Avalanche forecast

Purcells

Mar 10 – 11, 2016Avalanche Canada
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Archived · expired 11 years ago

Avalanche danger

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

The timing and intensity of solar radiation on Friday is uncertain. If the sun makes an appearance, the snowpack could destabilize quickly and the Avalanche Danger may be HIGH.

Avalanche problems

  • Cornices

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–4
    Cornices have become large and weak, and may fail naturally with solar radiation forecast for Friday. A cornice fall could also be the trigger for a destructive avalanche on the slope below.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3–5
    A crust/surface hoar layer buried approximately 60 cm below the surface is triggerable by people on sleds or on skis. This layer could also avalanche naturally during periods of solar radiation and produce large avalanches.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible - Likely
    Size
    1–3
    Recently formed storm slabs are expected to remain sensitive to light triggers on Friday. Strong winds have encouraged hard slab formation and wide propagations are possible. If the sun comes out on Friday, large loose wet avalanches will be likely.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

A ridge of high pressure will keep the region mainly dry and sunny for the early part of Friday. Over the weekend a series of frontal systems will make their way across the region bringing 5-10cm of new snow each day. Ridgetop winds will be light on Friday, increasing to moderate and southwesterly for the weekend. Freezing levels will hover around 1500m on Friday, climb to about 1800m on Saturday, and then drop to about 1500m by Sunday.

Avalanche Summary

On Tuesday, several large (size 3) avalanches were reported from south-facing slopes near Golden. The avalanches were thought to be solar-triggered and could have failed on any of the persistent weak layers mentioned in the snowpack discussion section. At the time of publishing no new avalanches were reported, but I'm sure that speaks more to the lack of observations rather than actual conditions. I expect a good round of wind/storm slab activity took place in response to new snow and strong winds on Thursday. I also expect ongoing potential for human triggering on Friday. Sun may also be the driver for avalanche activity on Friday. With that, you can add cornices and loose wet avalanches to the mix. Solar warming will also increase the likelihood of avalanches failing on deeper persistent layers.

Snowpack Summary

Snowfall accumulations on Thursday were in the 10-20cm range. Strong southerly winds redistributed these accumulations into touchy wind slabs at treeline and in the alpine. 30-70 cm recent storm snow overlies a prominent weak layer buried on or around Feb-27. The deepest snow amounts appear to be in the west central area near Kootenay Lake. The Feb-27 weak layer comprises surface hoar and a crust. It has been widely reported but recent snowpack test results are mixed, with some tests indicating this layer is gaining strength, while others indicating it can still fail with sudden "pop" results. A deeper weak layer from mid-February is now down 50-80cm. The early January surface hoar/facet layer is typically down 70-120cm. Triggering an avalanche on either of these layers has become unlikely but either still has the isolated potential to produce very large avalanches with a heavy trigger.

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