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Avalanche Forecast

Archived

Dec 11th, 2021–Dec 12th, 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, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Continue to choose mellow slopes protected from the wind.

Wind and new snow are building slabs that can be triggered by a rider. It is still uncertain how well this new snow is bonding to the snowpack.

Confidence

Low - Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

SATURDAY NIGHT: Mostly cloudy. 5-10cm of snow expected. Moderate southwest winds, possibly extreme on high peaks. Freezing levels between 1000 and 1300m, starting to drop again around midnight.

SUNDAY: Mostly cloudy. 0-7 cm snow expected. Moderate to strong southwest wind. Freezing levels below 1000m. Alpine highs of -6 C. 

MONDAY: Mostly cloudy. 0-5 cm snow expected. Light to moderate south winds. Warming through the day, alpine high around -7 C.

TUESDAY: Mostly cloudy. Possible trace of snow. Light to moderate SW winds. Alpine high around -8 C.

Avalanche Summary

No new avalanches observed. Expect human triggered avalanches to remain likely as the storm snow settles.

On Wednesday, natural and skier triggered slab avalanches were observed at treeline and above to size 1.5.

Explosive control work near Golden on Dec 5th produced a size 2 slab avalanche that failed to ground in a steep and unsupported terrain feature. A naturally triggered size 3 was also observed on a west facing slope in the Northern zone of the Purcells, believed to have occurred around the 2nd of December. 

Two Mountain Information Network (MIN) reports (MIN 1 and MIN 2) from Dec 3 in Quartz Creek also reported deep persistent avalanches failing at the base of the snowpack. 

While these observations are a week old, this deeply buried weak layer is still a concern.

Snowpack Summary

Moderate to strong southwest winds continue to redistribute 20-30cm of recent storm snow, forming reactive slabs.

The new storm snow overlies about 50cm of settling snow from the last storm. This sits on a thick, supportive melt freeze crust on all aspects up to 2400m (December 2nd). 

Where the December 2nd crust does exist, facets may be forming on top, making this a layer to watch.

A late October facet/crust layer of concern sits at the bottom of the snowpack above 1900m. This layer has been reactive to human triggers, producing large avalanches. This layer is widespread and will likely continue to be a layer of concern.

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

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Closely monitor how the new snow is bonding to the crust.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.

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.