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Avalanche Forecast

Archived

Dec 11th, 2019–Dec 12th, 2019

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

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Expect heavy snowfall combined with strong winds to create unstable conditions. Slab avalanches may become easily triggered and widespread on high elevation slopes. This potent winter storm will drastically change the previous snow cover. 

Discussion

We have limited information about the current snow and avalanche set up, but heres what we know: Mission Ridge and Blewett Pass did well with a storm in late November, opening up opportunities to recreate on snow at upper elevations. As of this writing, average snow depths of 12" to 20" can be found in the area. That is set to increase, perhaps double over the next couple of days. Areas closer to the crest stand to pickup substantially more snow than the eastern edge of the Wenatchee Mountains. As such, the slabs may be thicker, and the danger greater.

Along with potential instabilities within the storm layer, the snowpack structure does harbor some weak snow. Many observers have noted a layer of large surface hoar in the Mission Ridge area last weekend. On the 11th, this layer was found to have been preserved under a few inches of snow near Mission Ridge. On December 8, a large natural slab was observed below the Microwave on an north aspect at 6,700ft. The same day, an avalanche course nearby observed some notable collapses on east aspects near 6,800ft. This is enough to warrant suspicion of persistent layers with our snowpack, and it bears watching into the weekend as this storm tapers off. 

Snowpack Discussion

Regional Synopsis Coming Thursday, December 12, 2019

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.