Avalanche forecast

Purcells

Dec 3 – 4, 2021Avalanche Canada
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Avalanche danger

Sun
Mon
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Approach treeline and alpine terrain with caution in the wake of the storm. Wind loaded areas still hold the greatest hazard with reactive slabs. Small avalanches may step down to deeper instabilities.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    Storm slabs remain reactive to human triggers, with deeper and more sensitive deposits in high elevation wind loaded terrain features from the strong south/west winds.

    Storm slabs in the Toby Creek drainage may sit on a touchy surface hoar layer at treeline and below. This interface may result in larger avalanches, wider propagation and storm slabs that are more sensitive to human triggers.

    Take the time to investigate the bond between the storm slab and old snow surface. 

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely - Possible
    Size
    2–3.5

    A layer of facets and depth hoar exists at the base of the snowpack. These very weak crystals continue to be a concern, especially as new load is added to the snowpack. Deep persistent slabs have a lower probability of triggering, but a much higher consequence.

    Smaller avalanches have the potential to step down to these deeper weak layers, producing very large and destructive avalanches. 

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

The atmospheric river has finally left the building! The Purcells will be dry and cold until Saturday morning when an active pattern returns.

THURSDAY NIGHT: Clearing skies as flurries taper off. Winds ease to moderate westerlies. Temperatures drop, with freezing levels falling to valley bottom.

FRIDAY: Cold and partly cloudy with alpine highs around -7. Freezing levels stay below 1000m with light to moderate westerly winds.

SATURDAY: Light accumulations with strong southwesterly winds. Freezing levels remain below 1000m with alpine highs of -7.

SUNDAY: Clearing briefly before more snow on Monday. 

Avalanche Summary

A natural avalanche cycle was observed near Golden yesterday, including storm slabs and cornice failures to size 2 in north facing terrain. Explosive control work also produced several storm slabs. Nearby Glacier National Park observed a widespread natural avalanche cycle and produced up to size 3.5 avalanches with explosive control methods. 

Over the last week, numerous natural and explosive triggered storm slabs have been observed up to size 2 in wind effected terrain as a result of the heavy snowfall, rain and warm temperatures. 

Of note, several avalanches in the Toby Creek drainage are thought to have failed on a late November surface hoar layer. This is believed to be widespread in the Toby Creek drainage area, however data is sparse this early in the season and this layer may extend to other areas.

Snowpack Summary

Strong southwesterly winds have redistributed recent storm totals of up to 80mm into deep, reactive deposits at treeline and above. At lower elevations a melt freeze crust sits at the snow surface, over refrozen wet snow to ground. 

A surface hoar layer in the Toby Creek drainage sits down 30 to 60cm in sheltered features at treeline and below. We have limited knowledge of the distribution of this layer, however recent avalanche observations have shown this layer to be reactive. The mid-November rain crust exists throughout the region below 1900m, around 60-100cm deep in the snowpack.

The lower snowpack holds several early season crusts. A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. This layer continues to produce results on snowpack tests, and will likely continue to be an issue.

Average snowpack depth at treeline is 100cm - 180cm, with the deepest snowpack found near the Bugaboos. 

Terrain and Travel

  • Be alert to conditions that change with elevation and wind exposure.
  • Carefully monitor the bond between the new snow and old surface.
  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Regions covered