Avalanche Forecast

Archived

Jan 3rd, 2012–Jan 4th, 2012

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 avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Sea To Sky.

Confidence

Good - -1

Weather Forecast

Wednesday: Continued very heavy precipitation with up to 60-80cm of new snow expected for immediate coastal areas, but less as you move inland. Freezing levels as high 1700m but dropping throughout the day, and strong to extreme southwesterly winds. Thursday: another 10-20cm possible but tapering off throughout the day. Freezing levels as high as 1000m and moderate to strong southwesterly winds. Friday: Relatively dry, cold and calm.

Avalanche Summary

Persistent slabs and wind slabs are primed for human-triggering and heavy loading from snow wind and rain will likely cause natural avalanches. Three separate serious incidents that are relevant for the Sea-to-Sky region occurred on Monday afternoon in the Duffey Lake and Southern Chilcotin areas of the South Coast Inland region. All of incident sites had similar terrain characteristics with north through east facing steep treeline features. Two of the avalanches likely involved the mid-December persistent weakness, while the third was reported to have released on basal facets in a shallow snowpack area with a 50-75cm depth. All of the avalanches resulted in serious injury and at least two of them were human-triggered.

Snowpack Summary

The snowpack structure is highly variable and complex. Gusty winds are keeping wind slabs and cornices fresh and weak. Slabs overlaying various old snow surfaces, including crusts, facets, and surface hoar, buried mid-December are primed for avalanches in many parts of the region. The surface hoar is lurking generally down 80-100cm in sheltered treeline areas and below. In exposed treeline and alpine areas, weak facets with associated crusts are down generally 80-120cm, but wind-loading has resulted in highly variable slab thicknesses. A recently buried thin hard rain crust that extends into alpine elevations is also providing a poor bond to the overlying slab. Basal facets remain a concern in shallow snowpack areas, especially where the snowpack is overlaying summer firn on slopes that have already produced deep persistent slab avalanches and have been reloaded.

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.