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Avalanche Forecast

Archived

Dec 18th, 2020–Dec 19th, 2020

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
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.

Incoming weather will increase avalanche danger to HIGH this weekend. Avoid avalanche terrain by sticking to simple terrain with low angle slopes and no exposure to overhead hazard.

Confidence

High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

Weekend will be stormy with Monday looking dry and clear.

FRIDAY NIGHT: 10 to 20 cm overnight, highest amounts on the westerly side of the mountains. Strong southwest wind. Freezing level around 1000 - 1200 m.

SATURDAY: Break between systems so dry, cloudy with some afternoon clearing possible, continued strong southwest wind.

SUNDAY: wake up to new snow that fell overnight, around 10 to 25 cm in westerly and deeper snowpack areas. Light snow and cloudy during the day, continued moderate to strong southwest wind.

MONDAY: Clear, dry, light variable wind, and -10 C or colder temperatures.

Avalanche Summary

A natural avalanche cycle up to size 2 or 2.5 on Wednesday was reported in northern parts of the region. The key point is the weakness was surface hoar -- expect more and bigger events on this layer.

On Thursday a size 2.5 or 3 was reported in Pine Pass -- the key point is it was bigger (see previous sentence).

I expect a significant avalanche cycle this weekend, especially where surface hoar from last week exists below the recent snow.

Snowpack Summary

Upper snowpack is made up of new or recent snow and with the storm systems lined up to track through the region this storm snow will continue to build. The forecast wind will also build windslabs. In some areas in the north (Pine Pass, possibly Wolverine) there is surface hoar near the bottom of this recent snow -- which may be as deep as 50 or 60 cm.

Mid-pack is where you'll find a variety of crusts. Most reports haven't found them to be an issue.

Basal-pack, the basement also has some crusts. I suspect the layer to be potentially reactive in northern parts of the region, such as Pine Pass, but do not have observations to back this up.

Snow depths are approaching 200 cm around Pine Pass, Torpy and McBride and around 100 cm in the northeast around Tumbler Ridge.  

Terrain and Travel

  • Choose low-angled, sheltered terrain where new snow hasn't been wind-affected.
  • Avoid areas with overhead hazard.
  • 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.