Avalanche Forecast

Archived

Apr 5th, 2017–Apr 6th, 2017

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells.

Cornice falls have been responsible for most of the avalanches reported in the past few days. Wind slabs or cornices may trigger persistent weak layers resulting in large avalanches.

Confidence

High -

Weather Forecast

Overnight: A couple of cm of new snow with moderate southwest winds and freezing down to 1000 metres. Thursday: 3-5 cm of new snow with strong southwest winds and daytime freezing up to 2000 metres. Friday: Re-freeze down to 1200 metres, followed by a mix of sun and clouds with 3-5 cm of new snow. Saturday: 5-10 cm of new snow with light winds and a cooling trend that should start to bring freezing levels down.

Avalanche Summary

On Tuesday there was a natural cornice fall near Golden that did not release a slab on the slope below. On Monday there were several natural cornice falls up to size 2.5 that released storm or persistent slabs from the northerly aspect slopes below. On Sunday there was a natural cornice fall size 2.5 from extreme terrain.

Snowpack Summary

10-15 cm of recent snow now overlies temperature crusts below about 1900 metres and sun crust all the way into the alpine on solar aspects. This precipitation fell as rain below about 1600 metres. Below the new snow interface, storms over the past week brought 40-60 cm of snow to the region. Several other crusts as well as moist snow are likely to exist within this storm snow, mainly at lower elevations and on solar aspects. Moderate to strong southwest winds during and since the storm formed wind slabs on leeward slopes as well as fragile cornices along ridgelines. At higher elevations, the February crust/facet layer is now down around 130-150 cm and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These layers were active during a storm in mid-March and produced some very large avalanches. Occasional deep releases were also reported in late March and these deeply buried weaknesses remain a serious concern as solar radiation and warming temperatures begin to penetrate the snowpack at increasingly higher elevations.

Problems

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.