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Avalanche Forecast

Archived

Dec 21st, 2011–Dec 22nd, 2011

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

Regions

Purcells.

Confidence

Good - -1

Weather Forecast

Thursday: Expect dry conditions with mostly clear skies. Winds should shift from northerly to westerly and reach 40-60 km/h at ridgecrests. Freezing levels may reach 1300m. Friday: Dry conditions persist with continued westerly winds and freezing levels reaching 1500m. Saturday: Winds turn southwest and increase to strong values at ridgecrest with freezing levels climbing as high as 1800m.

Avalanche Summary

Isolated reports of windslab avalanches in immediate lee locations. Due to the well-developed nature of the surface hoar layer, touchy avalanche conditions will persist where the slab is sufficiently deep and avalanches have not already run.

Snowpack Summary

The Purcell region has been getting less snow in the recent storms than the Selkirks. This means in most areas we have not reached critical threshold on a layer of large surface hoar and near surface facets that was buried mid-December. An exception to this is in the Dogtooth range, where avalanches to size 2 have been releasing on this layer. In most areas the mid-December surface hoar layer currently lies about 20cm below the snow surface. Some areas have a sun crust in the alpine on steep south and west aspects. There is a rain crust that is buried between 40-55 cm in some areas that reaches up to about 2200 metres. Lower in the snowpack, the mid-layers are well-consolidated and strong. Near the base of the snowpack there are a few layers that have the potential to wake up with a really big storm or very heavy loads. These include a surface hoar layer from early November, a crust/facet combo from October and the interface on steep glaciated terrain with snow that did not melt over the summer.

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.

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.