Avalanche Forecast

Issued: Dec 14th, 2019 10:06AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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Recent avalanche activity indicates you can still trigger a thick slab layer on steep, unsupported terrain features. You might also trigger a loose dry avalanche on similar terrain. Be cautious of other terrain hazards such as rocks, which are shallowly submerged in the early season snowpack.

Summary

Discussion

Lots of folks were out in the highway 542 zone on Saturday. Recreationists triggered a handful of soft slabs on steep, unsupported terrain both Friday and Saturday. One notable observation released within weaker snow at the new/old snow interface.

Remotely triggered avalanche (D1.5) that occurred on a NE aspect at 5600 ft in the hwy 542 corridor. The avalanche ran 150 ft downslope and broke 40 ft wide and 2 ft deep on the old/new snow interface. 12/14/19 Photo: Dane Caldwell

On Thursday afternoon, a backcountry traveler in this zone triggered a slab avalanche in near treeline terrain, propagating 30 ft wide and running 500 ft of vertical as reported here.

The new snow depth is 6” at 2200 f, 1 ft at 3500 ft, 2 ft at 4500 ft, and 27 inches at 5500 ft. The new snow sits on dirt below 4000 ft and patchy old snow exists until it becomes more continuous around 5000 ft. The old snow base is a pre-requisite for slab activity on Sunday.

 

 

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. 

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.

 

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 ”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.

 

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.

Problems

Storm Slabs

An icon showing Storm Slabs

With 2+ ft of snow accumulated at mid and upper elevations, the most significant concern remains the nature of the old-new snow interface and has proven that it can propagate to produce avalanches. This interface concerns us most where it remained colder and drier, perhaps on northerly slopes 5500 ft and above. The gradually healing weaker snow formed from a skiff of snow that fell on Tuesday. You can identify this layer sandwiched between a thin crust from last Sunday and the heavy snowfall that fall Wednesday through Friday. While this layer may be stubborn to trigger, it has been producing small-large avalanches, particularly on steep or unsupported slopes approaching 40 degrees.  These are the same slopes that you should consider avoiding on Sunday. 

Keep in mind that in steep or unsupported terrain at any elevation you could trigger a shallow loose dry avalanche within the recent storm snow.

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

Valid until: Dec 15th, 2019 10:06AM