Avalanche forecast

Northwest Coastal

Jan 19, 2017Avalanche Canada
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Avalanche danger

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

The storm is coming to an end, but the avalanche danger will continue to be HIGH in the alpine for tomorrow.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    1–3
    It is still possible to trigger avalanches on buried weak layers (Dec 25, Dec 30, Jan 5th, Jan 12th surface hoar) ... or have a smaller avalanche 'step down' and trigger deeper instabilities.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–3
    Natural storm slab activity may end as the storm snow settles, however human triggering may continue to be very likely. Avoid large slopes with convexities, and any slope exposed to overhead hazards like cornices.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2
    Watch for moist or wet snow below treeline that may be easy to trigger or release naturally due to rising freezing levels and rain on previously dry snow.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Winds becoming moderate from the south overnight with 5-8 cm of new snow above a freezing level of about 600 metres. Another 3-5 cm of new snow combined with light southwest winds during the day on Thursday and a freezing level of 500 metres. A few more cm of new snow by Friday morning when skies begin to clear and freezing levels drop to near valley bottoms. Another small pulse of wind and snow is expected on Saturday.

Avalanche Summary

Natural storm slab avalanches were reported on Tuesday up to size 2.5 near Terrace, as well as loose wet avalanches to size 1.5 below treeline a little northwest of Terrace. I suspect that widespread natural activity will be reported once travel and visibility conditions improve.

Snowpack Summary

There is 40-60 cm of storm snow in the south of the region that has been transported by strong southwest winds in the alpine and at treeline over the past few days. The freezing level has been around 1200 metres, and below this level the snowpack is moist or wet. It is cooler in the north, and the storm totals were between 60-80 cm with significant wind transport. As the storm snow settles into a slab, we are concerned that several of the weak surface hoar or facet interfaces that developed during cold and clear weather may become reactive. The January 5th layer is down 50-80 cm , and the December 25th is now down close to 100 cm in deeper snowpack areas. These are not typical conditions for this coastal region, and it may take some time to gain confidence regarding the distribution and likelihood of triggering these deeply buried weak layers. Areas exposed to cornice fall should be avoided due to the possibility of a cornice triggering a deep weak layer. Below treeline the snow is moist or wet and may be easily triggered as a loose wet avalanche in steep terrain. I suggest a conservative approach to terrain while gathering info after the storm.

Regions covered