lower layer, found 90-140 cm down, is widespread and can present as surface hoar and/or a crust.

Avalanche activity on these layers has dwindled since last week and snowpack tests results have generally indicated the weak layers are slowly gaining strength. Avalanche activity resulting from the storm may shed some light on whether or not they remain a concern.

The lower snowpack is characterized by more crusts, the most notable is a rain crust from early November that is near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers in steep, shallow, rocky areas with a thin to thick snowpack.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid exposure to steep sun exposed slopes.

Régions couvertes