Bulletin d’avalanche

Northwest Coastal

Dec 27 – 28, 2019Avalanche Canada
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Archivé · expiré 7 years ago

Danger d’avalanche

Sun
Mon
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Points saillants

Make cautious terrain choices, buried weak layers are getting stressed by new snow and could produce large avalanches.

Problèmes avalancheux

  • Wind Slabs

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

    Moderate to strong wind has formed unstable slabs around ridges and in lee terrain features.

  • Persistent Slabs

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

    Additional loading from storm snow and wind will stress a weak layer of surface hoar, now anywhere from 40-80 cm deep.

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

THURSDAY NIGHT: Snow flurries with 5-10 cm of snow, 30-50 km/h wind from the west, alpine temperatures drop to -5 C.

FRIDAY: Scattered flurries with 5-15 cm of snow, 30-50 km/h wind from the southwest, alpine high temperatures around -2 C.

SATURDAY: Snow, heavy at times, 15-30 cm, 40-70 km/h wind from the south, alpine high temperatures around -1 C with freezing level up to 800 m.

SUNDAY: Scattered flurries with 5-10 cm of snow, 30-40 km/h wind from the south, alpine high temperatures around -3 C.

Avalanche Summary

Recent avalanche activity has been limited in size 1-2 avalanches in the uppermost layers of snow (storm slabs and wind slabs), however ongoing snowfall is adding stress to buried surface hoar layers. Even though no recent avalanches have been reported on these persistent weak layers, they could be reaching the tipping point for producing large avalanches.

On Thursday a few size 1-2 wind slabs were triggered naturally and with explosives. Similar activity has been reported during each snowfall event over the past week.

Snowpack Summary

10-30 cm of recent snow has been blown around by moderate to strong southwest wind. 20-40 cm of recent snow overlies a thin layer of surface hoar and a total of 40-80 cm of snow now overlies a more prominent weak layer of surface hoar, old faceted surfaces, and/or a crust on south/southwesterly alpine slopes. This more prominent surface hoar layer has been reactive in snowpack tests, providing evidence it is at a tipping point where any additional load (such as a person) could trigger an avalanche on this layer. Heavy snowfall could easily overload this layer. Reports from the Shames area suggest the surface hoar layer is on all aspects, but more prevalent on southeast to southwest aspects around 800-1400 m, and more likely in leeward and sheltered alpine areas. The lower snowpack is generally considered strong.

Terrain and Travel

  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
  • 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.

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