Persistent Deep Slab in the Tetons
The classic backcountry ski checklist – shovel, probe, beacon, brain – has also come to include the avalanche forecast, and most of us make the effort to check the forecast before we head into the mountains. But an important question that’s often overlooked is whether we are really absorbing the most important information contained in that forecast? Sure, the hazard rating is important, but it means even more when you also understand the “problem.”
If you stood at the trailhead for a popular backcountry destination and asked each person, “what’s the avalanche hazard today?” you would probably get the correct answer more often than not. However, if you were to ask these same skiers what the avalanche problems were for the day, you would most likely get a variety of different responses. Knowing the hazard is a good first step, but for those of us who want to ski in and around avalanche terrain, knowing the avalanche problem is critical to good decision-making.
The first step to understanding the avalanche problem is understanding what type of avalanche you are dealing with. If you haven’t taken a Level One Avalanche Course recently, the Canadian Avalanche Center has a helpful tutorial that does a good job of explaining the eight avalanche types and their basic characteristics. The eight types are: loose dry, loose wet, wet slab, storm snow, wind slab, persistent slab, persistent deep slab and cornice.
In addition to knowing the type of avanche, you need to look at the full picture offered by the avalanche forecast and find out what the “avalanche problem” is. The problem consists of the avalanche type combined with its location (aspect and elevation), likelihood of triggering and size. Check out the pictures below for an example of NWAC’s graphical representation of the avalanche problem (listed as Avalanche Concern in 2013/14).
For an experienced backcountry skier or rider a day like this seems pretty manageable. Avoid the solar aspects, especially late in the day. Watch out for small wind slabs in the alpine, specifically on SE to N facing terrain, and small storm slabs in the alpine on all aspects.
Compare that relatively easy day above to the day below, which had the exact same hazard rating (moderate all elevations).
This day would be much harder to manage. The biggest concern I would have would be the persistent deep slab. The likelihood of triggering this avalanche is unlikely but the resulting avalanche would be large to very large, so consequences of triggering this avalanche would be huge. In addition, this persistent deep slab is on all aspects and elevations, so avoiding the problem would be nearly impossible. The final two nails in the coffin would be the cornice and wet loose problems, both of which could be big enough to trigger the deeper instabilities.
This would probably be a good day to avoid all avalanche terrain even though the rating of moderate x 3 makes it seem relatively benign.
If you have been using the avalanche problem to help with your decision making and planning great, keep up the good work. If you haven't, I challenge you to know the problems each and every time you are out in the backcountry (or slackcountry) this winter. This will move you one step closer to understanding what's going on with the snowpack. We will never be able to know everything about the snowpack and there will always be surprises but putting in this extra effort will hopefully keep those surprises to a minimum.
Enjoy these last few weeks before the ski season starts and start getting ready. Winter is right around the corner!