Avalanche Forecast
Regions: Olympics.
Avalanche danger has increased quickly with significant snow and transport near ridgelines. Travel cautiously along ridges, where recently formed cornices can fail easily and break farther back than you expect. Enough new snow sits on the dry ground that storm slabs are possible to trigger on smooth slopes.
Discussion
On Thursday (12/12), NWAC and NP professionals were able to trigger a sensitive cornice near a 5000 ft ridgeline, which produced a 1 ft deep slab on the slope below. Although the slab didn't run far, it made the observers travel cautiously.Â
They found 10-14" of new snow, sitting above 5 inches of decomposing snow (deposited Saturday), capped by a melt-freeze crust (formed Sunday). The new snow contained a minor density inversion which may be a factor for triggering storm slabs on Friday.
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 there’s 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 â€deepest†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, you’ll 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 1-2’ 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. We’ll 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 there’s little layering of note at these elevations, avalanches aren’t 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, we’ll wait and see what the next round of weather brings.
Avalanche Problems
Cornices
Small cornices grew rapidly with moderate winds on Thursday and these may be large above treeline. They were sensitive, reactive, and 1-1.5 meters in size on Thursday and we expect this will continue on Friday. Travel cautiously along ridges and avoid traveling along slopes exposed to actively loading cornices. Cornices also indicate wind-loaded slopes below that you can trigger as a wind slab or may be triggered if you release a cornice.
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
Enough new snow has fallen to produce a large avalanche. However, the prevalence of terrain anchors still prevents this from becoming a widespread problem. Avoid smooth looking slopes steeper than 35 degrees or slopes where additional wind-loading increases the thickness of the slab; these are the places where you are most likely to trigger an avalanche.
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: Possible
Expected Size: 1 - 1