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Avalanche Forecast

Archived

Dec 1st, 2021–Dec 2nd, 2021

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Storm slabs remain reactive with the greatest hazard in wind loaded terrain. Choose conservative terrain while the snowpack gains strength. 

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

After a final wave of moisture Wednesday night, freezing levels drop and cloudy skies return. The next round of precipitation arrives Saturday morning.

WEDNESDAY NIGHT: Strong, easing to moderate southwest winds. Freezing levels drop to 1500m by Thursday morning. 10cm overnight, favouring the north. 

THURSDAY: Clearing skies as flurries taper off. Winds ease to moderate westerlies. Temperatures drop, with freezing levels falling to valley bottom in the afternoon. Alpine highs near -8.

FRIDAY: Cold and clear with alpine highs around -10. Freezing levels stay below 1000m with light to moderate westerly winds.

SATURDAY: Light accumulations with strong southwesterly winds. Freezing levels remain below 1500m. 

Avalanche Summary

Observations over the last week show consistent direct-action avalanche cycles, where new snowfall and winds produce natural avalanches throughout and immediately following the storm. 

On Tuesday November 30th, several naturally triggered size 1 storm slab avalanches were observed in north facing treeline and alpine terrain near Invermere. Multiple size 1 storm slabs were explosively triggered in the same area, in wind loaded north facing terrain features. 

Over the last 3 days, numerous natural and explosive triggered storm slabs have been observed up to size 2 in the north and central Purcells with the recent heavy snowfall. 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 above 1600m, however data is sparse this early in the season and this layer may extend to other areas.

Snowpack Summary

Up to 80mm of heavy snowfall and mixed precipitation over the last week have added a significant new load to the snowpack. Strong southwesterly winds have redistributed this load to create deeper deposits in lee features at treeline and above.

A surface hoar layer has been observed down 40 to 50cm in the Toby Creek drainage in wind sheltered features. Recent avalanche observations have shown this layer to be reactive. The mid-November rain crust exists throughout the region below 1900m, around 60-100cm down 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. Elevated freezing levels over the past week have created moist snow at lower elevations. 

Terrain and Travel

  • 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.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.