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Avalanche Forecast

Archived

Dec 9th, 2019–Dec 10th, 2019

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.

Regions

North Columbia.

Avalanche activity related to last week's storm has slowed down but several buried weak layers remain active. Use caution when pushing out into more complex terrain.

Confidence

Moderate - Uncertainty is due to the complexity of the snowpack’s structure.

Weather Forecast

Monday Night: Cloudy with clear periods, alpine low -12 moderate northwest wind.

Tuesday: Mix of sun and cloud, alpine high -5, moderate southwest wind.

Wednesday: Mainly cloudy with isolated flurries, alpine high -5, moderate west wind increasing to strong overnight.

Thursday: Flurries accumulating 5-10 cm, alpine high -5, strong southwest wind easing to moderate.

Avalanche Summary

On Sunday, neighboring Glacier National Park reported natural storm slab avalanches up to size 2.5 in steep south-facing gullies in the alpine, likely triggered by the first rays of sun to hit the recent storm snow. Natural storm slab activity has been in decline since the end of the storm. However, skier triggered storm slab avalanches up to size 2, and explosive triggered avalanches up to size 3 have been reported as recently as Monday. 

There have also been a few reports of persistent slab avalanches failing on a layer of surface hoar down 80-120 cm at treeline elevations within the past week. At least one of these was remotely triggered (from a distance). 

Snowpack Summary

30-60 cm of new snow from late last week has covered a layer of feathery surface hoar that has been found at all elevations. This surface hoar sits on soft snow that fell shortly before its formation (early last week) and was covered shortly after (mid last week). It may soon settle out, but for now we will continue to watch for it. A thick layer of faceted crystals, previously wind-affected surfaces, and/or surface hoar in sheltered areas (around treeline and below) is now 50-90 cm below the surface.

An additional layer of surface hoar may be found in sheltered areas around treeline down 70-120 cm. In some areas this may sitting on a thin crust. A variety of crusts from late October are buried deeper in the snowpack.

Problems

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.

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.