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Avalanche Forecast

Archived

Jan 5th, 2020–Jan 6th, 2020

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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 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.

Regions

South Coast Inland.

Many large avalanches have occurred in the region. The snowpack will require more time to heal from the recent snow and wind loading. Travel in alpine and treeline avalanche terrain is not recommended until the snowpack shows signs of stabilizing.

Confidence

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

Weather Forecast

SUNDAY NIGHT: Cloudy with some clearing and isolated flurries, local accumulation 5 to 10 cm, moderate west wind, alpine temperature -10 C, freezing level below valley bottom.

MONDAY: Cloudy with snowfall, accumulation 5 to 10 cm in the north of the region and 10 to 20 cm in the south of the region, moderate southwest wind, alpine temperature -7 C, freezing level rising to 800 m over the day.

TUESDAY: Cloudy with snowfall, accumulation 10 to 30 cm, moderate to strong southwest wind, alpine temperature -3 C, freezing level 1400 m.

WEDNESDAY: Morning snowfall and afternoon clearing, accumulation 5 to 10 cm, light northwest wind, alpine temperature -10 C, freezing level 500 m.

Avalanche Summary

On Sunday, storm slabs were reported as being touchy and having surprisingly wide propagation. They were most touchy on lee slopes at treeline and alpine elevations. Many natural avalanches released within the recent storm snow.

Large persistent slab avalanches continue to be triggered naturally, by humans, and explosives in the northern half of the region. On Saturday, the avalanches were 50 to 70 cm thick and released on all aspects between 1800 m and 2300 m. Some of the avalanches sympathetically triggered other avalanches, suggesting the potential for wide propagation.

Snowpack Summary

Around 50 to 80 cm of recent snow has formed touchy storm slabs in the region, with the thickest slabs found in the southern half of the region. The snow has been falling with strong southwest wind, redistributing it in exposed terrain. The snow may overly a touchy weak layer of feathery surface hoar, making storm slabs particularly touchy.

A persistent weak layer is present near the bottom of the snowpack in the northern half of the region (e.g., Duffey, Hurley). The weak layer of sugary faceted grains exists around a hard melt-freeze crust from mid-November, which is an indicative snowpack setup for large and destructive avalanches. The likelihood of natural and human-triggered avalanches on this layer remains elevated with the snow and wind loading occurring during this stormy period.

Terrain and Travel

  • Travel in alpine terrain is not recommended.
  • Avoid exposure to overhead avalanche terrain, avalanches may run surprisingly far.
  • Be aware of the potential for wide propagation.
  • Any steep opening in the trees should be treated as suspect right now.

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.