Avalanche forecast

Purcells

Feb 26 – 27, 2012Avalanche Canada
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Avalanche danger

Tue
Wed
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible - Likely
    Size
    1–5
    Old windslabs that developed last week above the Feb surface hoar/ facet/ crust combination have become harder to trigger, but continue to have wide propagations.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–7
    As the storm snow above this week layer consolidates into a cohesive slab, expect large avalanches with wide propagations. Areas where the weakness is on top of a crust may result in very large avalanches.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–4
    Loose dry storm snow is sliding easily and running fast on the old surface.

Confidence

Fair - Timing or intensity of solar radiation is uncertain on Monday

Weather Forecast

The ridge of high pressure should have moved into the region by Monday morning causing mostly clear skies and cooler overnight temperatures. Strong solar radiation may bring warm temperatures at or above freezing on southerly aspects. Temperatures are expected to drop down to near -14.0 in the alpine Monday night during clear skies and light northerly winds. High pressure should continue for Tuesday, and warm temperatures are expected on sunny alpine slopes. Shaded aspects are expected to remain cool and dry. Some cloud should move into the region by Tuesday evening. Forecast snowfall amounts are unsure for Wednesday at this time.

Avalanche Summary

Skier remote and natural avalanches continued to be reported on Saturday. Small natural avalanches up to size 1.5 are releasing in the storm snow. I expect that the new snow was not enough to cause another round of natural activity on the Feb persistent weak layer (PWL), but human triggering continues to be likely from light additional loads. Avalanches that release on or step down to the PWL could be very large and destructive.

Snowpack Summary

Another 8cm of dry light snow overnight brings the new storm total to about 20-25 cm of dry light snow above the recent stiff windslabs. The upper snowpack structure is very complex. There are buried layers of surface hoar, buried melt-freeze crusts, and some buried crusts with facets. These weak sliding layers are buried anywhere from 40 - 80 cm by several different storm layers. There are some shears in the storm layers on decomposed and fragmented snow crystals. As the snow above the surface hoar layers settles into a cohesive slab, we are seeing easier and more sudden shears that are a bit deeper. The surface hoar is more likely to produce wider propagations, and lower angle fractures in areas where it is associated with a crust. The crust has been reported to be 2-3 cm thick in some areas. The mid-pack is generally well settled. Basal facets have not been reactive, but operators continue to monitor this layer in tests. Triggering this deep persistent weak layer is unlikely, but shallow snowpack areas or shallow weak areas adjacent to deeper wind loaded slopes are the most suspect locations.

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