Bulletin d’avalanche

South Coast Inland

Mar 8 – 9, 2020Avalanche Canada
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Archivé · expiré 7 years ago

Danger d’avalanche

Tue
Wed
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Points saillants

Slabs formed from Thursday's snow may take a bit more time to bond, particularly in wind-loaded terrain features. Should the sun pop out, it could destabilize sun-exposed slopes and large cornices will weaken.

Problèmes avalancheux

  • Wind Slabs

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

    Wind slabs formed from Thursday's snow and strong south to southwest wind could still be triggered by riders. The most suspect terrain is immediately adjacent to ridges at treeline and alpine elevations. In sheltered terrain, the snow has likely consolidated into small storm slabs. During periods of sunny skies, the storm snow on sun-exposed slopes could rapidly deteriorate. Note that cornices are large, looming, and could fail during periods of intense sun or from the weight of a human.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely - Possible
    Taille
    1.5–2.5

    A weak layer of feathery surface hoar may be found around 40 to 80 cm deep on northerly aspects around treeline and lower alpine elevations. This layer may be spotty and only found in sheltered terrain features. The latest avalanche activity was in the north of the region on March 6. The layer has been dormant in the south (e.g. around Coquihalla), where snowpack tests are suggesting that the layer may be bonding. Assess the layer in the snowpack prior to committing to avalanche terrain or adopt a conservative mindset to avoid the problem.

    In the north of the region (e.g. Duffey, Hurley, Goldbridge), a persistent weak layer of faceted grains near the ground presents a lingering concern on steep, shallow slopes in the alpine.

Weather Forecast

SATURDAY NIGHT: Mostly cloudy with isolated flurries, accumulation 2 to 5 cm, light north wind, alpine temperature -10 C.

SUNDAY: Mix of sun and cloud, light northwest wind, alpine temperature -10 C, freezing level 900 m.

MONDAY: Clear skies, light west wind, alpine temperature -10 C, freezing level 1000 m.

TUESDAY: Cloudy with snowfall, accumulation 2 to 5 cm, moderate southwest wind, alpine temperature -6 C, freezing level 1200 m.

Confidence

High - Confidence is due to a stable weather pattern with little change expected.

Avalanche Summary

On Friday, explosives triggered cornices and 20 cm slabs within the recent storm snow, producing small (size 1 to 1.5) up to large (size 2) avalanches. The avalanches were on west to north aspects and at alpine elevations. A persistent slab avalanche was triggered by a rider in the northern part of the region, likely releasing on the surface hoar layer described in the Snowpack Summary. The avalanche was large (size 2), in alpine terrain, and on a northerly aspect.

Natural avalanche activity is expected to taper with a calmer weather pattern. The exception is during periods of strong sun, where the storm snow on sun-exposed slopes could moisten and form loose avalanches. Human triggering of the storm and wind slabs may remain possible this weekend until the snow bonds to previous surfaces.

Snowpack Summary

Thursday's storm dropped around 15 to 25 cm of snow across the region. The snow fell with strong south to southwest wind, forming wind slabs in lee terrain features at treeline and alpine elevations. In sheltered terrain, the snow is likely consolidating into a storm slab and starting to bond to underlying surfaces.

A weak layer of surface hoar crystals and/or faceted grains may be found around 40 to 80 cm deep on northerly aspects at treeline and lower alpine elevations. The layer may have been destroyed by strong wind immediately before burial on February 22nd, but it may still exist in sheltered terrain features. Where it exists, it has been reactive in some snowpack tests but not others, suggesting spatial variation. This persistent weak layer warrants investigation and a conservative terrain use strategy. Check out the latest forecaster blog that offers a deeper dive into these conditions.

In the northern part of the region (i.e., Duffey/Hurley/Goldbridge), a deep instability may linger at the base of the snowpack. It has not produced an avalanche since February 17th. Human-triggering is unlikely, though if one were to trigger the layer, it would likely be in a thin, shallow, rocky spot. A large load, such as from a cornice fall, or rapidly changing weather have the potential to trigger this layer.

Terrain and Travel

  • Carefully evaluate steep lines for wind slabs.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.
  • Minimize exposure to sun-exposed slopes when the solar radiation is strong.

Régions couvertes