It has been 4 days since a very warm storm slammed into our area dropping inches of rain up to 2,500′-3,000′ and several feet of snow above this. Since then, cold temperatures have cemented the wet snow into place and locked up any slab avalanche problem below 2,500′ and 3,000′ in areas. Therefore, our main concern lies at elevations above this where a ‘deep persistent slab’ problem exists. On these upper elevation slopes, a dense hard slab is sitting on a variety of weak layers in the mid pack (buried surface hoar) and old November facets near the ground. Triggering a deep slab will be more and more difficult as the days pass, but still possible. The most likely spots to trigger them are thin areas in the snow cover, often near rocks, or where the slope rolls over. High peaks, that see a lot of wind, can also be thinner and more likely to find a deep slab.
If you are headed out today, and the skies stay clear enough for easy travel above treeline, keep these points in mind:
– This is ‘low probability, high consequence’ situation – often refered to a ‘Scary Moderate’ avalanche danger
– Several tracks may be on a slope before someone finds a trigger spot
– Obvious signs of instability are not likely to been seen (such as whumpfing and cracking)
– Remote triggering is possible
– This issue can simply be avoided by sticking to terrain below 3000’ (which is a good portion of terrain at Turnagain) or choosing low-consequence terrain in the alpine.
Photo: Old cornice fall and avalanche from 1/16 near the Seattle Headwall region at 3,300′. Steep upper elevation slopes that have not slid are the ones most suspect for triggering a large slab avalanche.
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