Avalanche Discussion

Regardless of any layer below that release, I contend that the recreationalist has bigger problems than PWLs if he's surprised by an avalanche on a 40+ degree convex roller next to rocks in isothermal slop that's ~2 feet deep.  And that's my contribution to this thread.

I am grateful for the wisdom Gary has put on this post. I believe one issue that can supercede the best judgment are trigger points. The variability of terrain can upset the best observations, since the variability may not appear on the surface. Rudi's accident was triggered on a 10 degree ridge, a shallow point in the snow pack that was attached to an enormous 38 degree slope below.

I agree - and thanks - Matt for adding to this discussion. The problem is that by the time Rudi found himself in shallow, weak faceted snow it was too late. Probably especially with faceted structure identifying trigger points can be practically impossible because shallow and rocky areas may not appear to be so from any kind of a normal viewing perspective. Aireal viewing or knowledge of slope history are often the only clues because the once very shallow areas by the time they are being skied are often masked under a more homogenous layer of new snow.

We train ourselves to look for convexity's, concavities, unsupported slopes, random disturbances under the snow surface, yet the ridges can often be as dangerous when deep instabilities with cohesive layers thin out and extend well over the ridge often pulling the opposite slope with it.

And therein lies the problem, more often than not the trigger points we come to expect with new snow instabilities can be the exact opposite of trigger points with faceted structure.

Surface hoar is a bit different, the main reasons for variability of surface hoar development and maintenance prior to burying are a little easier to feret out - north and east aspects, more in low areas than higher on slopes, less on the windward part of bowls, more often in gladed terrain, etc.

The winter of 2006 in France surprised professionals and recreationalists alike with 56 fatalities(22 guides and instructors) from conditions similar to the BC 2003 season. Trigger points on shallow slopes attached to steeper, exposed  aspects caught a large number of skiers who thought they made a safe choice of lower angle terrain.

One study showed that professionals more often get caught in moderate hazard, recreationalists in considerable. It is very difficult to deal with persistent weak layers and all the more so because of remote triggering, extent of propagation, and potentially the triggering of deep instabilities.

No test will solve this problem, however careful observations of terrain configuration with good sense of scale can help either avoid or seek low consequence slopes. Last year I think one of the Hummels found this out near Fortress and walked away with a new awareness of testing this issue.

It has to be about consequences, particularly with weak layers.

This is the micro slope scale where the forecast is no longer the primary driver in the decision making. Neither are bonding tests. Jill Fredston talked about training to see what is wrong with the picture, and this is a skill that gets overlooked.

Without complete facts, the discussion about Durrand Glacier serves as a red herring because snowpack was only one factor in the accident. Considering the complete set of facts helps avoid bias. This avoidance of bias is, I think, the point made by SkyKilo in his recent posts.

[] How many people were in La Traviata at the time of the avalanche?
[] What was the spatial distribution of the individuals? 
[] Is there a terrain trap at the bottom of La Traviata?
[] How many avalanches occurred?
[] Were there multiple burials?

No one argues that a persistent weak layer was involved. However, a very biased discussion is the only possible outcome if the rest of the facts aren't considered. This constant discussion of the snowpack at La Traviata amounts to a "search for supportive evidence" that favours a single fact ( the accident involved a persistent weak layer ) over the complete set of facts. ***This is outright bias.***

This accident has been cited repeatedly in this thread as a reason to fear persistent weak layers. It's fine to use this accident as an example of what can happen with persistent weak layers ( avalanches can happen ) but the resulting discussion is HIGHLY BIASED without the remaining facts.

The complete set of facts about La Traviata provides an entirely different view of the accident. I don't think anyone here doubts that persistent weak layers are dangerous. No one doubts that persistent weak layers present significant challenges to the backcountry skier.

But honestly, how is discussion based on an incomplete set of facts helpful to anyone? Does this discussion have to include rank speculation such as "The problem is that by the time Rudi found himself in shallow, weak faceted snow it was too late." There were *MANY* other factors that should be considered in any discussion of this accident.

Here is a short summary that includes additional facts:

"21 skiers, traveling quite close to one another, simultaneously climbed a large avalanche path above a terrain trap. Three avalanches released on a buried rain crust. 13 people were buried in close proximity to one another."

Can someone start another thread if this accident must be discussed ad infinitum?

Interesting discussion, but I'm not sure it says very much. The Cookie guy seems adept at tearing down the arguments of others...that seems to be his motivation. Seems to me that this is all just mental masturbation and one-upmanship. Maybe we could actually discuss some real snowpack/avalanche observations that occured in the past few days?

I think you are off base here, Cookie Monster. I think you were the first one to mention the Durrand accident. Earlier I merely mentioned the winter of 2002-3 - as a classic example of snowpacks with persistent weaknesses.

Most recently Matt used La Traviata as such (an example), also. We were discussing persistent weak layers you will note if you read the thread more carefully.

The main point is that persistent weak layers require a different mind set than new snow instabilities because they are triggered and behave differently - in most cases - than new snow instabilities.

The current snowpack has potential to be troublesome and much more so if it snows enough in the near and intermediate future. Numerous trip reports and recent avalanche forecasts support this conclusion.

Thank you Gary, I agree. My addition about trigger points relates to what you mentioned about the change in behavior of the snowpack with buried surface hoar in relation to trigger points. The Cement Basin avalanche had seven people on the slope and the last person triggered the avalanche on buried surface on a 25 degree slope leading into a 38 degree convexity.

Skykilo references this point, he skied the same area then Hummel followed with different results and consequences. Rudi had a lot of people out on the slope yet it was triggered on a 10 degree slope near the top of the ridge, where the snowpack was shallow enough for Kelly to impact the weak layer.

One of the surprise factors of avalanches class 2 and larger is the size, often catching people on lower angle terrain and propagating much wider than anticipated. I believe knowing the layer exists and results have occurred on it, should be a red flag to not test this when there are possible consequences. When other people ski a slope with these buried weak layers, that can create a negative feedback loop, if other people skied it then it is okay. This is true of early season snowbridges over crevasses as well.

I appreciate this discussion, it opens up awareness to the history that helps us shape our decisions with knowledge gained here.