Northwest Mountains

Up to 2 feet of snow on a fragile weak layer spurred a widespread natural avalanche cycle over the weekend on all but the sunniest aspects.  On slopes that still hang in the balance, you can trigger large avalanches on or near steep terrain.  More natural avalanches are possible due to continued snow and windloading today.   Carefully choose your routes to avoid traveling on or directly below steep slopes that haven't already avalanched.  Collapses and shooting cracks are clear warning signs of danger.  

Yesterday, we got our first good look at the carnage resulting from our weekend storm.  Near and below treeline slopes saw the most widespread persistent slab activity on west through north through east aspects.  This should come as no surprise: the snow surface on these shadier, wind-sheltered slopes was incredibly fragile before the storm, and the subsequent 1 to 2 feet of snow easily tipped the balance.  We noted a fair amount of activity above treeline as well (Example A, Example B).  Saturday night's winds likely erased some evidence, but I suspect the damage wasn't as widespread because the weak layers in the alpine weren't quite as fragile.  Recall that the facet sluffing a week ago wasn't a widespread issue in the alpine because the snow surface wasn't as weak in areas buffed by the wind.  On many of these alpine slopes, or on aspects with a hint of south (such as southeast aspects), there are wind crusts or sun crusts capping the buried weak layers, which may require just a little more load, perhaps that of a skier or rider or today's windloading, to get the crust to collapse and produce an avalanche.  On shady aspects near and below treeline that saw sufficient snow (more than about a foot), triggering an avalanche appears to be almost a sure bet if you cross a steep slope that hasn't already slid.  Yesterday's observations provide ample evidence of that, with continued remote triggered slides, widespread collapsing, and shooting cracks.  

So here's the good news:  A lot of suspect terrain flushed naturally, thus diminishing the distribution of the problem.    The bad news is that there are still plenty of slopes hanging in the balance, waiting for a human trigger and being stressed by today's new snow and windloading.   I would treat any slope that hasn't already avalanched as guilty until proven innocent.   The most reliable way to avoid an encounter with today's persistent slab problem is to travel on slopes less than 30 degrees that aren't connected to steeper terrain above.  If you are considering stepping out into more aggressive terrain, choose south or southwest facing aspects, terrain that has already avalanched, or small / low consequence pitches where slabs won't be large enough to bury you and where you won't get pushed into a terrain trap.  The Southwest Mountains received less snow over the weekend and are expected to pick up less snow today.  For this reason, the threat posed by persistent slabs is generally smaller and a little less severe, especially at lower elevations.  

Saturday night's northerly winds and today's southwesterly winds have drifted snow at higher elevations and spurred a few wind slab instabilities.  The focus of our problem list is on the far more widespread and larger persistent slab problem. Today's wind loading is building on the same suspect slopes as the persistent slab problem.  Be aware that shallower wind slab instabilities are also developing if you are traveling into near and above treeline terrain.