Avalanche forecast

Purcells

Feb 19 – 20, 2016Avalanche Canada
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Avalanche danger

Sun
Mon
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Snowfall amounts overnight are uncertain. We are also unsure of when and where the sun will shine. Solar radiation on dry storm snow is a concern for increased avalanche activity.

Avalanche problems

  • Cornices

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely - Very Likely
    Size
    1–4
    Cornices may be large and fragile. Cornice growth continues with each pulse of new snow and wind. Natural cornice falls may trigger storm slab avalanches on the slopes below.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible - Likely
    Size
    1–3
    Storm slabs are deep and continue to load with each pulse of stormy weather. Buried surface hoar at the storm snow/crust interface may result in wide propagations and large avalanches.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    3–5
    The early January surface hoar may continue to be triggered by large loads like storm snow avalanches in motion, or light loads in shallow or weak spots on large terrain features. This problem is more likely in the north of the region near Golden.

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

Light winds and 5-10 cm of new snow overnight. Winds becoming moderate from the west during the day on Saturday as a weak ridge of high pressure builds after the storm. There may be some sunny breaks Saturday afternoon, but more likely to clear overnight. Freezing levels bouncing between valley bottoms and 1400 metres. Mostly sunny on Sunday with light winds and freezing levels up to 1200 metres. Cloudy with increasing southwest winds on Monday.

Avalanche Summary

On Thursday we had reports of natural storm slab avalanches up to size 2.0 and explosives controlled avalanches up to size 2.5. On Wednesday there were reports of natural cornice falls up to size 2.5, and explosives control released slab avalanches up to size 2.5. Most of the slab avalanches were in the alpine on east aspects. I suspect that new wind slabs and storms slabs were easy to trigger in the alpine on Friday.

Snowpack Summary

There has been 20-30 cm of new snow in the past few days that has added to the recent storm slab above the buried crust. About 30-60 cm below the surface, you'll likely find a melt freeze crust which exists in most places except for in high elevation shaded terrain. In general there seems to be a reasonable bond between the crust and the overlying snow. I'd still keep an eye on this interface as it has become reactive on some features. If the sun comes out this weekend, periods of strong solar radiation may increase the likelihood of triggering the storm slab. In isolated areas there may be a weak layer of surface hoar at the storm snow/crust interface that is touchy to human triggers and may result in wide propagations. The early January surface hoar/ facet layer is typically down 70-120 cm. Avalanche activity at this interface has tapered-off over the past week, but I'd be reluctant to trust this potentially destructive layer just yet. In general, the lower snowpack is well settled and strong, apart from some thin snowpack areas where basal facets exist.

Regions covered