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Avalanche Forecast

Archived

Mar 12th, 2012–Mar 13th, 2012

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
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.

Regions

Northwest Coastal.

Confidence

Fair - Intensity of incoming weather is uncertain

Weather Forecast

Tuesday: Light snowfall - strong to extreme southwest winds - freezing level at 200m Wednesday: Very light snowfall - moderate to strong south winds - freezing level at 400m Thursday: moderate snowfall - moderate to strong southeast winds - freezing level at 700m

Avalanche Summary

A skier released a size 1 hangfire slab (overburden from a previous avalanche) in the northern part of the region on Sunday. The avalanche occurred on a south aspect at 900m and is thought to have reacted on the early February interface. Most other relevant avalanche activity reported was from a natural storm cycle that occurred a few days ago.

Snowpack Summary

Continued strong and variable winds have redistributed light amounts of recent storm snow into thin wind slabs that most commonly exist at treeline and in the alpine. In the northern part of the region two persistent weak layers are on the radar of some operators: Surface hoar buried at the beginning of March is as much as 70cm deep. The early February persistent weak layers (surface hoar, facets, crusts) are over a metre down. Although not widespread throughout the region, persistent slabs would be destructive in nature and may be reactive to skier triggers, particularly below treeline on isolated and sheltered steep terrain where buried surface hoar may be preserved. Cornices in the region are reported to be very large and unstable.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.