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Avalanche Forecast

Archived

Dec 16th, 2012–Dec 17th, 2012

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 unlikely, human triggered possible.
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, human triggered possible.

Regions

Northwest Coastal.

Confidence

Fair - Due to limited field observations

Weather Forecast

Synopsis: On Monday & Tuesday the region will see a cool unstable onshore flow with light SW winds, light snowfall with no significant accumulation, and freezing level near valley bottom. The next system is not arriving until sometime Wednesday when a Gulf of Alaska Low starts sliding south off the coast.

Avalanche Summary

In areas with deeper storm snow accumulations (for example near Stewart) natural avalanches to size 2.5 were reported; otherwise no new naturals reported but visibility was limited.

Snowpack Summary

Storm snow accumulations in the north and near the coast (for example the mountains near Stewart) are as high as 65 cm; away from the coast there was much less. For example near Shames it was closer to 20 cm of storm snow but the total snowpack depth only added 5 cm due to settling. The upper snowpack should be mostly settling powder depending on elevation. I expect wind slabs or wind affected snow in higher elevation exposed areas. Deeper in the snowpack professionals continue to monitor a crust down near the ground. It seems this problem is confined to  alpine areas and likely needs a large trigger and / or a shallow snow pack feature to trigger.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.