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Avalanche Forecast

Archived

Dec 4th, 2012–Dec 5th, 2012

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 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, human triggered possible.

Regions

South Columbia.

Confidence

Fair - Due to limited field observations for the entire period

Weather Forecast

Wednesday: Moderate snowfall in the morning decreasing throughout the day with 5-10cm of accumulation, freezing levels around 1000m but cooling rapidly throughout the day, strong to extreme westerly winds easing throughout the day. Thursday and Friday: Light to moderate snowfall with 5-10cm of accumulation each day, freezing levels at valley bottoms and generally light southwesterly winds.

Avalanche Summary

Several 60-70cm deep storm slab avalanches up to Size 2.0 were ski cut on north facing unsupported treeline rolls on Monday. These all ran on surface hoar buried late last week. In the alpine, explosive controlled cornices triggered several 20-40cm thick Size 1.0-2.0 soft wind slab avalanches on west through northeast aspects, and many natural avalanches were heard running in very steep high alpine areas on north aspects.

Snowpack Summary

Another 20cm of snow in the Selkirks and Eastern Monashees brings the total snowpack depth to as much as 270cm at treeline. 3-10mm surface hoar buried late last week can be found down 50-80cm and has been reactive to light triggers on unsupported features at treeline elevations. The well-settled and strong mid-pack may be bridging deeper weaknesses; however, recent snowpack tests produced occasional hard but sudden results on the early November facet/crust combination down 100-190cm in treeline and alpine areas. Furthermore, favorable slab structure for step-down fractures can contribute to the persistence of this deep weakness. Meanwhile in the Okanagan, relatively warm temperatures are promoting a well consolidated 70-80cm treeline snowpack with isolated small thin wind slabs.

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.

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.