Bulletin d’avalanche

North Rockies

Feb 8 – 9, 2022Avalanche Canada
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Archivé · expiré 5 years ago

Danger d’avalanche

Thu
Fri
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

It is uncertain how reactive the recently buried weak layer is to human triggers. Once triggered, it will likely result in a large avalanche that can have serious consequences. This uncertainty is best managed with conservative terrain choices.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2.5

    The recent snow loaded a weak layer of surface hoar crystals, which is at a prime depth for human triggering. The layer is most prominent in areas sheltered from the wind at and below treeline. Example terrain features to particularly treat as suspect include the lee side of ridges, openings in trees, cut blocks, and burns. Large, wide-propagating avalanches could result where this layer is present. 

    The buried weak layer was reported to be present up to treeline in the Torpy Sande and Pine Pass areas and to be present in isolated areas in the Renshaw. 

  • Storm Slabs

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

    Expect to find wind slabs in lee terrain features at higher elevations from strong south and west wind. Storm slabs will be found in sheltered terrain. 

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to the limited number of field observations.

Weather Forecast

MONDAY NIGHT: Mostly cloudy with some clear periods, up to 5 cm new snow, moderate to strong southwest wind, alpine temperature -5 C, freezing level 1000 m. 

TUESDAY: Mostly cloudy, up to 5 cm new snow, moderate southwest wind, alpine temperature -2 C, freezing level 1500 m.

WEDNESDAY: Cloudy with snowfall, 10 to 20 cm new snow with the higher amounts in the south of the region, moderate to strong southwest wind, alpine temperature 0 C, freezing level rising to 2200 m.

THURSDAY: Mostly cloudy with snowfall, 10 to 20 cm new snow with the higher amounts in the west of the region, moderate to strong southwest wind, alpine temperature 0 C, freezing level slowly dropping to 1500 m.  

Avalanche Summary

On Sunday, a deep persistent slab avalanche was likely triggered naturally by intense warming and sun and released on depth hoar or the November crust. It occurred just south of the region (see this MIN report). Explosives triggered several large wind slab avalanches up to size 3.

On Saturday, several storm and wind slab avalanches up to size 2 were observed at treeline and in the alpine. Most of these avalanches released naturally, and two were likely triggered by riders.

On Friday a widespread natural avalanche cycle was reported with wind slab avalanches up to size 3 as well as storm slab avalanches up to size 1.5. Many avalanches were observed at and below treeline. A couple large (size 2) slab avalanches were reported at treeline on a southwest and an east aspect. 

Snowpack Summary

10 to 20 cm new snow fell, with higher amounts in the centre of the region. The new snow formed storm slabs in sheltered areas and wind slabs in lee terrain features from strong south and west wind. At lower elevations, the precipitation fell as rain and formed a rain crust. This new snow added to the previous storm which brought 40 to 60 cm for much of the region, with the most in the southwest of the region and the least to the east of the divide and in the north. 

The snow will be particularly touchy where it sits on a weak layer of surface hoar crystals. Reports suggest that the surface hoar is likely most prominent in sheltered openings at and below treeline but could extend into wind-sheltered terrain in the alpine. Example terrain features to treat as suspect include the lee side of protected ridges, openings in the trees, cut blocks, and burns.

The lower snowpack is generally strong and well-bonded. The base of the snowpack is expected to be weak and faceted in shallow, rocky slopes east of the divide.

Terrain and Travel

  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

Régions couvertes