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Avalanche Forecast

Archived

Dec 24th, 2019–Dec 25th, 2019

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 possible, human triggered probable.
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

Kootenay Boundary.

Several weak layers are active in the snowpack, producing avalanches and concerning test results. Successfully managing this complex snowpack will be less about picking out the most problematic layer and more about choosing conservative terrain while the snowpack strengthens.

Confidence

Moderate -

Weather Forecast

Tuesday night: Cloudy with continuing isolated flurries and a trace of new snow. Light southwest winds.

Wednesday: Mainly cloudy with cloud decreasing late in the day. Light north winds. Alpine high temperatures around -8.

Thursday: A mix of sun and cloud with cloud increasing over the day. Light to moderate southwest winds increasing over the day. Alpine high temperatures around -7.

Friday: A mix of sun and cloud. Light to moderate west winds. Alpine high temperatures around -8.

Avalanche Summary

Several more large persistent slabs were triggered with explosives control in the Nelson area on Monday. These slabs featured crown depths of 15-120 cm, giving some evidence of wind loading. These avalanches are suspected to have released on our early November facet/crust layer, which exists below at least two other persistent weak layers of concern.

A widespread avalanche cycle occurred on Saturday, with observations of large artificially triggered avalanches continuing into Sunday. Many of these recent avalanches released within the storm snow, and large avalanches have been observed on all aspects and generally above 2000 m. Numerous other large to very large avalanches were also triggered by explosives on the late-November layer described in our Snowpack Summary.

Snowpack Summary

The past several stormy days saw around 70 to 100 cm of snow deposited in the region, initially accompanied by strong west wind. This snow has loaded multiple weak layers, including:

  • a feathery surface hoar layer now buried around 70 to 110 cm.
  • an older surface hoar layer buried about 90 to 130 cm, associated with a melt-freeze crust on steep south aspects.
  • a complex layer of weak and sugary faceted snow, surface hoar, and melt-freeze crusts in the bottom half of the snowpack, buried in late November.

There is uncertainty on which of these layers will emerge as our primary persistent slab problem. However, given our extensive recent avalanche activity, it is prudent to expect that any one of these deeply buried layers could produce large and destructive avalanches with a human trigger. 

Terrain and Travel

  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Make conservative terrain choices and avoid overhead hazard.

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.