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Avalanche Forecast

Archived

Dec 4th, 2012–Dec 5th, 2012

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

Regions

Northwest Coastal.

Confidence

Poor - Due to limited field observations

Weather Forecast

An upper level trough will influence an onshore flow with several embedded systems. These systems will bring light-moderate amounts of precipitation. Winds will generally stay light and freezing levels will remain in valley bottoms through the forecast period.Wednesday: Snow amounts 10-15 cm, ridgetop winds SW 25 km/hr, alpine temps near -8.Thursday: Snow amounts 5-10 cm, ridgetop winds SW 20 km/hr, alpine temps near -10.Friday: Snow amounts 8-15 cm, ridgetop winds SW 10 km/hr, alpine temps near -10.

Avalanche Summary

On Monday a size 3 natural icefall avalanche was reported. Several natural sluffs size 1-1.5 initiated from steep terrain in below treeline elevations. Explosive avalanche control work also saw size 1-2 slab avalanches from North aspects, all failing in the recent storm snow. On Sunday in the Shames area, a fairly large (no size was listed) natural avalanche had was seen on a North aspect initiating from the alpine just below the ridgeline. A couple skier triggered size 1.5 slab avalanches were reported from the same group skiing down from the Dome. The slab initiated as the second skier made a turn over a convex slope. The slab was 20-40 cm deep, 20-30 m wide and ran approx. 50 m down slope. For more information and observations from the area, please check out the Regional Forum Posts.

Snowpack Summary

Up to 50 cm of new storm snow, accompanied by strong NW-SE winds has fallen over the region in the past 3 days. Wind slabs continue to dominate the upper snowpack. The snow surface conditions are variable with hard slabs, wind slabs and areas that are scoured. Recent test results done in the upper storm snow show an easy (RP) shear down 20-25 cm and a hard (RP) down 80 cm. The mid-pack is gaining strength and is well settled.Digging down deeper is the early November facet/crust layer. This crust sits near the base of the snowpack, and tests done in the Bear Pass area around 1100 m have shown this layer to be unreactive. Testing done in the Shames area on this interface have also shown no results, with moist snow below.Total snowpack depth above 1000 m is 150-200 cm deep. Below 1000 m the snowpack shows a sharp transition from 100 cm dropping to 50 cm, and is generally below threshold.

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.