Northwest Mountains

Large natural avalanches continue to release from windloading.  Avalanches that break in leeward start zones have the potential to run long distances. Avoid traveling on or below leeward terrain near and above treeline.   At lower elevations and in wind-protected terrain, skiers and riders can easily trigger slabs breaking several feet thick.  Avalanches have the potential to be remotely triggered and to break much wider and run further than you might expect.  Conservative terrain selection is paramount today.  The safest and most reliable way to manage today's dangerous snowpack is on slope angles less than 30 degrees that aren't connected to steeper terrain above.  

Yesterday's observations highlight continued natural persistent slab activity, both from wind loading (example A, example B, example C) and solar effects (example D).  With strong winds continuing today, expect more of the same.  The curve of natural avalanche activity is declining since its peak on Friday night as winds have less snow available for transport now.  Since Friday, we have documented 90 persistent slabs that broke on weak layers in the bottom half of the snowpack.  Nine of these were large enough to snap trees (D3 in size) and reached nearly full track.  Our media gallery below highlights just a fraction of the activity.  This graphic documents the observed distribution of avalanches since Friday.  West and northwest winds are raking snow off of the high, windward fetches and adding stress to slabs on leeward and crossloaded terrain at all elevations.  Those are the slopes where the threat of natural activity continues.  In the most sheltered terrain near and below treeline, slabs extend around the compass where the winds haven't played as strong of a role.  With more recent snow and less wind erosion in the Northwest Mountains, we are showing a broader distribution for slopes where you can find poor structure extending around the compass to windward aspects.  

Where slabs are thickest from favored snowfall (generally the Northwest Mountains), the sensitivity to human triggers will start to decrease now because weak layers are buried deeply and slabs are getting stiffer.  Obvious signs of instability underfoot will not be as common, but you can still easily trigger a slide from shallower spots on the slabs: perhaps halfway down the slope below the windloading, near a rock outcrop, or on the margins of a windloaded slab.   These land mines are difficult to spot and asses, so avoidance is still the best solution.  In the shallowest parts of our forecast area (the Southeast Mountains), slabs are thinner and will continue to be reactive and easy to trigger for quite some time from anywhere on the slope.     

Given the dangerous conditions, we've been highlighting low angle terrain as the most reliable way to travel safely all weekend.  My favorite observation from yesterday was that the snowpack was "stable on flat ground".   That's true....sort of.   From flat ground, you can remotely trigger avalanches that start in steep terrain above you.  This video and story from Utah is a great example of how you can get in trouble from below a steep slope.  With the threat of natural activity ongoing today, you can also get overrun by a natural while standing on flat ground.   Choose your terrain wisely today with careful consideration for overhead hazards.