Avalanche Forecast
Regions: Mt Hood.
Dangerous avalanche conditions continue on Mt Hood. Strong winds on Thursday night will quickly build new slabs that you can trigger near and above treeline. Early season conditions still exist and even small avalanches can be dangerous due to shallowly buried rocks, open creeks, and other hazards.
Discussion
A strong storm brought 15-20" of snow to Mt Hood Wednesday and Thursday. Strong winds and snow will continue tonight. This storm ushers in a significant change from the early season snowpack that we have had. Many slopes now have decent coverage and enough snow to avalanche. Differences in the snowpack between upper and lower elevations will become more pronounced. Carefully evaluate the snow as you travel and stay alert to changing conditions.
Snowpack Discussion
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.
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.
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
Wind Slabs
Strong winds will continue to rapidly transport snow, building large, dense slabs on leeward slopes. Avoid wind loaded start zones, or slopes leading into gullies, cliffs, or other terrain traps.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
Wednesday night and Thursday brought us a fresh dump of snow and a brief, surprise rain event up to around 6500’. You will be able to trigger small avalanches within the new snow tomorrow. Use caution around convex rolls and on slopes with terrain traps.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1