Northwest Mountains

Strong winds yesterday and overnight formed small wind slabs across the terrain favoring higher elevations.  Fresh drifts will be most dangerous above cliffs, trees, or in gullies. Watch for cracking underfoot or thick, smooth deposits in leeward terrain.  A large persistent slab was remotely triggered yesterday on Mt. Emmons, which is indicative of isolated upper snowpack issues involving faceted crusts.  

If you glanced out your window yesterday afternoon, you saw intense blowing snow off of all the peaks, with blowing snow mixing down into valley floors.  Although it appears that the wind transport was too strong to effectively load start zones in most terrain, there was some wind slab formation, evidenced by several natural wind slabs observed in the alpine yesterday.  After strong winds like that, sometimes wind slabs form in unusual locations, such as creek beds near the valley floor or behind tree fences mid-slope.  Use visual clues on the snow surface to help identify the problem.  This video from earlier this week describes an older, thicker generation of wind slabs that is no longer in play, but many of the same visual strategies still apply for today's hazards.  Wind slabs will be less sensitive today and will trend increasingly stubborn tomorrow.  

Yesterday, a group of skiers remotely triggered a large persistent slab near Red Lady Glades on Mount Emmons.   The slab broke on a faceted crust layer just over a foot deep on a below treeline slope that has seen some drifting from recent storms.  In the past couple of weeks, our forecast team has searched without success for signs of developing persistent slab instabilities in the upper snowpack on the transition aspects: East/Southeast and West, where periods of sunny weather formed thin, faceted crusts below our recent storms (Example A, Example B).  This avalanche comes as a bit of a surprise; it's the only large avalanche below treeline this month, and the only remotely triggered persistent slab avalanche since the Snodgrass remote trigger 3 weeks ago on a similar southeast aspect.  Although this appears to be an isolated issue for now, it's a good reminder that there is always spatial variability and uncertainty with a regional forecast:  slope-specific assessments are an important part of backcountry travel.   

Looking ahead, winds are running out of snow available for transport and the danger should trend down tomorrow as wind slab instabilities become more isolated and stubborn.  That trend will dramatically reverse on Tuesday night with a strong winter storm that is expected to bring dangerous avalanche conditions by Wednesday.