Avalanche Forecast

If the sun comes out on Saturday and you start seeing rollerball activity, limit your exposure to steep south-facing slopes where even a shallow slide could have significant consequences in early season conditions. Rollerball activity was noted by NP employees on Friday, but we expect more sun on Saturday as we transition away from the recent storm.

Small cornices grew rapidly with moderate winds on Thursday. NWAC and ONP professionals noted that they were sensitive, reactive, and 1-1.5 meters in size and one such cornice released and triggered a 1 ft deep slab on the slope below. Although the slab didn't run far, it made the observers travel cautiously and this advice extends to all who travel on or near ridgelines. 

The snowpack consists of 17" of storm snow, sitting above 5 inches of decomposing snow (deposited Saturday), capped by a melt-freeze crust (formed Sunday). Thursday's observation found that the snow may contain a minor density inversion that could still be a factor for triggering storm slabs on Saturday.

Professionals observed 1-1.5 meter cornices that were forming rapidly on Thursday and were capable of triggering small-large avalanches on the slopes below. Photo Credit: Matt Schonwald

Please consider supporting your forecast by submitting your observations on the NWAC site. It's simple, quick, and tangibly improves our forecasts.

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.