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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

Snoqualmie Pass.

Conditions are changing in the mountains. The first major snow event of the season will increase avalanche danger and cover the existing thin snowpack or bare ground. Temper your excitement, be conservative with your terrain choices, and ease into the winter.

Discussion

We have little information on the Snoqualmie Pass area prior to this storm, but expect most issues to occur where new snow is falling on existing snow. Smooth slopes were recently observed above treeline along with some recent small avalanche activity. Recognize that until this storm, very low snow conditions existed, and early season hazards will persist despite the new snow. Be conservative and ease into terrain by starting with small test slopes and evaluating what is happening with the snow. Tomorrow will be stormy with low visibility, it is not a good day to travel under features that you cannot see the top of.

This storm has the ability to change the avalanche danger quickly over space and time. With periods of high precip rates and fluctuating freezing levels, pay attention to how the storm snow sets up. Is it rightside up or upside down? Are there interfaces within the storm snow as winds or precip rates change? As the snow piles up, a day that starts out as moderate can develop into considerable quickly. Anchors will become buried, and slab depth will increase throughout the storm. Pay attention to wind affected snow and precipitation changes between rain and snow.

Low snow conditions in the Alpental Valley prior to the storm. Photo: Jeremy Allyn 12/9/19

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.