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Avalanche Forecast

Archived

Dec 9th, 2021–Dec 10th, 2021

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

Regions

Purcells.

The best riding will be found in sheltered terrain. Keep your head up in wind loaded features near ridgeline - expect reactivity where slabs sit over a crust. 

Avoid shallow rocky start zones, deeper weak layers are still a concern. 

Confidence

Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack. We have a good understanding of the snowpack structure and confidence in the weather forecast

Weather Forecast

A front pushes through the interior on Saturday bringing warmer weather, moderate to heavy snowfall and very strong ridgeline winds.

THURSDAY NIGHT: Cloudy with a chance of flurries, moderate westerlies. Freezing levels at valley bottom.

FRIDAY: Trace amounts of snow, most cloudy. Freezing level at valley bottom, alpine highs of -9. Light westerlies. 

Snow begins overnight, bringing up to 5cm by morning. 

SATURDAY: 5-15cm over the day. Freezing levels climb to 1500m. Alpine highs of -3. Strong to extreme southwest winds. 

SUNDAY: Light snow continues Saturday night (3-10cm), and tapers throughout the day Sunday (2-5cm). Winds decrease to moderate southwesterlies. Freezing levels below 1000m. Alpine highs of -6. 

Avalanche Summary

Yesterday, 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 almost a week old, this deeply buried weak layer is still a concern.

Snowpack Summary

Up to 50cm of storm snow sits over a thick and supportive melt freeze crust on all aspects up to 2400m. Westerly winds have redistributed new snow into deeper deposits in wind loaded features at higher elevations.

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

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of highly variable recent wind loading patterns.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.