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. The combination of 1-3" of new snow and wind is building isolated wind slabs in exposed terrain. These slabs are likely to be small, but touchy, thanks to the faceted surfaces they are being deposited on. At middle and lower elevations, and on more wind-sheltered slopes, the snowpack is generally stable and avalanches are unlikely.

Let's spend a minute thinking about crusts, facets, and the way the wind transports the snow:

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. 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 given property over a certain 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 faceted snow grains, or facets for short. We call them facets because they have sharp, angular edges on them, much like a cut gemstone. Facets are problematic because they don’t bond well to the snow around them, which makes them a “great” weak layer. This is a significant simplification of the processes involved here, but I hope it helps you grasp the basic points.

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 once they're deposited. And just like that, you've got a wind slab. The speed of the wind plays a critical role in this process. Steady, moderate winds are great at building widespread, thick slabs. But 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 yesterday and last night have been more on the gusty, and strong end of this spectrum. 

Keep these two patterns in mind — an overlapping patchwork of crusts, facets, and wind slabs. 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. Be alert to this as you travel through the backcountry today, and don't let the lack of touchy slabs in one location lead you to believe that this problem doesn't exist anywhere.