Avalanche Forecast
Issued: Dec 18th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
This is a good time to be patient and keep your terrain very conservative. You can trigger avalanches in the new snow on open slopes greater than 35 degrees. Avalanches could step down into deeper snow layers causing very large and destructive avalanches.
Summary
Discussion
Reports of natural and triggered avalanches continue in the adjacent Mt. Baker area. Periods of clear skies over the weekend allowed observers to view evidence of a unique and impressive avalanche cycle. Numerous avalanches were very large and destructive (some could destroy a house). Several ran full track to valley bottom. Many had very wide propagation (up to ¼ mile). The majority of avalanches failed on a weak layer (surface hoar and/or facets) buried 4-6 ft deep.
Snowpack Discussion
Regional Synopsis: December 18, 2018
Why has the avalanche danger been so high for so long? Two reasons: A very active and wet weather pattern and widespread persistent weak layer.
For perspective, the approximate snow totals from 12/9-12/18 are:
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Mt Baker: 93”
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Washington Pass: 45”
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Stevens Pass: 66” mid-mountain
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Snoqualmie Pass: 59” mid-mountain
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Crystal Mountain 57” Green Valley
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Paradise: 70”
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Mt Hood Meadows: 37” mid-mountain
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Olympics: 20”
In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause in numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess … in short, the scenario is tricky and getting more dangerous by the day.
Higher snow totals along the Hwy 542 corridor /Mt Baker area have driven several avalanche cycles during this period. Loading from recent storms has been more incremental to the east and south and we have not yet experienced a widespread cycle …
When will we reach the breaking point? It’s hard to say. What we do know is we have a deep weak layer, reports of very large explosives triggered slides at Mission Ridge and Crystal Mountain, and more storms on the way.
Be patient and continue to stick to lower angle slopes with nothing above you. This is a good time to avoid areas where avalanches can start, run, and stop.
We’d like to thank all of you who have sent NWAC your observations. If you are out in the mountains, let us know what you see.
Problems
Storm Slabs
Heavy snow and wind loading have created large slabs storm slabs at all elevations above the rain/snow line. Near ridgelines, these slabs may be enhanced by the wind. Avoid open slopes steeper than 35 degrees and navigate around wind-loaded terrain features.
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: All elevations.
Likelihood
Expected Size
Deep Persistent Slabs
A concerning weak layer is buried under 3+ feet of new snow at mid and upper elevations. You or storm-related snow loading may trigger the avalanche, which will be very large and destructive. Thes snowpack has been loaded with 2-3" of water in the last 36 hours and it needs time to adjust to the load. These avalanches could break near the ground and act in surprising ways. You may be able to trigger these avalanches remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty and avoid large avalanche paths at all elevations. Watch for shooting cracks, collapses, and recent avalanches. Choose moderately angled and supported terrain away from overhead hazards. Recent snow may cover signs of recent instability on this layer.
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 or near the ground. 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. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 19th, 2018 10:00AM
