Boletín de aludes

South Rockies

Feb 21 – 22, 2021Avalanche Canada
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Archivado · caducó 6 years ago

Peligro de aludes

Tue
Wed
Alta montaña
3 · Notable
Límite del bosque
3 · Notable
Bajo el límite
2 · Moderado
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Lo esencial

Reactive wind slabs have formed in lee terrain features with more wind on the away. As winds increase, slabs may form lower on slopes, into open tree line areas, or in other unusual places. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–2.5

    Strong west-southwest wind will impact loose snow and build reactive slabs in lee features. As winds increase, slabs may form lower on slopes, into tree line areas, or in other unusual places. Older wind slabs formed under recent variable winds, be mindful that the upper snowpack could have a few layers of wind slab.

    Wind slabs are slow to bond where they sit over cold sugary facets. There is also potential for wind slabs to step down to the late January persistent weak layer. 

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2.5

    This layer continues to be easily triggered by skiers and riders. 40-65 cm of snow sits above a buried weak layer of surface hoar, facets, and a crust (depending on elevation and aspect). Reports suggest the persistent weak layer at treeline in the Elk Valley is the biggest repeat offender, and things have been most reactive on northerly and easterly aspects, but don't let your guard down elsewhere.

    Small terrain features are producing small avalanche results, bigger features can produce larger more dangerous avalanches. These kinds of avalanches can happen in surprisingly mellow terrain and be triggered from a distance away.

Terrain and Travel

  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks or recent avalanches.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

SATURDAY NIGHT: Flurries, trace amounts, moderate southwest wind, temperature low -13 C.

SUNDAY: Cloudy with scattered flurries, 5 cm, strong to extreme southwest wind, temperature high -2 C, freezing level rising to 1700 m. 

MONDAY: Snow, 10-15 cm snow with rain at lower elevations, strong to extreme southwest wind, temperature high +1 C, freezing level at 1800 m.

TUESDAY: Pockets of sunshine, light northwest wind, temperature high -8 C, freezing level at 1000 m.

Avalanche Summary

On Friday, skiers easily ski cut fresh size 1 wind slabs. Whumpfing and cracking were reported in wind affected terrain.

Several explosives triggered slab avalanches up to size 2 and several natural and skier triggered dry loose and wind slab avalanches up to size 1.5 were reported on Thursday. 

On Tuesday and Wednesday a few wind slab avalanches up to size 2 were triggered by explosives and released naturally. 

Several wind slab avalanches up to size 2 were triggered by skiers and released naturally at treeline and in the alpine on Monday. Human triggered avalanches were reported north of Crowsnest Pass. The avalanches were triggered on east aspects on mellow slopes in the alpine and at the treeline/alpine interface (see this MIN report). The failure plane of these avalanches is likely the late January persistent weak layer consisting of surface hoar, sugary facets or a crust/facet combination. 

Snowpack Summary

Wind slabs are found in open areas, moderate to strong winds are redistributing 5-15 cm recent snow. This overlies wind affected snow surfaces in the alpine and open areas at treeline and a facet layer which is up to 20-30 cm thick in sheltered areas. A persistent weak layer lurks 40-65 cm below the surface. In some places it consists of surface hoar, in other places just facets, or crust/facet combinations. 

A solid mid-pack sits above deeply buried decomposing crust and facet layers near the bottom of the snowpack. Though unreactive under the current conditions, steep rocky slopes and shallow snowpacks should still be approached with caution.

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