Avalanche forecast

Northwest Coastal

Mar 19 – 20, 2013Avalanche Canada
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Avalanche danger

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

Avalanche problems

  • Cornices

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–6
    Cornices are reported to be large and unstable, especially when temperatures become warm or the sun is out. Watch for reverse loading from outflow winds.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    3–7
    A weak layer comprising surface hoar crystals is buried around 50 cm below the surface and is reacting readily to natural and human triggers. This layer appears to have the ability to propagate over large areas of terrain.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely - Very Likely
    Size
    2–6
    Recent new snow has consolidated into a slab and is reacting on steep terrain. Watch for deep and weak wind slabs to form in North through East facing terrain well below ridge crests.

Confidence

Fair - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Synopsis: A frontal system is spreading moderate snowfall to the region today and through Wednesday. Conditions remain unsettled in the wake of the system on Thursday before high pressure develops on Friday resulting in sunshine.  Tonight and Wednesday: 10-15 cm (maybe more near Bear Pass). The freezing level is around 400-500 m. Winds ease to moderate from the southwest, shifting to northwest.  Thursday: Mainly cloudy with a chance of flurries. The freezing level drops to valley bottom and winds ease to moderate from the west. Friday: Mainly sunny. Temperatures remain cool with the freezing level at valley bottom. Overnight lows near treeline could be in the -15 range. Watch for outflow winds to develop.

Avalanche Summary

Natural, human triggered, and remotely triggered avalanches up to size 2 continued on Monday with most avalanches releasing on the March 9th layer. A very large (size 3.5) avalanche was triggered in Cherry Bowl in the Shames backcountry on Sunday. Click here for a full report. This slide illustrates clearly the potential for deep and destructive releases on the March 9th surface hoar layer. On Saturday, natural slab avalanches were reported up to size 3 on all aspects in the alpine.

Snowpack Summary

30-60 cm of recent storm snow rests on a variety of old snow surfaces, including crusts, previous wind slabs and surface hoar. Warmer temperatures and recent strong southwest winds have now set this new snow into a reactive slab, with wind slabs building in exposed lee areas. The surface hoar (buried March 9th) is reported to be well-developed and fairly widespread, at treeline and alpine elevations. Recent snowpack tests have produced sudden failures with moderate loading force at this interface. The mid snowpack is generally well settled and strong. Cornices are large and untrustworthy, especially when the sun is out.

Regions covered