Soldier & Wood River Valley Mtns

Triggering an avalanche that fails on weak snow buried 1-2 feet below the surface is possible You are most likely to encounter this problem in areas where the wind has built thicker drifts on top of the weak snow underneath. Avalanches are also possible in wind-sheltered terrain, particularly in the snowier portions of this zone to the west of the Wood River Valley. Reduce your odds of triggering a slide by sticking to lower-angled terrain without steep slopes above you. Avalanches may be triggered remotely—from below, above, and to the sides of steep slopes.

Avalanche danger will rise overnight as an intense winter storm impacts our area. If you are out in the early morning hours on Wednesday, the likelihood of both human-triggered and natural avalanches will have increased significantly, particularly in the hills above the Wood River Valley.

Yesterday, I used my Forecast Discussion to talk about slabs (link here if you want to get caught up on the two-part mini-series). Today, we will be thinking about weak layers. 

WEAK LAYERS: Up to this point in the year we have mainly had just one weak layer to worry about: the thick stack of facets that we buried in December.  In the professional snow community, we name the layers in the snow based on the date that they are buried. On the 11th of December, after a 3 week stretch of cold temperatures, light winds, and clear skies, this thick layer of weak, early season snow was finally put to bed. And thus, 12/11 was formed. Ever since its burial, 12/11 has caused us nothing but headaches. The general weather pattern that created this weak layer (early season snowfall + early season drought) isn't unusual for our area. However, the weak, faceted snow that developed this year is unique, both in how reactive it has been and how long it has remained reactive. In the six weeks since this layer was buried, it has been responsible for nearly all of the 400 avalanches we've recorded thus far this season. 

There is an interplay between slabs and weak layers. The weight and the characteristics of the overlying slab affect the development of a weak layer after burial. In our northern areas (Banner Summit) the weak snow has been buried beneath a 3+ foot thick slab of snow for several weeks now. During this time, the weak snow has been mechanically compacted by the weight of the overlying slab. In addition, deeper snowpacks tend to create conditions where faceted grains in the snowpack can round and gain strength. Because of this, we are beginning to see signs of healing on the 12/11 weak layer in this area. 

As you travel further south, the slab overlying the weak layer is thinner and the weight has not been enough to significantly change the properties of the weak snow. Also, since the snowpack has remained thin, strong gradients in the snowpack have helped this weak layer to stay very weak. As if that weren't enough, we are also suffering from complications due to the nature of the loading. Instead of putting one big load of snow on at once and giving the weak layer time to adjust, we have gradually built a slab in small doses. This loading style, known as cumulative or incremental loading, tends to help a weak layer stay reactive for extended time periods.

As I write this, a deep low-pressure center is making landfall on the California coast. This slow-moving storm will be impacting our area for the next few days, putting the weak layer to the test. Where the snowpack is the weakest the storm is predicted to be the most intense. If the storm verifies I have no doubt that the snowpack will not be "passing this test". Further north, where less snow is anticipated and the snowpack is stronger, many places may be able to bear the weight of the additional load. Of course, much of this depends on how this storm plays out over the coming days. Stay tuned, we will keep you posted.