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Avalanche Forecast

Archived

Mar 6th, 2013–Mar 7th, 2013

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

Regions

Kootenay Boundary.

Confidence

Good

Weather Forecast

As the low off the Oregon coast moves inland and South, light-moderate snowfall amounts are expected through Thursday. The low deteriorates by Friday morning and is being replaced by a ridge, bringing dry, sunny conditions with very little wind. Overnight into Thursday: Light-moderate snow accumulations. Ridgetop winds will blow light from the East. Alpine temperatures near -4 and freezing levels at 1300 m.Friday and Saturday: Ridgetop winds will remain light from the NE switching to the West on Saturday. Alpine temperatures near -4 and freezing levels rising to 1500 m in the afternoon then falling to valley bottom overnight.

Avalanche Summary

On Tuesday a couple of skier triggered avalanches occurred on Southerly aspects above 2000 m. These were reported to fail on the early March facet/crust interface which currently has about 25 cm of snow sitting above it. No new natural avalanches have been reported.

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 when the sun comes out on Friday.A strong melt-freeze crust exists at 1700 m and below and is bridging deeper instabilities. If temperatures warm this crust will likely break down causing the snowpack will weaken. 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.