Avalanche Forecast
Regions: Cascades - West.
The first major snow event of the season continues to increase the avalanche danger. Temper your excitement, be conservative with your terrain choices, and ease into winter.
Discussion
So far this season, avalanche activity has been minor and infrequent due to low snow. This storm is bringing rapid change and creating dangerous avalanche conditions at upper elevations. The zone has already received anywhere between 6â to over a foot of new snow at upper elevations. Snow totals, along with the avalanche danger, will noticeably increase as you gain elevation.
Donât let the excitement of this first major winter storm distract you from making informed and conservative decisions. Be cautious if you decide to venture into the backcountry on Friday. Ease into terrain slowly and gather information along the way. Getting caught in even a small avalanche would be a rough ride with high consequences in these early season conditions. Expect to encounter shallowly buried objects and challenging travel conditions, especially below 5000ft where the new snow will fall on bare ground.
Snowpack Discussion
December 12, 2019
After a dry November, this week marks the beginning of more winterlike weather across the region with snow at middle and pass level elevations. While thereâs uncertainty in the weather forecast, it does appear that the snowpack will continue to build to some extent over the coming week.Â
Throughout much of the region, the slow start to winter has left avalanche conditions distinctly defined by elevation. Slopes above 5,500-6,000ft hold a layered snowpack that provides enough coverage for winter travel and avalanches. Below this elevation, most slopes were bare until the past week. At these lower elevations, it will likely take another round of storms before larger avalanches are possible and travel becomes easier.
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Image courtesy of the Natural Resources Conservation Services interactive map showing Snotel weather stations measuring well below normal snow water equivalent for this season so far.
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Upper Elevations
The lack of significant snowfall has resulted in commonalities throughout the region. Slopes above about 5,500ft currently hold the âdeepestâ snow cover and the most layered snowpack. Until the lower elevation terrain builds a more substantial snowpack, the upper elevations will hold the most potential for producing large avalanches. If you dig into the snow in these areas, youâll find a range of height of snow and a variety of layers. Here are a few layers to note:
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The interface of older snow and any new incoming snow would be the first interface to check.
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Snowfall around December 7th and 11th may have buried surface hoar and near-surface facets in some locations. As of Dec 12th, these interfaces can be found 1-2â below the surface.
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A layer of facets can be found near the middle of the snowpack, buried just before Thanksgiving. Where found, the facets are often rounded or have even undergone some melt-freeze metamorphism from liquid water.
While these layers give you something to look at in snow profiles, they may not be your main snowpack concern for the day. Continue to check the daily zone forecasts for the most up to date avalanche conditions. Weâll monitor these layers as future weather brings changes to the snow and avalanche conditions.
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A layer of facets in the middle of the snowpack resulted in sudden test results on Dec 11th. Rock Mtn, N, 6270ft. Photo: Josh Hirshberg.
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Middle and Lower Elevations
At most locations below 5,500ft, slopes are still building uniform snow cover. In many zones, the hazard of hitting rocks or shallowly buried objects may be more significant than the avalanche danger. Depending on future weather, the snowpack could continue to form or could, unfortunately, melt out to the ground. While thereâs little layering of note at these elevations, avalanches arenât completely out of the question with the right weather input. Further low elevation snowfall or warming could drive avalanche activity. The East North forecast zone, including Washington Pass, has more low elevation snow than other zones and therefore more potential for avalanches at these elevations. As with the upper elevations, weâll wait and see what the next round of weather brings.
Avalanche Problems
Storm Slabs
Heavy snowfall will build fresh storm slabs near and above treeline on all aspects. Avalanches are most likely where new snow falls on upper elevation slopes with pre-existing snow cover. Wind-loaded terrain will develop thicker and more reactive slabs. The new snow may not bond well to old snow surfaces, and mid-storm layers may be problematic as well. Watch for shooting cracks, collapses, and recent avalanches. It may be a good day to avoid going into avalanche terrain altogether, steering clear of big terrain features like 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