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Avalanche Forecast

Archived

Dec 2nd, 2020–Dec 4th, 2020

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

Regions

North Rockies.

Forecast freezing levels and warm alpine temperatures are uncertain for the North Rockies. Carefully monitor conditions and be extra cautious if you see temperatures above zero, as warm temperatures could result in large avalanches that may catch you by surprise.

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

High winds and a strong temperature inversion causing unseasonably warm temperatures up high will dominate the weather pattern for a few days.

WEDNESDAY NIGHT - Clear / southwest wind, 40-60 km/h / alpine low temperature near +3

THURSDAY - Sunny with cloudy periods / southwest wind, 40-60 km/h / alpine high temperature near +4 and cooling in the afternoon

FRIDAY - A mix of sun and cloud with isolated flurries / southwest wind, 50-70 km/h / alpine high temperature near 0 / freezing level 2000 m

SATURDAY - Sunny with cloudy periods / southwest wind, 60-80 km/h / alpine high temperature near +5  

Avalanche Summary

There has been a report of an avalanche fatality near Pine Pass on Saturday, November 28. Reports indicate a size 3 deep persistent slab avalanche at approximately 1600 m elevation on a northeast aspect. The avalanche was approximately 55 cm deep, 800 m wide and ran 400 m in length. The avalanche may have run on a weak layer of faceted crystals sitting on a crust that was buried in early November.

On Tuesday, there were reports of a few explosives triggered wind slab avalanches, size 1-2.5.

If you get out into the mountains, please consider sharing your observations on the Mountain Information Network.

Snowpack Summary

The North Rockies have seen between 30-70 cm of recent storm snow in the past week. Strong southwest winds have likely created widespread wind slabs in the alpine and open areas at treeline. In some areas, the windward slopes have been stripped of the recent snow. Fresh wind slabs may be reactive to human triggers.

A crust that was buried in early November is near the base of the snowpack. In some areas, weak, sugary facets are sitting on this crust. This layer is of most concern on steep terrain in thinner snowpack areas.

Snow depths are approximately 150 cm in the northwest of the region (Pine Pass), 180 cm in the south (Renshaw, Kakwa) and 100 cm in the the northeast (Tumbler ridge).  

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.
  • Avoid steep convexities or areas with a thin, rocky, or variable snowpack.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger 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.