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Avalanche Forecast

Archived

Feb 22nd, 2013–Feb 23rd, 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 possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

Confidence

Fair - Intensity of incoming weather is uncertain

Weather Forecast

Synopsis: There is a brief break between weather systems on Saturday before the next frontal system arrives late Saturday and into Sunday.  Saturday: Cloudy with light snow and possible sunny breaks. The freezing level is around 400 m and winds ease to moderate from the southwest. Sunday: Moderate snowfall – 5-15 cm. The freezing level is around 700 m and winds are moderate to strong from the southwest.Monday: Cloudy but drier. The freezing level remains around 500 m and winds are light from the west.

Avalanche Summary

There are no new reports of avalanches; however, observations have been limited due to poor visibility and weather. I expect natural avalanches in steep lee terrain and the potential for human triggered avalanches.

Snowpack Summary

10-15 cm of new snow has fallen in the past 24 hours. This new snow was accompanied by strong southwest winds forming wind slabs in exposed terrain in lee of ridges, in gullies, and behind terrain features. There are a variety of old interfaces now down 40 - 100cm which include facets, crusts and isolated pockets of surface hoar. There is very limited information about the nature of this interface. I would stress the importance of digging down to find and test weak layers before committing to larger and steeper slopes.A strong mid-pack currently overlies a weak base layer of facets/depth hoar. It is worth noting that the snowpack in general is quite shallow compared to averages; triggering the basal weakness may still be possible from thin spots, rocky outcrops or under the weight of larger triggers such as cornice fall.

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.