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Avalanche Forecast

Archived

Mar 4th, 2013–Mar 5th, 2013

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

Regions

South Coast.

Confidence

Fair

Weather Forecast

Synopsis: A weak weather system will bring more cloud and light precipitation to the South Coast for Tuesday and Wednesday. A ridge of high pressure should build in for Thursday and Friday bringing mainly sunny skies. Tuesday: Increasing cloud with light precipitation (2-4 cm). The freezing level will rise to around 1100 m during the day. Winds are light from the southeast. Wednesday: Cloudy with light precipitation (4-8 cm). The freezing level is around 1200 m and winds are light from the southeast. Thursday: A mix of sun and cloud. The freezing level is steady around 1400 m.

Avalanche Summary

A widespread natural avalanche cycle to size 3.5 occurred on Friday and Saturday. Numerous large slabs failed on a surface hoar/crust weakness, many of which took out mature forest. Natural avalanche activity decreased on Sunday but explosive control on the Duffey Lake road produced several avalanches up to size 3 from northwest aspects. Most of these were suspected to have released on the late February surface hoar layer.

Snowpack Summary

The recent storm delivered 60-70 mm of precipitation, which fell with warming temperatures, dropping rain below about 1600-1800 m. A subsequent drop in temperature has left a supportive frozen crust below treeline. Strong south to south-westerly winds during the storm created touchy wind slabs on lee terrain. A weak layer of buried surface hoar (February 20th) and/or a crust was overloaded during the storm, creating a widespread avalanche cycle. The lower snowpack is well settled.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.