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Avalanche Forecast

Archived

Mar 20th, 2013–Mar 21st, 2013

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
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.

Regions

Little Yoho.

May see another 10-20 cm of snow tonight with the possibility of some reverse loading onto Southerly aspects towards the end of the storm.  Keep to conservative route choices.

Weather Forecast

A system tonight could deliver another 10-20 cm with moderate west wind creating new wind slabs and adding to existing slabs.  In some areas, this could be enough to overload existing weaknesses and another natural cycle may result.  Winds will flip to moderate NW as the system passes and temperatures will cool.  Skies will clear for the weekend.

Snowpack Summary

Over the past week, 60-80 cm of snow has fallen in the region with strong West winds.  The storm snow is starting to bond to underlying layers West of the divide, but still may be of concern steeper features where the interface is a suncrust.  East the divide, storm snow is a cohesive slab resting on a weak faceted snowpack.

Avalanche Summary

Yesterday a size 2.5 was noted in Redoubt bowl (near Lake Louise ski area) with a 400 m wide propagation with a crown 60-80 cm thick. Generally, natural activity has tapered, only a few naturals to size 2 were reported in the area yesterday in steeper alpine lee terrain.  Skier triggering of recently developed slabs remains possible to likely.

Confidence

Intensity of incoming weather systems is uncertain

Problems

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.

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.