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Avalanche Forecast

Archived

Dec 27th, 2011–Dec 28th, 2011

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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

Kootenay Boundary.

Confidence

Fair - Forecast snowfall amounts are uncertain

Weather Forecast

Tuesday night: 20-30cm. Wednesday: 5-15cm. Freezing level rising to 1500m. Strong south-west winds. Thursday: Moderate snow. Strong south-west winds. Friday: Generally drier conditions. Weather forecasters have low confidence at the moment (read more in the Forecaster's Blog). Be prepared for local variations in weather, and assume the avalanche danger is elevated in areas with enhanced snow, rain or wind.

Avalanche Summary

A natural and skier-triggered avalanche cycle with slabs up to size 2 was observed over the last couple of days. These were failing on the mid-December surface hoar on north through east aspects from 1700m to the peaks. Wind loading from the south-west overnight Sunday led to naturally-triggered wind-slabs on lee slopes on Monday morning. Touchy conditions are likely to continue through the forecast period and avalanches are likely to increase in size as snow-loading builds.

Snowpack Summary

Up to 30cm of new snow has been redistributed by westerly winds into touchy wind-slabs on lee slopes. Two buried surface hoar layers are causing concern in the upper snowpack. The most recently buried of these is only about 10-15cm deep but is reactive in snowpack tests. The second, a buried surface hoar layer from mid-December around 30-40cm down is now highly reactive, with enough snow above it to put it in the prime range for human-triggering (especially in areas which have received enhanced snowfall. It may take a little bit longer for the Rossland Range to get to this point). It is well-preserved with 10-15mm crystals and is most reactive well below ridges in protected terrain. The mid-pack is generally well settled. Facets exist at the base of the snowpack.

Problems

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.

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.