Tahoe Slides 1/14/2016, Coldstream(JT Holmes) and Tallac Corkscrew

[whatsupdoc]

I was in the party that triggered the avalanche on Tallac 1/14. I hadn't seen this thread until recently. I can offer my perspective on what went down that day and lessons learned. We felt obligated to post to SAC immediately (my friend wrote up the SAC report, not me) but I hadn't planned on offering my thoughts to the public in any other forum, but this seems like a good place to do it (apart from the SAC incident report) so others can learn from our mistakes.

First, being involved in even a small slide such as this one is quite humbling. Besides kicking off some small wind slabs on test slopes, I'd never seen a true slab avalanche such as this one. Seeing that snow moving downhill so quickly is downright frightening and eye opening. Oddly, my overwhelming emotion after this incident was mainly one of disappointment. Disappointment in myself that I made poor decisions. Embarrassment wouldn't be too strong a word. Certainly these feelings need to be taken in the context that no one was injured or killed. If this were the case, my feelings would be much different.

We fell prey to several cognitive traps, mainly familiarity (I've skied this line many times and have never had problems), commitment/consistency (we've come this far we might as well go the whole way), and social proof (others are skiing it so it must be safe). I myself have skied this line probably at least a dozen times.

One factor that likely played a role here was that the entire group was skiing together for the first time. There were 2 of us that knew each other very well, and three of us that knew each other well, but all 5 of us had never been out together. There wasn't a specific moment where I feel this came into play, but it likely shaped the course of the day. Basically it's tougher to speak out when you're not as familiar with someone. We've all been there I imagine if you ski with new partners with any frequency. That being said, let me be clear that everyone was solid in their own right IMO. I would (and actually have) ski with these guys again. But it's definitely a factor.

We read the avy bulletin and understood the heightened danger and the fact that this snowpack was unlike a typical Tahoe snowpack. We had made tentative plans to just ski the trees on the ascent ridge and stop short of the ridge. But we never came out and said with 100% solidarity that this was the plan and we're not deviating from it. We could have been much more deliberate here.

The climb was uneventful. Dug some mit pits and everything seemed OK. No cracking/whumphing on the ascent. We stopped just short of the ridge and dug a quick pit. We did 2 compression tests but nothing to test possible propagation such as an ECT or PST. We did ID a reactive layer about 10-12 inches down. My CT reacted on tap 14. But we mistakenly ignored this data in favor of other data that pushed us higher.

As an aside, if my lame username isn't an indication, I'm a physician. I've heard a few lectures about cognitive biases that can make you prone to errors. Anchoring, diagnostic momentum, satisfaction of search, etc. There's a lot of similarities between the 2 fields at least in this regard. Just being aware of these traps is crucial. You don't think about them every day but if you don't even have awareness of these traps you won't realize when you're falling into them. Ignoring data that refutes your opinion in favor of data that only supports your opinion is a common error. Confirmation bias I believe is what it's called. This is what we experienced with our pit data.

Anyways, we were like 50 vertical feet from the ridge at this point. We saw 2 skiers drop in from the ridge from a fairly low angle start point. It was likely TahoeBC and his partner. It seemed like a mellow entrance so we figured we might as well just head to the ridge. We just kind of kept going. Indecision instead of decision. The start point was like 100 feet away from our pit. But from here we broke our tentative plans for the day because the entrance looked mellow and no one else seemed to have any problems. As another aside, there were multiple parties skiing in this area that day, not just our group and TahoeBCs group. No one seemed to be having any problems. Of course this is a huge error but certainly contributed to our fate. We finished the climb, transitioned, and dropped in skiers right side of the corkscrew bowl from a fairly low angle entrance, I'd say 30 degrees. No signs of instability at all here. We were pretty much at the same entrance as who I presume was TahoeBC.

The first lap was great skiing in the skiers right side of the Corkscrew. Psyche was high. Another factor that led to our poor decision. Good skiing tends to distract you from the dangers at hand. We got down to the bench about halfway up the corkscrew and transitioned after lunch. Plenty of laughs were had at the transition. Everyone was getting along. We made plans to check out the skiers left side of the corkscrew. There was a track here (from a solo skier nonetheless) which gave us a false sense of security. Mistake. If anyone's been there it's not exactly extreme terrain. It has perhaps 2 turns off the ridge at 35 or so degrees and pretty quickly mellows out. We all generally regarded the line as safe from past experiences which is an error but definitely contributed to our decision making. But we were reassured by the fact that this entrance isn't really that steep and we'd done it before. I guess we thought we were mitigating the danger by terrain management, but our choice of terrain didn't match the danger for that particular day. A crucial lesson to take away from this situation.

Winds picked up the second lap. By the time we gained the ridge the wind was howling and it was overcast/cloudy. A red flag but we ignored this in favor of other data that satisfied our decision to ski the line. We went skiers left of the rockbands in the middle of the corkscrew bowl to what we thought was a pretty mellow entrance. Obviously we were wrong. We stomped on the rollovers along the way and nothing cracked. We dropped a couple of small cornices and nothing happened. We also split up here. Perhaps part of the group dynamic. Skiers 1 and 2 entered on the right side of the bowl and skied down uneventfully. No major discussion was really had about this, it seemed fine to everyone.

I was skier 3. I was however the first person in our group to ski the path that eventually slid. I entered towards the left of the path on what was likely the least steep part of the path. It maybe approaches 35 degrees for a couple of turns. We didn't measure the exact angle at the crown since we wanted to get the hell out after this happened. But of course it's steep enough to slide. I entered 5 feet from the track that was already there from the previous solo skier. I did see a small shooting crack and I did make my group aware of this, but I think the momentum of the decision takes over at that point and we were committed to this line. I never specifically told them to bail, I just told them what I saw. A mistake by me. This was however our first sign of instability (aside from the limited pit data).

I skied to the other 2 members who were already down. We were in a safe area behind some trees. Skier 4 dropped to the right of where I dropped and skied down without issues. Skier 4 skied past the three of us into mellow terrain. I took some pictures of him as he skied by. He ended well below the slide path. Skier 5 entered the line between our 2 tracks and triggered the slide.

I heard the warning from skier 5 and immediately saw what was happening. I skied down and slightly right to make sure I was out of the path. Skiers 1-3 remained out of the path. Skier 4 was well below the path. No one could ID skier 5. His dog easily swam out of the debris.

At this point, we had no ID on skier 5. We had a feeling he might be above us, which was a sinking feeling, since we were near the bottom of the debris path, which ran maybe 250 feet. We went into search mode. We made the decision to have me ski down the path doing a search. Skier 1 put on skins to start climbing up if needed. Skier 2 was ready to act if needed to dig.

The person who wrote the SAC report (skier 1) didn't know skier 4 was well below the path (he had skied by us for some photos, I know, I know, not the best way to manage terrain). I knew this however since I took the photos. He thought skier 4 was also missing but I shouted to him I had eyes on skier 4 well below us and the path.

I searched the remainder of the path and didn't get a signal. I can't even describe how this felt, knowing the victim was probably above us and we hadn't ID'd him. It was windy which made communication more difficult.

After maybe 60 seconds (which seemed like an eternity) we made voice contact with skier 5 who had triggered the slide. He had grabbed a tree early on and remained well above the runout. Some snow ran up his back and buried his legs. Since he was kind of pressed against the tree and his legs were buried, he couldn't get out of his snowboard bindings. Skier 1 (the person who skinned up) ultimately reached him and dug out his legs to free his bindings. Skier 5 was uninjured. Another party did ski down to us from the skiers right side of the corkscrew (roughly the first line we skied) and asked if we needed help but we were fine by that point. We regrouped and then very cautiously descended back to the car. We did debrief quickly when we hit the road.

[whatsupdoc]

The slide itself wasn't huge. I'd say D2R2 but I'm not am expert in grading these things. I don't know if it could have fully buried a person. Maybe in a tree well but the debris wasn't terribly deep. But certainly it could've killed someone by trauma or buried them in a terrain trap, like a tree well. The crown averaged 12 inches I'd say. A wind slab failed on the weak layer we ID'd in our pit. Likely buried SH but the SH crystals were quite small.

I'm not trying to make light of the situation by saying the slide wasn't too big BTW. Just the reality of the situation. It was big enough to scare the shit out of me and forever change my outlook in these matters.


That's it. I understand we will be open to critique. We fucked up. No excuse. Like I said, it's embarrassing to admit this and make this matter (somewhat) public.

This incident highlights the cognitive errors that often contribute to avalanches. Familiarity (I've skied this a dozen times before and it's always been fine), committment (we've come all this way we might as well finish), confirmation bias (ignoring the negative data in favor of the data that supports your decision) and and social proof (there are tracks there so it must be safe). We ignored data that should've absolutely swayed us in the other direction (the presence of a PWL on the forecast, fresh wind slabs, our brief pit findings) due to these errors. We broke our plan for the day not really deliberately but more in a passive way by going with the flow and just saying hell we might as well go to the ridge which is like 50 feet away. Had we just skied the trees like TahoeBC and his partner we wouldn't have to explain our mistakes.

But we were all given the opportunity to learn a tremendously valuable lesson without anyone getting hurt that day. Hopefully you all can learn from our mistakes to make better decisions in the backcountry. Stay safe and have fun doing it.

Mike

A photo from our first lap that day:

[whatsupdoc]

So after reading TahoeBC's post again maybe it wasn't his group that dropped in off the ridge before our first lap. He seemed to indicate he stopped short of the ridge. Regardless, we saw someone enter from the ridge and this swayed us into just heading up a bit higher and following their lead by entering our first lap in this region. We didn't see any signs of instability on our first lap, which bolstered our confidence. Our second lap was the one that triggered the slide.

[Unfrozen Caveman Lawyer]

Whatsupdoc - Thanks very much for posting up your detailed account (and thank you very much for your partner submitting the prompt incident report to SAC). Real-world incidents such as these can serve as great learning resources and get a dialogue and internal thinking going, particularly when they turn out as they did here without any injury.

Importantly, I can see the mistakes you made happening in my own touring, and in fact, they have time and time again. Anyone that ski tours has been (and likely will be) subject to human errors and group dynamics such as these. Only in constantly recognizing that they exist, and doing everything in our power to develop systems to help combat them, can one even begin to attempt to mitigate their effects.

Having now had some time to digest this incident, I would be curious what steps you think you might take in the future to attempt to address some of the heuristic traps you identified above? It would likely be very helpful to this discussion, if you are willing to share.

Thanks again.

[whatsupdoc]

That's a great question. Of course I'm still digesting this so this is probably all over the place but...

Just recognizing their existence (the heuristic traps) and how they can fool you is huge. Knowing is half the battle, right? I mean I knew about them to the extent that they're taught in Level 1 and you read about them in reports etc, but having first hand experience is so much more reinforcing.


I plan on being more deliberate in my decision making and communication, especially with new partners. The dangers and the plan need to be verbalized. Out loud. Formally.

It would also be helpful to try to identify these traps ahead of time. "Hey, we might see folks skiing off the ridge but let's just stick to our plan of skiing the skin track out. The danger is high and the PWLs are unpredictable. Let's not get lured by other tracks and stoke."

One of the most important steps in ski touring is just deciding what the hell to do that day. I'm talking before even leaving the house. This depends on the dangers (the problems themselves and how likely they are), conditions, party experience, etc etc. Of course you're trying to ski good snow safely given the conditions you have at hand. Make good decisions here and stick to the plan. Like someone else said, have a list of permanently green terrain for high danger days. Certain days, like Jan 14, mandate a very deliberate and conservative approach. PWLs are unpredictable and obviously to be feared. The danger was heightened. So when we decide on a tour that I feel fits the conditions well, I will be more apt to speak up and say no, we're not deviating from the plan today. I don't care if someone else skied it, the danger hasn't changed and we need to stick to the plan.

I've skied once with Schralph, and his approach is tremendous. I should have learned more. Before we set off it was "what's the problem?" and "what are we going to do about it?" Someone else referenced this earlier in the thread. What's the problem and how can I kill it (or something to that effect)? I think it's worthwhile to have this conversation instead of just "Did you read the report today?" By getting this out there it makes it easier during the tour to return to this original plan when forces are swaying you.

But I'm realistic. Some days the danger is essentially nil. Hero corn days with a consolidated snowpack for instance. Those are the days to explore new terrain, drink a beer, and bask in the glory that is ski touring. Those are the days to perhaps be lured by an unskied line. Those are the days to be a bit more whimsical.

[adrenalated]

Thanks for sharing your experience, whatsupdoc.

"What's the problem, where do I find it, how do I kill it." I was the one who brought that up and you mentioned it so repeating it here.

I completely agree that the mistakes you made are mistakes that I have made before as well (and I suspect anyone that travels in the BC regularly has made).

[whatsupdoc]

One more thing to consider in all of this is that all of these errors we made were relative to the danger at hand.

Our behavior didn't suit the danger at hand, both the problem and the likelihood of occurrence.

Some observations like multiple parties dropping off a ridge without issue are likely of value in a typical maritime setup where wind slabs and storm slabs are the primary danger. They are much easier to predict and mitigate.

But these observations served to only fool us that day.

So that's why it's important to identify the specific dangers at the car and what you're going to do about them. When PWLs are involved, terrain choice is paramount and you can't afford to be swayed by these traps.

[TahoeJ]

Whatsupdoc - I appreciate that your group felt obligated to promptly post a report with SAC to warn others of potential dangers. As opposed to waiting a week and doing a podcast.......

[mcski]

I laughed

[lepistoir]

Whatsupdoc - I appreciate that your group felt obligated to promptly post a report with SAC to warn others of potential dangers. As opposed to waiting a week and doing a podcast.......

Hear hear

[CookieMonster]

After reading this thread, the only mistakes I really see are that the parties either ignored margin of safety or simply didn't make it large enough. It doesn't seem like anyone really ignored the danger or made bad decisions. Many of the "heuristic traps" in the account by whatsupdoc could have been totally avoided by simply saying "despite what we're seeing, we don't know what could happen, so we should take that into account beforehand".

In my opinion, most people are much better about considering margin of safety on the up than the down. I wonder if this is because we feel more vulnerable and less confident going up.

[bodywhomper]

Whatsupdoc, thanks a ton for sharing!

Im curious about your take at the time, and now after the fact, about the cornice knocking that you mentioned prior to your fateful descent.

A question for the larger audience, to me, a buried weak layer that's a foot down would be cataloged by me as a "surface" instability if there has been recent snow. Some of the questions to me become whether there is a slab on top and the reactivity/energy of the weakness. I start to think about it differently, but sometimes treat it the same if the instability is deeper and/or been present longer. How do others think of this?

[fogey]

Coming out of lurker-dom to thank whatsupdoc and the other posters in this thread. This is the kind of stuff that makes me think the internet is worthwhile after all.

Also, the common feature of these two incidents that stands out for me is that both of the groups that triggered the avalanches had dug and evaluated pits. Years ago when I took Avy 1, they didn't teach any of the standard pit tests--the reason given was the one or two pits a recreational group was likely to dig weren't enough to evidence that the snow was safe to ski--you would need a bunch of pits in a variety of locations. At the time, that sounded silly to me--logically, knowing more couldn't hurt, right? The truth is that in the face of temptation, we're not all that logical. In both these situations, it sounds like the party had some doubt about the safety of the line. Digging a pit gave them only a little more evidence, but enough more confidence to ski what they wanted to ski.

Hindsight is 20-20, and both these groups have more experience and made more sophisticated judgements than I have the skill to make. But there is something here to think about.

[skifishbum]

thanks doc
strong

[FormerKnuckleDragger]

Thanks, whatsupdoc, for sharing.

Question for the folks who are more experienced in the Tahoe backcountry. Are these areas (Tallac, Cold Stream) thought of as generally safe in higher avalanche danger? I've heard a lot about other areas (Jakes, Rubicon) being safe to ski even on considerable days due to trees (lower chance of SH) and skier compaction. I'm not sure I buy into that, but my risk tolerance is probably a lot lower than most (young kids at home...).

[JayTe]

First off, I want to thank Doc for the thoughtful insight to what happened that day. Also, thanks to Shraph for putting this out there. Fortunately, no one was hurt, and we may be able to learn something. I may be able to add some perspective to this conversation, as I was the solo skier mentioned in Doc,s post. I am also somewhat of an Avi geek. The morning began for me by checking the weather. We had approx. 8"+ new reported from around the area. It also came in warm and wet, with sustained winds of 40-60 mph, gusts to 80. Another storm was due late that day, with more strong winds. I arrived at the trailhead at 7am. There was already another group ahead of me. I followed there track up the far right side of bottom of corkscrew, and eventually passed them at the bench after they had crossed over to the left ridge. As I climbed, I dug a number of hand pits, finding poor bonding of the new snow, as well as a couple of other weak layers. As I approached the ridge line, I kept climbers left, on the wind scoured up track, to avoid the wind loaded start zone. Once on top, I noticed what looked to be a natural release, which may have happened late the previous afternoon or evening. By this point, the crown had been almost filled in by wind deposit. I could see piles of snow up against the trees below. This is fairly common in the center of the bowl, as this area gets the strongest winds, as well as being the steepest section off the ridgeline. As I usually do when I'm the first up right after a storm, I drop in close to my up track, to avoid the ridgeline start zone.
The snow was dense and wind affected. Skiing was fast and good. I transitioned at the bench and returned for my second lap.
As I got to the top, I noticed a group digging a pit, right underneath what I would consider one of the first start zones along the ridge from that up track. At first, I was thinking it was some sort of class, as one of the guys seemed to be in control and what I would consider a large group. I did not walk by them, but rather followed what I consider to be a safer route to the ridge. This area is commonly cut thru to the ridgeline.
When I got to the ridge, the wind was picking up, so I stopped behind some trees for shelter on the ridge, directly above the group digging. My timing was good, because just as I was putting on my jacket, I was able to watch the guy do a compression test. By my count, a saw 4 taps from the wrist, and then failure. I started moving on, and I noticed that they had immediately packed up and continued cutting across to the ridge. By this time, there were a few other track besides mine in this area.
I wanted to go inspect the natural release below the rocks in the center of the bowl. I continued over and confirmed what I thought I saw. The area had slid, maybe 100' wide just below ridgeline. The snow was piled up on the trees below approx. 6' deep. The area before the rocks is a bit flatter, so I entered on the lower angled entry skiers right of the natural. As I frequently do when conditions dictate, I stay off any trigger points, and just point it full speed down into the trees before making any kind of turn that may impact the slope. In these kinds of zones, I prefer this method, instead of a ski cut, since it is easier to avoid trigger points. Ok, time for another lap. This time I go to the other side of the natural. Over on the skiers left of the natural, there are some trees right on the edge of the ridge, that have nice flat entry points. I used the same entry method described above.
This may be the track that lured them in. To my right was a nice little steep wall of snow, maybe 60' wide up to the wind filled crown of the natural. I estimate this section to be approx. 38 deg. Mellowing out gradually before the trees.
Time for another lap. Now the winds are really howling. I notice on my way up, a group transitioning on that nice little section.
Next view of the ridge as I approach the top, I see a crown. I hurry over, yell down. Everyone ok. Went down to check it out and see if I could help. Two guys were digging out a snowboard from the underside of a group of trees. The board was approx 6' under the snow from the downhill side. Actually looks deeper
this is the snow piled up against the tree.
those poles are about 42"

I asked the guy in the orange jacket, who I think was the one doing the compression test what his score was. He said CT 3.
I asked if he had done any other tests, and he said no. I also asked if he would report it to SAC, and he said yes. He asked for my pictures, which he used in his SAC report.

[SchralphMacchio]

Wow, amazing developments in this thread regarding the Tallac incident. I personally know people from 3 of the parties (TBC's group that changed terrain, Whatsupdoc, and JayTe rolling solo).

I was in the party that triggered the avalanche on Tallac 1/14 ... First, being involved in even a small slide such as this one is quite humbling ... my overwhelming emotion after this incident was disappointment in myself that I made poor decision

As discussed on the phone, I hope you've quit beating yourself up over this. For an analytical person, the hindsight is overwhelming. It's really easy to be overwhelmed by a litany of "little mistakes" and you have to take a step back and focus on the big picture. Along CookieMonster's commentary, the point is to get the big picture correct and leave yourself enough room/margin to make mistakes, because if you expect to get every little detail 100% "right" as far as best practices or "could have/should have/would have" scenarios then you're not setting yourself up for success.

Here is the link to the TAR Issue that contained my article. It's on Page 20.
http://ameriavalanche.wpengine.com/p...April_2014.pdf

I suspect the article has a completely different context for those who have been through an accident vs. those who haven't. I wish I knew how to write it so that it has just as much meaning for those who have yet to be involved in an accident.

the common feature of these two incidents that stands out for me is that both of the groups that triggered the avalanches had dug and evaluated pits.

First response to this: I think the context of the pits is different in each accident. The Coldstream group was guided, and in my experience guides don't share pit data details with clients. I've been with Pacific Crest Snowcats before and it's SOP for the guide to dig pits, but I don't know their decision making frameworks or protocols with respect to pits, terrain selection, etc.

The Tallac group was recreational and a CT test was performed. The results indicated a sensitive snowpack. It sounds like further analysis or updating the travel plan based on new information (understanding of the avalanche problem changed? unexpected results? etc) did not occur.

Second point: Pits should not really be used for green-lighting terrain - only red-lighting it. The terrain should be chosen based on the avalanche problem, balancing the group's certainty that they would not run into the problem (or could manage the problem) on this terrain against the consequences of the avalanche problem breaking loose on this terrain. What the pit does, it gives you more information about the avalanche problem compared to what you are anticipating, or possibly changes your characterization or understanding of the avalanche problem. The Tahoe area has lots of available data that goes into forming your hypothesis of the avalanche problem before you even get on the snow. The pit adjusts your expectation of the avalanche problem, for example maybe you were expecting 6" windslab but instead found facets under crust 1.5 feet down which was a more serious problem you didn't anticipate. A single pit should not give you enough certainty that a targeted slope is stable, but it can help you re-characterize the problem or tell you when the slope is obviously unstable. Other observations go into formulating your certainty of stability in this terrain.

Question for the folks who are more experienced in the Tahoe backcountry. Are these areas (Tallac, Cold Stream) thought of as generally safe in higher avalanche danger? I've heard a lot about other areas (Jakes, Rubicon) being safe to ski even on considerable days due to trees (lower chance of SH) and skier compaction.

First point: if you re-read the avalanche bulletin linked in the top and the bulletins from days preceeding this accident, you will see that the surface hoar problem was identified in locations below treeline (higher chance of SH).

Second point: Despite regular skier traffic in some of these zones, I would never buy into any "skier compaction" fantasy to provide confidence that an avlanche problem is not existing in a specific zone. This is not the Utah backcountry and we're not talking about moguls forming in these zones, daily patrol slope cuts, regular bombing, etc.

Regarding the zones and how people consider them:
Tallac is a big mountain with lots of aspects and includes extremely exposed and steep terrain, open bowls, and well anchored snow below treeline. There are some areas below treeline that are considered safer when lee aspects near and above treeline are being or have been recently loaded by new snow. The very top of Corkscrew bowl where the accident occured would be considered a near/above treeline area.

Coldstream Canyon has a variety of terrain, ranging from lower angle well anchored runs to wind loaded, corniced bowls that contain cliff bands, rollovers and other terrain complexity.

You've heard about the northeast aspect of Jakes, which is pretty much all below treeline except the very top. Yes I would say that many people consider this is a safer area when dealing with storm slab and windslab avalanche problems. However, there are some terrain traps and terrain complexities in the northeast aspect that can cause problems, and Jakes is steep enough that I would consider a lot of the upper part avalanche terrain, even though below treeline.

[adrenalated]

Second point: Pits should not really be used for green-lighting terrain - only red-lighting it. The terrain should be chosen based on the avalanche problem, balancing the group's certainty that they would not run into the problem (or could manage the problem) on this terrain against the consequences of the avalanche problem breaking loose on this terrain. What the pit does, it gives you more information about the avalanche problem compared to what you are anticipating, or possibly changes your characterization or understanding of the avalanche problem. The Tahoe area has lots of available data that goes into forming your hypothesis of the avalanche problem before you even get on the snow. The pit adjusts your expectation of the avalanche problem, for example maybe you were expecting 6" windslab but instead found facets under crust 1.5 feet down which was a more serious problem you didn't anticipate. A single pit should not give you enough certainty that a targeted slope is stable, but it can help you re-characterize the problem or tell you when the slope is obviously unstable. Other observations go into formulating your certainty of stability in this terrain.

Never toured in Tahoe, but we deal with persistent slab problems a lot in CO.

I think Schralph makes a very good point about certainty vs. consequences. I personally refer to is as likelihood vs consequences and is something I think about every day out in the backcountry. First, what is the likelihood that the slope will slide - slope angle, slope configuration (anchoring, trigger points, etc), and snowpack structure all play into this. Second, what are the consequences if the slope does slide - slope configuration again plays a role (are there terrain traps, strainer trees, etc) as well as slope size and potential slab depth and propagation potential. So if the likelihood is low but the consequences are high, I am likely to avoid that slope. Same if the likelihood is high but consequences are low. So I'm basically looking for low consequence, low likelihood slopes for most of the year. Anytime I'm targeting a higher consequence line, my certainty that the likelihood is very low must be very good or I will not be skiing that slope.

For many persistent slab problems, the consequences side of that equation is often high just due to propagation potential so all the other factors are irrelevant. I then avoid slopes that might harbor the persistent slab problem entirely.

As for pits - yes, agree they can only be used to red-light terrain, not green light. I make a plan first based on the forecast and avalanche problems for the day. Then, if I dig a pit, I'm looking for snowpack that is different from what I'm expecting to see. For example, let's say there is a PS problem near and above treeline. Generally there is no slab below treeline. So I plan on skiing a below treeline slope. I dig a pit below treeline and find no slab. I continue with the plan. But if I find a slab in that pit, then I'm adjusting my terrain back to lower angles.

For stability tests - I rarely do CTs anymore. They're good for identifying a weak layer but not much more. I'm really preferring the ECT these days. I really don't pay much attention to how many taps it takes, the main thing I'm looking for is propagation. If the ECT propagates across the column, I'm out of there.

[TahoeJ]

Pits should not really be used for green-lighting terrain - only red-lighting it.

Really like this point, given the inherent unreliability of a single snow pit (or 2 or 10 for that matter). Kind of a neat thread with so many involved - or adjacent - parties to the incident showing up to check in. A great example of why this forum is worth hanging around.

[bodywhomper]

following the tactic of not using pits to green light (devil's advocate here), say you headed into deep creek this past tuesday , having observed active avi activity in castle peak area on Monday. You enter with the preconception of observing many potential weakness in the snow pack: two reactive SH layers and faceting around the rain crust. your plan is to have a nice tour observe those layers and ski some 30* or less slopes that are not exposed to larger/steeper slopes above, where you could trigger a slab from the bottom. basically, keeping it conservative. you dig many pits and poke around. you consistently find no significant instabilities, basically a well bonded and uniform snowpack. the SH is rounding and being non-reactive to snowpack tests, no reactions around the rain crusts and the top 6-12" is unconsolidated and cold. What's the call? stick to the protected and low angle slopes or alter plans and take higher consequence (GLORIOUS!) lines?

yes, i realize that the scenario include obs beyond just myopic of a single pit.

[SchralphMacchio]

The question is whether those observations are applicable to the higher consequence lines (loading pattern, wx history) to reformulate your identification of the Avalanche problem, and most of all if you feel that data gives you enough certainty to take on the higher consequences. Do your partners agree with the homework and share in the certainty?

I didn't make up consequences vs. certainty, I'm borrowing it from Tremper's latest field guide. It's very conceptual and difficult to apply until you embrace the approach.

[swissiphic]

Relevant to the variability of snow instability test results....I remember reading this paper years ago. Twas an eye opener on the concept of spatial variability.

http://schulich.ucalgary.ca/asarc/fi...bellThesis.pdf

[bodywhomper]

I remember seeing that ^^^^ article a while back and having some long conversations with friends. it definitely changed my perspective.

The question is whether those observations are applicable to the higher consequence lines (loading pattern, wx history) to reformulate your identification of the Avalanche problem, and most of all if you feel that data gives you enough certainty to take on the higher consequences. Do your partners agree with the homework and share in the certainty?

the scenario that i provided was a compilation of a few obs on the SAC site (or their FB page). this week, we've experienced weather that has stabilized a lot of the tahoe snowpack. for me, part of understanding that stabilization would be through observations in the snowpack looking at structure and re-activity/energy/propagation, especially of the SH weaknesses that had been (variably) reactive for the past several weeks. digging is a great way to know for this type of circumstance. i would not hit higher consequence terrain w/o personal observations of what was going on with those layers.

that SH layer was still probably lurking and reactive in a few places here and there until tonight/tomorrow when it'll (hopefully) be hosed.

[shakyknees]

Thanks to all those who have offered first-hand accounts here, especially whatsupdoc. It sounds like this slide had potential to injure or kill someone. As someone who likes skiing corkscrew bowl in powder, this discussion is a valuable close-up of the event that I worry about but have never witnessed first-hand.

[TahoeJ]

the scenario that i provided was a compilation of a few obs on the SAC site (or their FB page). this week, we've experienced weather that has stabilized a lot of the tahoe snowpack. for me, part of understanding that stabilization would be through observations in the snowpack looking at structure and re-activity/energy/propagation, especially of the SH weaknesses that had been (variably) reactive for the past several weeks. digging is a great way to know for this type of circumstance. i would not hit higher consequence terrain w/o personal observations of what was going on with those layers.

But I would say that you would be heading out with some degree of optimism / trust regarding the SAC removal of the PWL as an avalanche problem, and digging the pit looking for that layer to show instability and therefore possibly red light your plans of anything steeper. Right?

Edit: nevermind, you said Tuesday and SAC didn't make that update until Wednesday.

Even so, with only the anecdotal data of a pit or two, I probably wouldn't trust my findings until there was more widespread confirmation of the same. But we all have our varying tolerances.