Avalanche forecast

Purcells

Dec 4 – 5, 2021Avalanche Canada
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Avalanche danger

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

Increasing winds will impact flurries and loose, dry snow. Approach avalanche terrain with a conservative mindset as the best riding areas may overlap with the greatest avalanche hazard.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2.5

    Storm slabs have been recently reactive to human triggers, with more sensitive deposits at upper treeline and above. Fresh slabs will form Saturday as increasing winds impact new flurries and loose, dry snow. Expect to find the deepest deposits and most reactive slabs around ridges and in open terrain.

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

  • 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 with any new load 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 extremely variable snowpack conditions reported through the region.

Weather Forecast

FRIDAY NIGHT: Increasing clouds and flurries. Light west wind up to 15 km/hr. Alpine low temperature -16 C. Freezing level valley bottom.

SATURDAY: Flurries starting overnight, up to 10 cm accumulation through the day. Increasingly gusty southwest wind, 25-45 km/hr. Alpine high temperature -5 C. Freezing level valley bottom. 

SUNDAY: Partly cloudy with sunny breaks. Northwest-west wind, 20-30 km/hr. Alpine high temperature -7 C. Freezing level valley bottom. 

MONDAY: Mostly cloudy with flurries, trace to 5 cm accumulation. Variable wind 10-20 km/hr. Alpine high temperature -9 C. Freezing level valley bottom. 

Avalanche Summary

On Thursday, Dec 2 evidence of a natural avalanche cycle to size 3 occurring overnight Wed-Thurs was observed around the Golden area and north end of the forecast region. Through the day (and throughout the region), large storm slabs (size 2) and large deep persistent slabs (size 2-2.5) were triggered by explosives.

On Wednesday, Dec 1 a natural avalanche cycle was observed near Golden, 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

The snowpack is cooling and drying from the recent deluge of weather. With freezing temperatures a surface crust formed, which is now covered by 10-40 cm of snowfall from the past few days. 

The mid-November rain crust exists throughout the region below 1900m, around 60-100cm deep in the snowpack. A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. This layer continues to produce results with explosives and in snowpack tests, and will likely continue to be an issue.

Around the Toby Creek drainage, a surface hoar layer 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. 

Average snowpack depth at treeline is 100-180cm, 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.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • In times of uncertainty conservative terrain choices are our best defense.

Regions covered