Avalanche Forecast

Archived

Nov 25th, 2011–Nov 26th, 2011

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 and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

Sea To Sky.

Confidence

Fair - -1

Weather Forecast

Up to 30 cms of snow is expected for saturday accompanied by strong southwesterly winds. Freezing levels are expected to rise to 1500m. The system will continue through sunday bringing another 20-40 cms of snow. Winds are expected to remain strong but switch to north/northwest with freezing levels dropping to surface late in the day. A ridge should develop through sunday evening bringing mostly cold and clear conditions for monday.

Avalanche Summary

Explosives control in the region has produced numerous size 2 and 3 avalanches suspected to have stepped down to both of the facet/crust interfaces. Recent weather has limited the ability to make field observations, but there has probably been wide-spread natural activity as well due to recent snow and wind.

Snowpack Summary

The snowpack sits at approximately 150 cms just below treeline. At treeline and above, expect variable snow depths with much deeper wind deposits. In the lower snowpack there is a crust/ facet layer just above the ground that has been reactive to explosive testing in the region. In the mid snowpack, there has been widespread surface hoar observed at all elevations although reactivity on this layer seems to be decreasing. A more persistent weakness of facets with an associated crust can be found in the mid snowpack from 40-70 cms above the ground. This weakness exists at treeline and into the alpine where it is sensitive to human triggers and has a high propensity to propagate fractures. There's been over 120 cms of new snow since Sunday. Fluctuating freezing levels and varying wind and precipitation intensity during the recent storms has probably resulted in various storm snow weaknesses.

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.

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.