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Avalanche Forecast

Archived

Dec 20th, 2020–Dec 21st, 2020

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

Regions

Kootenay Boundary.

An incoming storm will likely cause an avalanche cycle to occur by Tuesday morning. Expect the likelihood of triggering large avalanches to increase as the day progresses. Be conservative and step away from avalanche terrain if you find more than 25 cm of snow accumulation.

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain.

Weather Forecast

SUNDAY NIGHT: Partly cloudy, 30 km/h northwest wind, alpine temperature -6 C, freezing level 1000 m.

MONDAY: Snow, accumulation 10 to 20 cm, 30 to 50 km/h south wind, alpine temperature -7 C.

TUESDAY: Early-morning snowfall then clearing, accumulation 20 to 40 cm, 30 to 60 km/h northwest wind, alpine temperature -2 C dropping to -6 C by morning, freezing level 1500 m dropping to 1000 m.

WEDNESDAY: Clear skies, 10 to 20 km/h west wind, alpine temperature -4 C.

Avalanche Summary

Numerous large persistent slab avalanches were triggered by skiers and explosives on Saturday Sunday. They released on the weak layer described in the snowpack summary. They were on all aspects and at an elevation around 2100 m. See this MIN for a great example of what people are seeing in the region.

Snowpack Summary

A hefty storm is forecast for the region from Monday morning to Tuesday morning. Around 10 to 20 cm is forecast for Monday to 4 PM followed by another 20 to 40 cm Monday night, with associated strong south to southwest wind. Expect storm and wind slabs to form over the day as the snow accumulates.

This snow will continue to load a weak layer around 30 to 60 cm deep. The layer may be composed of feathery surface hoar or sugary faceted grains and it overlies a hard melt-freeze crust. Riders have reported substantial avalanche activity and whumpfing on this layer as well as surprising snowpack test results. This layer remains a major concern, as humans may be able to trigger large and destructive avalanches on it.

Another crust with associated faceted grains may be found near the base of the snowpack. There have been no recent avalanches reported on this layer but it remains on our radar. The most likely place to trigger it would be from shallow, rocky terrain. 

Terrain and Travel

  • Travel in alpine terrain is not recommended.
  • Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
  • Make conservative terrain choices and avoid overhead hazard.
  • Avalanche hazard is expected to to increase througout the day, think carefully about your egress.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.