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Avalanche Forecast

Archived

Mar 4th, 2013–Mar 5th, 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, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Kootenay Boundary.

Confidence

Fair - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Tonight and Tuesday: A weak ridge will remain over interior ranges with generally dry conditions. Ridgetop winds will blow moderate from the South. Light snow amounts. Alpine temperatures near -8 and freezing levels 900 m.Tuesday night and Wednesday: A low pressure system will move inland. Most of this system will remain South of the border, but some cloud and light precipitation will spread into the Southern Interior. Ridgetop winds will be light from the South. Alpine temperatures near -6 and freezing levels at 1400 m, falling to valley bottom overnight.Thursday: Generally clear and dry. Ridgetop winds light from the SW. Alpine temperatures near -6 and freezing levels 1400 m and then dropping overnight.

Avalanche Summary

Numerous natural slab avalanches were reported on Saturday. These occurred mainly on N-E aspects, above 2200 m and ranging from size 1.5-2.5, failing within the recent storm snow, and suspect of failing on older surface hoar layers buried down 50-60 cm. Explosive control initiated numerous size 1.5-2.5 slab avalanches on SE-SW aspects above 1600m.No new avalanche observations on Monday.

Snowpack Summary

In much of the region, up to 80 cm recent snow overlies older interfaces comprising of surface hoar and sun crusts. The storm slab above these interfaces has the potential for wide propagations and surprisingly large avalanches.Wind slabs and new cornice development exist at ridgelines and on lee slopes. These are likely sensitive to human triggers. Cornices may become weak under the influence of the sun, and could trigger slabs on slopes below.A strong melt-freeze crust exists at 1700 m and below and is generally bridging any deeper instabilities. The average treeline snow depths sit near 250 cm.

Problems

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.

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.