Avalanche forecast

South Coast

Mar 10, 2017Avalanche Canada
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Avalanche danger

Fri
Sat
Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

Persistent weak layers are not typical on the South Coast. Backcountry users in the region need to keep in mind the potential for storm slab avalanches to 'step down' well beyond the depth of new snow.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–4
    Storm snow from the past week sits over a buried persistent weak layer. While this layer has gained a lot of strength, very large avalanches may still be possible in isolated terrain. This likelihood increases while new snow adds load to the snowpack

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1–2
    Another 30-60 cm of new snow is likely to accumulate in the South Coast mountains by Friday afternoon. Warm temperatures and strong to extreme winds will promote the formation of touchy storm slabs that will be primed for human triggering on Friday.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2
    Wet snow and rain at lower elevations will allow surface snow to sluff easily from steep terrain. These sluffs may entrain significant mass and become a serious hazard around terrain traps like cliffs or above terrain depressions.

Confidence

Moderate - Forecast snowfall amounts are uncertain on Friday

Weather Forecast

Thursday night: Periods of snow and wet snow bringing 20-40 cm of new snow to higher elevations. Strong to extreme southwest winds. Freezing level around 1500 metres with alpine temperatures around 0. Friday: Continuing snowfall and wet snow bringing 10-20 cm of new snow to higher elevations. Moderate to strong southwest winds. Freezing level to 1400 metres with alpine temperatures around 0. Saturday: Another pulse of wet snow and rain bringing up to 20 cm of new snow to higher elevations. Strong to extreme south winds. Freezing level to 1400 metres with alpine temperatures around 0. Sunday: Continuing wet flurries and rain bringing up to 20 cm of new snow to higher elevations. Moderate to strong southwest winds. Freezing level to 1500 metres and alpine temperatures around +1. Forecast snowfall amounts are subject to freezing levels. Wet snow can generally be expected to accumulate 300-400 metres below the freezing level.

Avalanche Summary

On Wednesday, explosives triggered two size 1 storm slabs which released down 10-25 cm. These were described as having minimal propagation and minimal entrainment. On Monday, a report includes whumphing and a ski cut triggering a size 2 slab which released on the crust layer. Explosives also triggered numerous storm slabs 20-30 cm thick and one stepped down to the crust layer down 40-60 cm. On Thursday, recently formed storm slabs may still be reactive to human triggering, especially in wind loaded terrain. Large persistent slab avalanches have generally become unlikely but still may be possible in isolated areas. Natural sluffing is possible from steep sun exposed slopes.

Snowpack Summary

Another 15-30 cm of snow fell in the North Shore mountains on Wednesday, bringing the typical storm accumulation in the past week to 115-230 cm. Recent winds have been from a variety of directions and wind slabs should be expected on all aspects; some may exist below less wind affected new snow accumulations. This recent snow overlies the late-February interface that is composed of a sun crust on southerly aspects and surface hoar on shaded aspects. An observation on Wednesday suggests this layer has gained a lot of strength since the weekend. However, conservative choices are still recommended until we are sure this layer has become unreactive. We could be in a low probability, high consequence scenario where very large avalanche are still possible in isolated areas. Substantial snowfall and warming in the forecast are two factors that may promote increasing reactivity at this interface over the weekend. the Below this interface, the mid and lower snowpack appear to be well settled and strong.

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