Avalanche Forecast
Regions: Snoqualmie Pass.
The snowpack changes dramatically as you go up in elevation. If you travel to higher elevation terrain be suspicious of unsupported slopes, rocky terrain, and areas below cornices where you may be more likely to trigger a lingering avalanche. Early season travel conditions exist at all elevations and could be your greatest hazard of the day.
Snowpack Discussion
December 12th, 2019 (The regional synopsis is updated every Thursday @ 6 pm)
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.
Â
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 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:
-
The interface of older snow and any new incoming snow would be the first interface to check.
-
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.
-
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
Â
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 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
Storm Slabs
Recent storms were generous to the high elevation terrain around Snoqualmie Pass depositing more than 2 feet of snow in some areas. We still don’t have much information about the upper elevation snowpack due to difficult access. However, observations from nearby zones suggest that weak layers within and at the base of the storm snow could still produce avalanches. If you travel above 5500 ft, remember it’s a different snowpack. Use small low consequence slopes to investigate potential weak layers. Be leery of unsupported terrain, wind loaded slopes, and rocky areas steeper than 40 degrees where you are more likely to trigger an avalanche.
At all elevations, numerous obstacles exist including trees, rocks, and open creeks. Expect difficult travel due to low snow coverage. Give yourself time and flexibility to navigate these early season conditions.
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.
Likelihood: Possible
Expected Size: 1 - 1