Avalanche Forecast

Archived

Apr 17th, 2013–Apr 18th, 2013

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

South Coast.

Confidence

Poor - Intensity of incoming weather is uncertain

Weather Forecast

Thursday: Cloudy skies with temperatures reaching -2 and freezing levels climbing to 1900m. Expect light westerly winds and a chance of isolated flurries with limited accumulations.Friday: Expect continued unsettled skies with more isolated flurries, temperatures around -1 and light westerly winds. Freezing levels may again reach 1900m.Saturday: Clouds linger with more intermittent flurries. Alpine temperatures should be around -3 with continuing light west-southwest winds.

Avalanche Summary

Numerous rider triggered and isolated remote and sympathetic events have been reported in high (2400-2700m) north and northeast facing slopes up to size 2.5, all associated with preserved surface hoar.

Snowpack Summary

North and Northeast aspects above 2000m continue to hold dry snow; otherwise, the surface condition is crusts. The carrying strength of the crusts varies depending on aspect and elevation as well as proximity to rocks, etc.... Generally speaking, the crusts have been going moist in the afternoon and then refreezing at night in all but the lowest terrain. The recent storm snow has settled into a cohesive slab between 30-60cm deep. At the interface of this storm snow lies the April 05 layers. On solar aspects, the layer is a strong melt-freeze crust, with some faceting observed above and below. Reactivity on this layer has been inconsistent, with occasional sudden results and some whumphing. On North and Northeast aspects above 2350m, this interface may be preserved surface hoar (up to 15mm). Where the surface hoar is present we have reports of sudden results in stability tests and have seen rider triggered, sympathetic and remote events up to size 2.5. Professional operators express consistent concern for triggering this layer.Cornices are huge and will continue to grow with the forecasted precipitation.

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.

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.