Slope Instability (snow)

[greenonion]

First off, I have Not taken the AAIRE courses and do have a lot more to learn about avy conditions and assessment. I notice in winter that the conclusions of folks about slope instability comes up fairly often. Meaning, it seems to get thrown around a bit that one concludes a slope to not be unstable. Is this potentially a dangerous misleading statement, or are there specific ways to asses instability visually, and without digging to check things out more thoroughly? I know cracks, womphs, and cornices are to be looked and listened for, but can’t a slope be ripe for sliding without obvious visual clues? Just wondering if some slope instability statements are sometimes made too easily or quickly?? I also know underlying faceting can set up dangerous conditions, but I assume that is not an easily defined situation on visual assessment alone. I have a lot to learn here, so let me have it, if you will.

[Dave B]

Some signs of instability are visual (cracking, obvious wind slabs, adequate slope angle, observations of surface hoar pre-storm), others are audible (whoomphs), some require digging (faceted or ice layers, assessing slab "energy"). But there is no such thing as a holistic assessment of snowpack stability so any travel onto avy terrain carries with it a degree of risk - and thus travel depends on the risk tolerance of the individual or group.

I spend a lot of time digging around in the snowpack during the winter, mostly as part of water resource research where we measure snow depth and density to get snow water equivalent (SWE; the amount of water held in the snow pack). We do dig pits and measured vertical variation in density, but rarely do avy-specific tests like the ECT (mostly because we purposely sample from low slope angle locations). Regardless, the one thing I can assure you is that the snowpack in CO is characterized by spatial variability above anything else. Not only variability in depth, but also variability in density and how layers form and change. This leads to spatial variability in factors leading to avy risk and danger.

The point being, never ever ever ever ever ever make go/no-go decisions from a single pit or a single observation. Decisions need to be made as an aggregation of all factors and any signs of instability/danger need to be weighted far heavier than signs of stability.

The Avaluator is an excellent rubric for decision making

[Jorts]

rarely do avy-specific tests like the ECT (mostly because we purposely sample from low slope angle locations).

It's notable for the OP that ECTs are reliable as stability indicators on low angle slopes though.

For more on this: https://avalanche.org/wp-content/upload ... opeAng.pdf

are there specific ways to asses instability visually, and without digging to check things out more thoroughly?

Other signs of avalanche activity on slopes at similar elevations and aspects is the best indicator. For moderate hazard where natural slides are unlikely and human triggered are possible, you can look for windloading for possible instability or conversely wind compaction and stripping for greater stability. In deeper snowpacks with a known buried weak layer, the cohesive strong layer is more likely to be connected around terrain features like ridge lines and tree cover. That's the kind of snowpack where you want to be aware of what's overhead. Aim for 20 deg or less for an alpha angle if you're on flat terrain and concerned about overhead hazard.

I'm not sure I know what you mean by instability statements being made too easily or quickly. Stability certainty is rare. A slope can be skied many times without avalanching and then rip upon the fifth skiers descent. Be leery of active loading and new loading.

[daway8]

I assume the reference is to various CR's etc where it's somewhat common to see people make some remark about whether or not any signs of instability were noted.

I occasionally make such remarks on some of my updates, as do a variety of folks on here. I *think* I usually qualify any such statements I make to be phrased something like: "no obvious signs of instability."

The idea is that it can be fairly easy to confirm that there ARE potentially dangerous conditions (like some of the things the OP mentioned) but not nearly so easy to conclusively prove that there are NOT dangerous conditions. It helps to dig into the CAIC forecast beyond just the basic color coded level of danger to look at the specific problems, aspects, probabilities, sizes, etc as well as read the discussion to see where things are trending and what sort of issues have been observed recently.

Taking classes can certainly increase your awareness of potential issues but one thing (among many others) they warn you about in AAIRE is the "expert halo" - where you can trust that such and such slope won't slide because so-and-so said it wouldn't and he's an "expert."

Bottom line - it's your life on the line (and possibly the lives of others on the mountain that day) so choose carefully how you evaluate danger.

[greenonion]

I assume the reference is to various CR's etc where it's somewhat common to see people make some remark about whether or not any signs of instability were noted.

I occasionally make such remarks on some of my updates, as do a variety of folks on here. I *think* I usually qualify any such statements I make to be phrased something like: "no obvious signs of instability."

The idea is that it can be fairly easy to confirm that there ARE potentially dangerous conditions (like some of the things the OP mentioned) but not nearly so easy to conclusively prove that there are NOT dangerous conditions. It helps to dig into the CAIC forecast beyond just the basic color coded level of danger to look at the specific problems, aspects, probabilities, sizes, etc as well as read the discussion to see where things are trending and what sort of issues have been observed recently.

Taking classes can certainly increase your awareness of potential issues but one thing (among many others) they warn you about in AAIRE is the "expert halo" - where you can trust that such and such slope won't slide because so-and-so said it wouldn't and he's an "expert."

Bottom line - it's your life on the line (and possibly the lives of others on the mountain that day) so choose carefully how you evaluate danger.

Thanks daway. You nailed my question’s intention. And Dave B and Jorts helped a bunch too. I was concerned about the willy nilly statements at times in CRs that a visual inspection of a slope appears stable. Maybe, maybe not… given all the variables involved as well described by these responders. Appreciate those responses. I’ve learned and hope to increase awareness a bit by the original post, uh… op. Thanks again

[Jorts]

Ahh. Didn’t realize CRs prompted that question.

If there are no new slides in the vicinity; no cracking or collapsing while walking across a slope; or if hucking rocks or sawing a cornice on to it doesn’t result in a slide, it’s not unreasonable to declare “no signs of instability”.

But “scary” moderate is that somewhat deeply buried persistent weak layer that isn’t particularly reactive but that can be affected if you hit that sweet spot or can be dormant until new loading reaches critical mass making it more reactive.

I wouldn’t give much credence to any broad statements about stability posted on CRs. Check out CAIC obs for detailed, reliable info.

[curt86iroc]

But “scary” moderate is that somewhat deeply buried persistent weak layer that isn’t particularly reactive but that can be affected if you hit that sweet spot

Here's an example of what scary moderate looks like in real life:

https://www.youtube.com/watch?v=LzXKVGhoZ-A

[greenonion]

But “scary” moderate is that somewhat deeply buried persistent weak layer that isn’t particularly reactive but that can be affected if you hit that sweet spot

Here's an example of what scary moderate looks like in real life:

https://www.youtube.com/watch?v=LzXKVGhoZ-A

EVERYONE going into the backcountry in winter needs to watch that. 14 very important minutes of video. Thanks for sharing

[GravityPilot]

If people are interested in the "obvious signs of instability" I suggest you read this paper that came out this year from Jason Konigsberg of the CAIC. Hopefully, it provides insight into why and when forecasters alter their message regarding cracking and collapsing.
https://arc.lib.montana.edu/snow-scienc ... hp?id=3008

[daway8]

If people are interested in the "obvious signs of instability" I suggest you read this paper that came out this year from Jason Konigsberg of the CAIC. Hopefully, it provides insight into why and when forecasters alter their message regarding cracking and collapsing.
https://arc.lib.montana.edu/snow-scienc ... hp?id=3008

I think I must have seen a preliminary presentation of some of this or similar data last winter at an event where CAIC was present - I recall some snazzy graphics showing a good whump-to-avalanche correlation early in the season but less so later in the season. That's basically what's being referred to in this summary statement at the end of the abstract:

"Our data show that although whumpfs generally indicate unstable conditions and correlate with avalanche activity, the largest avalanches of the winter may not always be preceded by whumpfing."

Key takeaway: avalanches are complex phenomenons - don't rely too much on any one bit of data (which color the avy map is, presence or lack of whumping/cracking, etc) but instead always take full advantage of the complete amount of details on every page of the CAIC forecast along with careful observations in the field.

[Jorts]

Anecdotally it seems like hikers generally trigger dangerous slides in the compression zone beneath a slide path later in the season. I think of the multiple accidents on Torreys and the one on Hoosier Pass a couple years ago. Hikers on summer trails triggering slides from below. Best to simply avoid undercutting suspect terrain by giving it a wide berth even if that means breaking trail off of the established trail or turning around and finding an alternate route. Alpha angle can be estimated with the inclinometer on a phone by extending your arm. Read CAIC’s forecast like scripture. Forecasters are generally most aware of conditions that can lead to remote triggers.

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