Avalanche Forecast

Issued: Dec 11th, 2019 10:18AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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Expect dangerous avalanche conditions on Thursday. Strong winds along with heavy snowfall will create unstable snow at upper elevations. This potent storm will drastically change the previous snow cover. Even small avalanches can cause injury if you take a ride through shallowly buried rocks, stumps or other ground hazards.

Summary

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

An icon showing Storm Slabs

Snowfall overnight into Thursday, along with strong westerly winds are likely to create unstable slabs as the day progresses. The most dangerous areas may be found closer to the crest and above 5,500ft, where more snow will accumulate. Exposed slopes at higher elevations will also experience more wind drifting. Below treeline, there is generally insufficient snow for avalanches, but that may change. You are likely to experience stormy weather, poor visibility, and quickly changing conditions in the mountains. If you do go out, look for signs of instability such as cracking, collapsing, and hollow sounding snow. It may be a good day to avoid going in to avalanche terrain altogether, steering clear from big features such as large bowls and chutes. 

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Dec 12th, 2019 10:18AM