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Every winter we get mugged. Here in Colorado we get snow, oh wonderful fluffy powder. Then the bad guy shows up. A thick avalanche slab hangs there for days, weeks, waiting to kill someone. How do we deal with such “persistent slabs”? We watch slope angles, dig a few pits to see just how tender the slab is, listen to the avalanche forecaster.
School time at Cripple Creek Backcountry.
We are fortunate here in our HQ town of Carbondale, Colorado that our local ski shop, Cripple Creek, hosts a series of lectures by our area forecaster Blase Reardon. Main takeaways from his recent impartation of wisdom: Persistent slab does EVENTUALLY go away or get buried so deep it’s not dangerous, but the dragon must be watched lest he comes out of his cave while you’re not looking.
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Interestingly, a persistent slab can go “dormant” then come alive again with just a small load of new snow. The balance gets tipped, if you will. Perhaps the most important reminder I got from Reardon’s lecture was that persistent slabs are part and parcel to “remote triggering,” that ego destroying encounter with forces of nature when the kiss of your skis or stomp of a boot causes an avalanche to break above you — and take you out if you’re not watching your location.
Where the concept of remote triggering hits home with me is I’m a fan of ski cutting, but a persistent slab can break above you while you’re doing a cut. Thus, ski cutting: Only done on lower consequence slopes preferably _without_ a persistent slab, and use a rope whenever possible. Another thing I’ve been thinking about is how persistent slabs have weak spots often close to exposed rocks or vegetation, but how often have I “skied over by the trees” because I thought doing so kept me out of the main slide danger zone? Imaginary safety based on mythology?
One of our most tragic “remote trigger persistent slab” accidents in Colorado was the 2013 Sheep Creek disaster. Good idea to read up on Sheep Creek, regarding everything from slope angles, to alpha angle, to route finding thoughts. As always, we attempt to cover such accidents in a way that saves lives, through education by example. As always, condolences to the friends and loved ones of those lost. Speaking of which, a shout out to the IAN Fund, a sweet non-profit founded in memory of one of the Sheep Creek victims.
Thanks Cripple Creek and CAIC for hosting Blase.
5 comments
Great to see so many younglings at Cripple Creek. Lou, we skewed the average age curve that night.
Sunday we witnessed a lone skier remote trigger a slide. He was skiing in a small tree glade when the slope broke ~50 feet distant. This is the new the slide between Alley and Noname Peaks near the saddle. The slide erased 4 sets of tracks from earlier this week. So the persistent weak layer discussion is spot on. We guess the southerly exposure and warm temperatures changed the weak layer between Saturday and Sunday.
triggering in the transition from deeper to thinner snow is well explained and defined in the German avalanche safety book “Lawine” – which somebody ought to really translate….
There are a couple of spots where you can nearly count on the phenomenon – so much so that one can go and watch it happen. The Zischgeles in Tirol, which is a place you’re familiar with, I believe, is a place to see this almost yearly.
Don’t go where the snow is wind-drifted and deep. Don’t go where there are rocks and it’s thin. Don’t go when its too hot. Don’t go when it’s too cold.
Just stay where everything is JUUUUST Right.
It’s the Goldilocks method of avalanche prevention! 🙂
Hi Lou,
persistent weak layer is widespread in the Alps, especially in the areas with low snow cover. VERY bad in parts of Tyrol (Tux, Stubai, Zillertal, Ötztal), 5 fatalities in one accident this saturday. In fact, I think the only reason there aren’t more fatalities is the fact that we had exactly the same situation last year and most people still remember the well-mediatised accidents and keep it low.
Lenka K.
An online English version of “Lawine” (Avalanche) by Mair & Nairz can be found here: https://lawine.tirol.gv.at/en/basics/dangerpatterns/#c170677
Klaus, thanks for the link. I’ve always been a fan of knowing the “seasons of the snowpack.” Lou
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