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Avalanche Forecast

Archived

Dec 21st, 2020–Dec 22nd, 2020

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 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

Lizard-Flathead.

The incoming storm will likely cause a widespread avalanche cycle overnight Monday and into Tuesday. This is good time to stick to non-avalanche terrain free from overhead hazards. Avalanche paths may run into below treeline terrain. 

Confidence

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

Weather Forecast

An intense Pacific frontal system will track across BC today bringing the region strong to extreme winds and heavy snowfall amounts. High pressure will start to set up Tuesday afternoon and we can expect a clearing trend.

Overnight Monday: Heavy snow 20-30 cm. Alpine temperatures near -6 with a brief above freezing layer 1300-1500 m. Ridgetop winds strong to extreme from the southwest.

Tuesday: A mix of sun and cloud. Alpine temperature near -7 and freezing levels at the valley bottom. Ridgetop winds generally light with strong gusts from the West.

Wednesday: Mix of sun and cloud. Air temperatures falling to -12 in the alpine. Ridgetop winds light from the West.

Thursday: Sunny. Alpine temperatures -6 with an alpine inversion and freezing levels in the valley bottom. 

Avalanche Summary

On Monday several loose dry avalanches up to size 1 were reported. Tomorrow will likley be a different story with new snow and wind overnight. A natural widespread avalanche cycle is expected to happen overnight and/ or tomorrow.

There have also been a few accidentally skier triggered avalanches involving both storm snow and the December crust layer such as this one from the MIN.

Please consider sharing your observations on the Mountain Information Network. Thank you to those that have already submitted this winter!

 

Snowpack Summary

By Monday morning up to 20 cm of storm snow hit the region. The big story is the next system arriving today and tonight bringing a lot of new snow by Tuesday. Large storm slabs will be reactive, especially where they sit above a weak interface. Depending on aspect and elevation 60-100 cm now sits on the early December crust. There is increasing evidence that this persistent weak layer, with facetted crystals and surface hoar crystals above and/or below it are reaching a tipping point. 

Deeper in the snowpack are two hard melt-freeze crusts that formed in November that may have some weak crystals around them. There is uncertainty if and where in the region this may be a problem. 

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

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.

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.