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Avalanche Forecast

Archived

Dec 25th, 2011–Dec 26th, 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 and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.

Regions

Northwest Coastal.

Confidence

Poor - Timing, track, or intensity of incoming weather is uncertain

Weather Forecast

A series of frontal systems will affect the region over the next several days bringing periods of moderate to heavy precipitation and very strong winds. Monday: 20-30cm of snow. Very strong S-SW winds. Freezing level (FL) rising to around 1000m. Tuesday: 10-15cm. Moderate to strong SW winds. FL around 1000m. Wednesday: 10-25cm. Strong SW winds. FL rising to 1500m.

Avalanche Summary

Expect widespread natural avalanche activity throughout the forecast period with forecast snow and wind. Several natural slab and loose snow avalanches up to Size 2.5 were observed at lower elevations around Shames and west of Terrace on Saturday. Recent observations have been limited to below treeline.

Snowpack Summary

Over the past week the northwest has been slammed by snow, strong to extreme winds, and fluctuating freezing levels. An additional 25-30cm on Saturday has brought total snowfall over the past week to almost 150cm near Terrace. Locations to the north have seen a little less (Stewart up to 90cms). 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.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.