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Avalanche Forecast

Archived

Dec 20th, 2019–Dec 21st, 2019

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

Regions

South Coast Inland.

Over 100 cm of snow fell in the south of the region on Friday, over 30 cm in the north of the region, and more is on its way. Both natural and human triggering of avalanches will remain very likely on Saturday. Best to avoid avalanche terrain.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

FRIDAY NIGHT: Cloudy with snowfall, accumulation 10 cm in the north of the region and 20 cm in the south of the region, moderate to strong southwest wind, alpine temperature -5 C, freezing level 1000 m.

SATURDAY: Mix of sun and cloud in the north of the region and cloudy with snowfall in the south of the region, accumulation 10 to 15 cm, moderate southwest wind, alpine temperature -7 C, freezing level 1000 m.

SUNDAY: Mix of sun and cloud, light southwest wind, alpine temperature -7 C, freezing level 600 m.

MONDAY: Mix of sun and cloud, light southwest wind, alpine temperature -9 C, freezing level below valley bottom.

Avalanche Summary

A widespread avalanche cycle was observed across the region on Friday. Avalanches were reported to be running to valley-bottom in the north of the region. The cycle may continue into Friday night and Saturday, particularly in the south of the region where more snow is forecast.

Snowpack Summary

Over 100 cm of snow has accumulated in the south of the region with this storm and over 30 cm in the north of the region. The snow fell with strong southwest wind. More snow is expected in the south of the region on Friday night and Saturday while the skies dry up in the north. This snow will likely be very touchy and may produce both natural and skier-triggered avalanches on Saturday.

In the north of the region, the snow is loading a weak layer of sugary faceted grains and hard melt-freeze crust buried mid-November, which is a recipe for large and destructive avalanches and a problem that may persist for weeks to months.

In the south of the region, the snow is loading a weak layer of feathery surface hoar buried about 150 cm.

Avalanches triggered in the storm snow could step down to these buried weak layers, producing large and destructive avalanches.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Avoid exposure to overhead avalanche terrain, avalanches may run surprisingly far.
  • 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.