Avalanche Forecast
Regions: Stevens Pass.
Significant winter storms are continuing to create very dangerous avalanche conditions. Stay out of areas where avalanches can start, run, and stop. Avalanche danger will rise and peak during times of heavy snow and warming temperatures.
Discussion
During the day on December 11th, professional observers reported natural, and remotely triggered avalanches within the storm snow and on an old snow layer. These were on a variety of aspects and as low as 4700ft. On December 12, professional observers reported whumphs and collapses. Some of these collapses propagated long distances through a weak layer in the snowpack. With another round of heavy snowfall, a warming trend and strong winds, we may tip the scales for very large avalanches on this layer.
Since December 10, the total snowpack depth has more than doubled on Stevens Pass. 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. Currently, this layer is two to three feet down in the snowpack and tests are showing that if triggered, it will likely propagate.
Snowpack Discussion
Regional Synopsis 20181211
Welcome winter!!!
The NW snowpack is evolving quickly and a near constant pattern of winter storms with fluctuating freezing levels is making things interesting for our 1st week of forecasting.
Here’s what we know:
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Tuesday saw a significant avalanche cycle.
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We have a lot of new snow...2-3’ above 4500ft.
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We’ve gotten a lot of wind.
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We have weak layers near the ground.
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We are going to get more snow before the weekend.
All in all that’s enough to indicate that dangerous avalanche conditions will persist over the next couple of days. That said, it is early season and the flavor of avalanche you may run into is likely a function of elevation, timing, and dumb luck.
Here are some basic emerging patterns:
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Storm total (Monday morning to Wednesday evening):
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Mt. Baker: 24â€
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Washington Pass: 22â€
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Snoqualmie Pass: 10â€
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Paradise: 17â€
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Mt. Hood Meadows: 10â€
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Upper versus Lower Elevations: The change in the snowpack is still pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile.
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East versus West: Loading along the East slope has been more incremental, and a  variety of buried facet and surface hoard layers may be found. This is most pronounced near WA Pass. Although significant snowfalls and precip totals have resulted in thicker, more homogeneous snowpack in the western zones, lingering weak layers near the ground will persist through the week.
Avalanche Problems
Storm Slabs
Natural activity within the new snow is likely to spike with the heaviest precipitation rates on Thursday morning. This will coincide with a rapid warmup. In addition, slabs from three to four feet thick are sitting on old weak snow. It is uncertain whether or not this storm will overload this layer to a breaking point, but it may. Any failure on this deep, weak layer would be surprising and scary. It will be prudent to avoid avalanche terrain and avoid entering terrain with overhead hazard.
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: Likely
Expected Size: 1 - 1
Loose Wet
At lower elevations, the danger from loose wet avalanches will rise in the morning as a result of rising freezing levels. Stay out from underneath steep slopes if you are noticing roller balls, very wet surface snow, and of course fan shaped avalanches!
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1