Galena Summit & Eastern Mtns

Triggering an avalanche on weak snow near the ground remains a possibility on very steep, rocky, wind-affected slopes. You'll be most likely to encounter this problem at upper elevations in the White Cloud and Boulder Mountains. At middle and lower elevations, and on more wind-sheltered slopes, the snowpack is generally stable and avalanches are unlikely. Small, recent slabs exist in upper elevation terrain exposed to the wind which may be sensitive to your weight today.

If you are going to run into a slab avalanche problem today, chances are it will be in an area where recent, thin wind drifts are resting on weak snow left behind in the wake of last week's cold, clear weather. In light of that, let's spend a minute thinking about crusts, facets, and the way the wind transports the snow. The following is a simplification of complex processes that take place in the snowpack, but I think it will help you to better understand the basic points:

CRUSTS:Crusts in the snowpack form when dry snow is melted and then frozen again. The most common ways these crusts form are with warm air temperatures, direct solar input, or rain. Right now, the type of crust that we are dealing with is a sun crust. Sun crusts are distributed in a predictable pattern: slopes that directly face the sun will have the thickest crusts, slopes on the margins (E-SE and W) will have thinner crusts, and slopes on the northern half of the compass won't have any crust at all. Crusts are often associated with faceted snow grains, which are described below. The combination of crusts and facets commonly form weak interfaces in the snowpack.

FACETS:Snow grains are in a constant state of change. Even before they hit the ground the structure of these crystals is rearranging in order to satisfy the whims of the laws of physics. Under weak gradients (the change in a certain property over a distance), snow trends towards a more rounded state. Rounded grains tend to pack nicely with each other and form strong bonds. However, when placed under strong gradients, snow trends to a more angular state (facets). Cold, clear nights and days, like the ones we saw last week, create conditions that are conducive to the formation of these faceted snow grains. Facets are problematic because they don’t bond well to the snow around them. They are frequently found in conjunction with crusts—the presence of the crust tends to increase the strength of the gradients in the immediate vicinity of the crust, which helps generate faceted snow grains. And just like that, you've got an ugly crust+facet combination that could make for a "great" weak layer. Now we just need a slab...

WIND:When the wind is sufficiently strong, it picks up snow and transports it to another location. While the snow particles blow through the air or bounce along the snow surface, they break up and become smaller. The smaller grains pack well into drifts or slabs once they're deposited. The speed of the wind plays a critical role in this process. Steady, moderate winds are great at building widespread, thick slabs. Shifting, variable, gusty winds don't do nearly as good of a job. If the wind doesn't blow hard enough it can't pick up much snow, but if it is blowing too hard it will throw the snow up into the air where it sublimates, never to rejoin the snowpack. The winds blowing during the recent storm have been more on the gusty and variable end of this spectrum, leaving an irregular pattern of loaded and scoured areas. 

Keep these patterns in mind as you travel through the mountains today. Triggering a wind slab is most likely where the presence of a crust+facet combination coincides with a fresh wind slab. Because of this, the problem will be isolated to specific terrain. In many areas, you won't find any touchy wind slabs. But in the right terrain, you will. The wind slab that was triggered on Titus Ridge yesterday is a great example of this. Don't let the lack of touchy slabs in one location lead you to believe that this problem doesn't exist anywhere.