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Avalanche Forecast

Archived

Dec 12th, 2018–Dec 13th, 2018

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

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Recent new snow and wind have covered a pronounced weak layer, creating a recipe for dangerous avalanches conditions. Safe travel in avalanche terrain will require a cautious approach and conservative decision making.

Discussion

Avalanche Summary

The first 9 days in December were clear and cold. Surface hoar grew large during this time, and facets developed near the surface of the cold, low density snow. On December 9th this layer was covered up and preserved on many aspects and elevations. On December 12 an observer reported shooting cracks at Mission Ridge. In adjacent regions numerous avalanches and signs of instability were observed on December 11th. A professional observer reported a large natural avalanche in the Spire Gully on Washington Pass with an impressive powder cloud. Near here, the observer reported triggering avalanches remotely (from a distance) and widespread collapses on the old snow layer near the hairpin on Hwy 20. Test results indicated the potential for wide propagation on this layer of buried surface hoar and near surface facets.

You are likely to trigger an avalanche that will break on the weak, old snow interface which can be found one to two feet down, and could surprise you. Look for signs of instability such as whumphing, shooting cracks through the snow, and recent avalanches. 

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.

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.