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Avalanche Forecast

Archived

Dec 25th, 2011–Dec 26th, 2011

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

Regions

Kootenay Boundary.

Confidence

Fair - Intensity of incoming weather is uncertain

Weather Forecast

Monday: Light snowfall. Moderate south-westerly winds. Freezing level near valley bottom. Tuesday: 5-10cm. Freezing level around 1000m, rising later in the day. Strong westerly winds. Wednesday: 15-20cm, with the freezing level rising as high as 2000m. Strong westerly flow.

Avalanche Summary

Several natural and skier-triggered size 1 soft slabs were observed on Saturday. In uncompacted terrain, these were failing on the surface hoar interface buried in mid-December.

Snowpack Summary

Up to 20cm of new snow is bonding poorly to the old surface. A surface hoar layer from mid-December around 30cm down is giving sudden test results. Avalanche activity on the mid-December buried surface hoar layer has increased in the South Columbia region over the last two days as critical amounts of snow have been reached. I expect to see this trend of rising avalanche occurrences and increased size/consequence reach the Kootenay Boundary region soon. In some parts of the region, it's probably already occurring. Soft and hard wind-slabs can be found on certain slopes in the alpine and around treeline. Facets at the base of the snowpack could create a deep persistent slab problem that deserves attention in shallow snowpack areas on planar rocky slopes.

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.