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Avalanche Forecast

Archived

Dec 21st, 2019–Dec 22nd, 2019

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 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.

Substantial snowfall triggered avalanche cycles on Friday and Saturday. This snow will likely remain touchy to human traffic and it has loaded buried weak layers that will need time to stabilize. Best to avoid alpine avalanche terrain and travel conservatively below.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

SATURDAY NIGHT: Mostly cloudy, light southwest wind, alpine temperature -8 C, freezing level below valley bottom.

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

MONDAY: Mostly clear skies, light southwest wind, alpine temperature -7 C, freezing level below valley bottom.

TUESDAY: Mix of sun and cloud with isolated flurries, 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 and Saturday. Avalanches were reported to be running to valley-bottom in the north of the region.

Human-triggered avalanches are expected to be very likely to occur on Sunday, with the potential to fail in both the recent storm snow and potentially in the weak layers described in the Snowpack Summary. We recommend avoiding alpine avalanche terrain and to travel conservatively at and below treeline until the snowpack strengthens.

Snowpack Summary

Over 120 cm of snow has accumulated in the south of the region with this storm and over 40 cm in the north of the region. The snow fell with strong southwest wind. This snow may remain touchy to human traffic on Sunday.

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 may overly a weak layer of feathery surface hoar buried about 150 cm, in areas where the storm snow hasn't already caused it to fail.

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

Terrain and Travel

  • Travel in alpine terrain is not recommended.
  • 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.

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.