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Avalanche Forecast

Archived

Dec 29th, 2011–Dec 30th, 2011

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

Regions

Northwest Coastal.

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertainfor the entire period

Weather Forecast

Friday: Light snowfall, perhaps 10-15cm accumulation. Moderate westerlies.Saturday: Light snow. Freezing level near surface. Moderate westerlies. Sunday: Moderate precipitation. Freezing level rising to 1400m in the afternoon.

Avalanche Summary

Highways avalanche control produced numerous Size 2-3.5 slab avalanches. The largest of these failed at ground, up to 4m deep.

Snowpack Summary

Over the past week the northwest has been slammed by snow, strong to extreme winds, and fluctuating freezing levels. The week's tally is closing in on 200cm near Terrace, with areas to the north seeing a little less (Stewart up to 120cm). New snow has been blown around by strong south to southwest winds creating wind slabs on lee slopes and scoured windward slopes.In addition to the more obvious direct-action storm instabilities, local avalanche professionals have some other concerns: Surface hoar that formed during the winter solstice sits approximately 50-70cm below the surface. As well, the crust-facet combo (extends up to alpine elevations in the south and to 1000m in the north) from the early December dry-spell sits about 160cm below the surface and has not gone away. Any avalanches on this layer would be highly destructive and are probably waiting for the right load or trigger. Add the new observations of avalanches failing at ground and we have quite a package.

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.