Avalanche forecast

Northwest Inland

Apr 4 – 5, 2016Avalanche Canada
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Avalanche danger

Wed
Thu
Alpine
4 · High
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Heavy storm loading from snow and then rain will push the Avalanche Danger to HIGH. I'd wait this one out from a safe distance.

Avalanche problems

  • Cornices

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    2–5
    Cooling over the weekend has likely added strength to cornices. That said, ongoing cornice growth is expected with stormy weather. Fresh cornice tabs may be highly sensitive to human triggering.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1–4
    Heavy storm loading on Tuesday will form deep and dense storm slabs which should be most reactive in high elevation lee terrain. In addition to creating potent new storm slabs, the mix of rain and snow will test deeper more destructive layers.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–3
    Rain at lower elevations should promote ongoing loose wet avalanches in steep terrain. Destructive wet slab avalanches are also possible, especially in areas where rain-soaked snow overlies a hard crust.

Avalanche Summary

No new avalanches were reported on Sunday. Heavy storm loading will spark a new round of storm slab avalanche activity on Tuesday and Wednesday while heavy rain at lower elevations will encourage loose wet avalanche activity. If the near surface crust breaks down because of rain at treeline or in the alpine, deeper, more destructive persistent weak layers may come back to life.

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Tuesday

Weather Forecast

On Tuesday and Wednesday an intense Pacific frontal system will bring moderate to locally heavy moist snowfall (20-40cm on Tuesday/ ~ 10-20cm on Wednesday) to higher elevations. A dry ridge of high pressure will deliver mainly clear skies on Thursday. Ridgetop winds will be extreme for the southwest with the storm, and then become light on Thursday. The freezing level will climb from 1200m to about 1800m over the course of Tuesday, hover around 1300m on Wednesday, and then spike to about 3000m by Thursday.

Snowpack Summary

On Tuesday and Wednesday, moderate to locally heavy snowfall will form new storm slabs while rain at lower elevations will further saturate the snowpack. Extreme winds and progressive warming throughout the storm will encourage the formation of a slabby, "upside-down" upper snowpack. Cornice growth is also expected to be significant during the storm. New accumulations will overlie a widespread melt-freeze crust that formed as a result of last week's warm weather and more recent cooling. The warm temperatures and sun over the last week woke up deeply buried weak layers within the snowpack. This includes a weak crust/surface hoar layer which was buried down 20-30cm in the north of the region, a widespread crust/facet layer buried in early February down up to 1m, a lingering surface hoar layer from January down around a meter, and weak basal facets at the bottom of the snowpack. Cooling should dramatically limit the reactivity of these old layers; however, they may come back to life during future periods of warming, solar radiation or heavy storm loading.

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