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Avalanche Forecast

Archived

Dec 5th, 2020–Dec 7th, 2020

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.

Regions

North Rockies.

  

Expect hazard to be one-step higher in the areas around Pine Pass that received more than 15 cm. of new snow Friday night. 15-25 cm of new snow in the Pine Pass area has formed localized storm slabs reactive to human triggers.

Confidence

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

Weather Forecast

  

SATURDAY NIGHT: Mostly cloudy with isolated flurries; 0-3 cm. / Strong, southwest ridgetop wind / alpine low temperature -8 / Freezing level 1500 m.

SUNDAY: Cloudy / Strong, southwest ridgetop wind / alpine high temperature -2 / Freezing level 1500 m.

MONDAY: Snow; 15-25 cm. / Strong, southwest ridgetop wind / alpine high temperature -2 / Freezing level 1500 m.

TUESDAY: Mix of sun and cloud / Light, west ridgetop wind / alpine high temperature -3 / Freezing level 1500 m.

Avalanche Summary

The new snow Friday night around Pine Pass is expected to form storm slabs reactive to human triggers.

There was 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.

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

Snowpack Summary

  

A surprise storm Friday night dropped up to 25 cm. of snow in the Pine Pass area which formed fresh storm slabs reactive to human triggers at treeline and above.

The snowpack is highly variable due to recent strong southwest winds. In some areas, the windward slopes have been stripped of any recent snow. On lee aspects, wind deposited snow drifts may be up to 90 cm deep. 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. It is most likely to be reactive to human triggers on south facing slopes and on steep, shallow, rocky, snowpack areas in the alpine.

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.
  • Closely monitor how the new snow is bonding to the crust.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid steep convexities or areas with a thin, rocky, or variable snowpack.

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.

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.