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Avalanche Forecast

Archived

Mar 25th, 2013–Mar 26th, 2013

Alpine
Natural avalanches possible, human triggered probable.
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.

Confidence

Fair - Timing or intensity of solar radiation is uncertain on Tuesday

Weather Forecast

The ridge of High pressure is expected to continue to influence the interior ranges for the next few days. As the ridge weakens we may see high cloud reducing the exposure to strong solar radiation. Nights are expected to continue to be clear and cool. Freezing levels in the North should rise during the day to about 1800 metres, and to about 2000 metres in the South. Some light precipitation may start to move in from the coast on Thursday afternoon.

Avalanche Summary

Natural and explosive cornice releases are a major theme. Explosives control in several parts of the region resulted in some large cornice falls that released some slab avalanches in the tracks that were restricted to the recent storm snow.A size 3 avalanche taking out the whole basin was triggered by snowmobilers Saturday in Hell Roaring Creek, just south of the region resulting in one fatality. Details are limited at this time; it may be indicative of the persistent slab problem (surface hoar and/or crust buried about 70 - 120 cm) we continue to describe. There was also a skier accidental size 2.0 avalanche on Sunday in an adjacent region (Glacier National Park), that resulted in one fatality.

Snowpack Summary

Moist snow may be found on solar aspects up to about 2000 metres; surface faceting elsewhere. New melt/freeze crusts are developing due to cold clear nights and periods of strong solar radiation during the day. The new storm slab is about 30-50 cms deep, with wind transported pockets that are about 70 cms deep. Downflowing or katabatic winds have created small slabs in gulley and atypical low alpine and treeline elevations. Recent cornice growth is reported to be poorly bonded and breaks off easily with light triggers. A weak layer of surface hoar and/or a crust is buried down about 70-120 cms in the alpine and treeline elevations; this layer continues to produce "sudden" results. This surface hoar layer is producing variable results in snow profile tests, and continues to be a concern for professionals.

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.

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.

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.