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

Regions

Purcells.

Confidence

Good

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 -10 and freezing levels 1100 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 ranges. Ridgetop winds light from the West. Alpine temperatures near -8 and freezing levels at 1400 m, falling to valley bottom overnight.Thursday: Generally clear and dry. Ridgetop winds light from the West. Alpine temperatures near -7 and freezing levels 1600 m the dropping overnight.

Avalanche Summary

Over the weekend the region saw a widespread large natural avalanche cycle, with avalanches up to size 3.5. On Sunday, numerous natural slab avalanches were reported up to size 2.5 on a a variety of aspects. Explosive avalanche control in region revealed several large slab avalanches up size 3.5.On Monday, explosive avalanche control produced a size 2.5 on a West aspect around 2900 m. No natural avalanche activity has been reported.

Snowpack Summary

Over the past week 60-80 cm snow fell. This new snow added to an ongoing storm snow instability, particularly on the west side of the region where accumulations have been highest. Buried surface hoar and sun crust layers exist up to 80cm below the surface and seem most prevalent on the west side of the region. Large avalanches initiated on these weak layers during the weekend storm. Large looming cornices exist on ridgelines and may become weak, especially when the sun comes out. Cornice fall could trigger slab avalanches on slopes below.A surface hoar layer buried on January 23 is still on the radar of some operators. This layer has been less likely to trigger by skiers, but may still be sensitive to large loads like avalanches in motion or cornice fall.Below treeline exists a melt-freeze crust which may be bridging instabilities deeper in the snowpack. If temperatures warm, you'll notice the crust breaking down and the snowpack becoming weaker.

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.