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Avalanche Forecast

Archived

Dec 5th, 2012–Dec 6th, 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 unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

South Columbia.

Confidence

Fair - Forecast snowfall amounts are uncertain on Thursday

Weather Forecast

Overnight and Thursday: Cloudy with some convective flurries in the west areas of the region. Mostly cloudy on Thursday. Some light precipitation during the day bringing 3-5 cms to higher elevations. Light westerly winds are expected.Friday: Mostly cloudy with cooler temperatures in the alpine and freezing levels down to the valleys. Some light convective flurries in the morning.Saturday: Cloudy with valley fog in the morning. Some sunny periods in the afternoon.

Avalanche Summary

Some small stiff wind slab releases were reported from explosive control during the storm on Tuesday. There is one report of a size 3.0 avalanche from very steep un-skiable terrain on a lee aspect in the northeast of the region in the Selkirks. There is also a report from the north of the region in the Monashees of a size 3.0 avalanche on a 30-40 degree slope NE aspect at 2200 metres that was 100-150 cms deep and ran on the early november rain crust.

Snowpack Summary

Very strong winds and warm temperatures on Tuesday created some stiff wind slabs in the alpine and open areas at treeline. The Selkirks and Eastern Monashees total snowpack depth has reached 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.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.