Avalanche Forecast

Archived

Dec 28th, 2011–Dec 29th, 2011

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 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.

Regions

Northwest Coastal.

Confidence

Poor - Timing of incoming weather is uncertain

Weather Forecast

Wednesday night and Thursday: Another system arrives bringing 20-30cm of snow, tapering off on Thursday afternoon. The freezing level drops from 1000m to valley bottom overnight. Winds are strong from the SW. Friday and Saturday: Mainly cloudy with light snowfall (maybe 5-10cm each day). Freezing level at valley bottom. Winds are light to moderate from the SW.

Avalanche Summary

Highways avalanche control produced numerous Size 2-3.5 slab avalanches on Tuesday. I suspect there was a fairly widespread avalanche cycle in backcountry areas on Monday and/or Tuesday, and this cycle may continue for another day or two as another system is forecast to bring moderate to heavy snow and strong winds again tonight.

Snowpack Summary

Over the past week the northwest has been slammed by snow, strong to extreme winds, and fluctuating freezing levels. The weeks tally is closing in on 200cm near Terrace, with areas to the north seeing a little less (Stewart up to 120cms). Alpine observations have been limited and wind sensors have been disabled by rime, but I suspect new snow has been blown around by strong southwest winds creating wind slabs on lee slopes and scoured windward slopes.In addition to the more obvious direct-action storm instabilities that are expected with the forecast weather, local avalanche professionals have some other concerns: Surface hoar that formed during the winter solstice sits approximately 40-60cm below the surface and may become more reactive with more wind and snow. 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 150cm 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.

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.