Does a good avy cycle really flush out a weak layer?

[Bronic]

Talking about a faceted weak layer in particular. For example, here in Colorado we'll probably go through a pretty good cycle after this weekend's snow on a pretty weak snowpack....december...

I hear the idea thrown around a lot that a good size avy cycle is good cause it'll give a new clean slate in some cases. I can see how that would be the case for some weak interfaces, mainly if they're thin. But say we got 8 inches of basal facets, and then something rips to the ground. Even though there may not be 8 inches anymore, there's still gonna be plenty of leftover shitty grains. Right?

Thoughts from the collective?

[CookieMonster]

Talking about a faceted weak layer in particular. For example, here in Colorado we'll probably go through a pretty good cycle after this weekend's snow on a pretty weak snowpack....december...

I hear the idea thrown around a lot that a good size avy cycle is good cause it'll give a new clean slate in some cases. I can see how that would be the case for some weak interfaces, mainly if they're thin. But say we got 8 inches of basal facets, and then something rips to the ground. Even though there may not be 8 inches anymore, there's still gonna be plenty of leftover shitty grains. Right?

Thoughts from the collective?

Here's something I wrote six + years ago:

The size of the forecast region is the most important factor, with precise answers only available for very small areas. However, even for small areas, the chaotic interaction between terrain and eather
makes it difficult to predict the effects of widespread avalanching on future snowpack instability. The following scenario, which is just one possibility out of many, hints at the overall complexity of this
forecasting problem.

Instability will persist when a bed surface composed of faceted crystals is immediately reloaded during a storm. On the other hand, future snowpack instability on that slope will be very different if the faceted crystals exposed by avalanching are subjected to multiple melt/freeze cycles prior to the next storm. Melt/freeze activity is often limited by aspect, so it is possible for the faceting process to ontinue on cold aspects, while faceted crystals on warm aspects undergo rounding as a result of melt/freeze metamorphism. In this highly general scenario, the weather builds new patterns of nowpack instability that are difficult to uncover without careful observations.

Therefore, for most recreational skiers, knowledge of a recent avalanche cycle is a very general and imprecise piece of information. General information often has a dangerous and unwarranted influence on individual or group beliefs about the presence of instability and its parameters. Without abundant information, expert knowledge, and significant experience, a recent avalanche cycle should not exert undue influence on recreational travel choices and decision-making at any operational scale. More than anything, incremental changes to the snowpack caused by synoptic scale weather events will alter the characteristics of the danger but won’t eliminate it.

* The chaotic relationship between terrain and weather is a primary source of uncertainty.
* Incremental changes to the snowpack are a primary source of uncertainty.
* Avalanches remove weak snow from some, but not all, slopes.
* Avalanches may or may not remove all the weak snow from a specific slope.
* Use multiple sources of information to determine the likelihood of avalanche formation.
* An avalanche cycle over a large area certainly does not mean a specific slope is safe.
* Proactively managing uncertainty is essential to safe decisions.

Here's what I would write now:

* From a geophysical perspective, avalanche forecasting involves scales of space and time.
* Space. Micro/slope, meso, synoptic scale.
* Time. Now, tomorrow, the day after tomorrow, next week. For backcountry skiing, we're usually interested in 'now'.
* It's very difficult to predict the result of an avalanche cycle on instability over the long-term.
* One needs detailed information about the snowpack, winter history, and terrain.
* Even then, there will be significant uncertainty.
* An avalanche will flush out any internal weaknesses in the slab itself. ( Because the slab is destroyed. )
* An avalanche cycle will not immediately decrease instability on slopes that do not slide.
* New snow increases the weight and pressure on weak layers.
* In the very short term, instability on these slopes will sharply increase.
* Over the longer-term, the increase in overburden pressure can reduce instability associated with certain crystal forms and interfaces.
* Typical physical changes from overburden: decreased pore space, sintering, higher number of bonds per unit volume.
* Thicker snowpacks tend to have a smaller temperature gradient, which can promote rounding / ensure that rounding is the dominant metamorphic regime.
* End result: does density and hardness increase with depth? Good.
* Or are there discontinuities in hardness and depth? Bad.
* An avalanche cycle will alter the nature and distribution of instability.
* Sometimes this can make your life easier. Ex: everything slid, buried surface hoar gone on these slopes.
* One would need to directly observe that this is the case. Never assume.
* Sometimes this will increase uncertainty about the spatial and temporal distribution of weaknesses. Ex: everything E/NE slid, buried surface hoar is inexplicably touchier on other slopes.
* For experts at assessing snowpack stability, including ski guides, and people with intimate knowledge of the terrain and detailed knowledge of the current winter history:
* An avalanche cycle will reduce danger on some slopes.
* An avalanche cycle may cause changes to snowpack that limit fracture propogation potential.
* An avalanche cycle can cause avalanches to run that trigger other avalanches that result in widespread reduction of instability at the drainage scale.
* Yes, there will often by plenty of shitty grains left over.
* This is often referred to as 'reloading the bed surface'.
* Long-term you want deep snow and moderate temperatures.
* Thin snow and cold temperatures = bad.

Snowpack is formed by the chaotic interaction of terrain and weather. The chaotic nature of these interactions is an enormous engine of uncertainty and the result is significant spatial and temporal variations in the distribution of weaknesses. Unless you know the slopes in question very well, have made numerous detailed observations, then the fact that an avalanche cycle has occurred should not be used as an input for decision-making EXCEPT to rule slopes out. Ex: "there was an avalanche cycle recently, and some slopes slid to ground. The slopes that didn't slide haven't adjusted to the increased weight, so these slopes are a no go."

* TL;DR A recent avalanche cycle changes nothing. Always follow best practices.

[Lindahl]

IMO, for depth hoar on Nish slopes, a recent avalanche cycle early in the season is often worse for a slope. You just lost a few feet of snowpack that could otherwise be, after later storms, contributing to lowering temperature gradient and healing the hoar.

[goldenboy]

What CM said, if you can digest all of it.

In my experience, widespread events in CO might be more likely to have repeater slides than becoming more stable. Let's say we get 4' in the next week. If a path fails with 2 of those 4', then it will only have 2' of base once the storm is over. Add a few cold nights with what is now a shallow snowpack, and you just have more facets, ready for the next storm.

[homemadesalsa]

This question was raised a number of years ago here (http://www.tetongravity.com/forums/s...g+the+snowpack),

that's where Cookie Monster's first response came from, and then I took it to The Avalanche Review, where we printed this: http://www.avalanche.org/moonstone/F...ck.TAR28.4.pdf

[Bronic]

More than I was asking for but thanks for the big info dump Cookiemonster. Lots of good stuff in there.

I was thinking along the lines of what GB and Lindahl said, most likely not a good "refresh"

I'll add that issue to the loooong list of TAR's to read. Such a valuable resource

Also, shame on me for my piss poor searching skills. I'll try to jong less next time

[baron]

The hidden problem that cant be forecasted into the equation is the old summer leftovers hidden way below the snowpack..i heard them described as a ball bearing base layer...and can cause slides in unexpected areas.

Maybe this was in 1 the long articles? I read little

[Bunion 2020]

^^^ Uh What?

[panchosdad]

To paraphrase, it depends.

[baron]

^^^ Uh What?

the leftover previous years snow turns rotten, pretty much ,through all the melting cycles...when winter dumps new layers on top they fail to hold the weight of the new snow ,and slide.

Snow flakes turn to a round shape, on the molecular lvl when sun hits them>u can google pics from microscopes lol...makes for super weak base layer

[homemadesalsa]

the leftover previous years snow turns rotten, pretty much ,through all the melting cycles...when winter dumps new layers on top they fail to hold the weight of the new snow ,and slide.

Snow flakes turn to a round shape, on the molecular lvl when sun hits them>u can google pics from microscopes lol...makes for super weak base layer

I don't have time to go through your reasoning step by step, but you might want to reread Tremper or the Avalanche Handbook, or even re-take an avalanche class and focus especially on the process and situations where faceting occurs. Go read the TAR articles- it doesn't get much clearer than that.

[panchosdad]

the leftover previous years snow turns rotten, pretty much ,through all the melting cycles...when winter dumps new layers on top they fail to hold the weight of the new snow ,and slide.

Snow flakes turn to a round shape, on the molecular lvl when sun hits them>u can google pics from microscopes lol...makes for super weak base layer

Uhhh, what?

[baron]

This thread is about Avalanche cycle related to slope stability....anyone can say >I agree w the article

We Know....
Every mountain slope is different ,state to state hill to hill; adjacent slopes can hold snow different.
& Avalanches are a redflag warning of unstable slope-not the opposite

[raisingarizona13]

Here's something I wrote six + years ago:
Snowpack is formed by the chaotic interaction of terrain and weather. The chaotic nature of these interactions is an enormous engine of uncertainty and the result is significant spatial and temporal variations in the distribution of weaknesses. Unless you know the slopes in question very well, have made numerous detailed observations, then the fact that an avalanche cycle has occurred should not be used as an input for decision-making EXCEPT to rule slopes out. Ex: "there was an avalanche cycle recently, and some slopes slid to ground. The slopes that didn't slide haven't adjusted to the increased weight, so these slopes are a no go."

* TL;DR A recent avalanche cycle changes nothing. Always follow best practices.

Yes. Spatial variation is a big one imho.

Being on a snow pack that you have been on and around since the fall (home playing field) and understanding it from day 1 of the season is a completely different deal than showing up mid season in an unfamiliar range/pack. If there is any uncertainty and you don't have a trustworthy local companion/guide then we should probably approach terrain and travel choices with extreme caution and expect the worst.

"So many things have come to light, lots of in's and outs and what have yous." The Dude

[raisingarizona13]

the leftover previous years snow turns rotten, pretty much ,through all the melting cycles...when winter dumps new layers on top they fail to hold the weight of the new snow ,and slide.

Snow flakes turn to a round shape, on the molecular lvl when sun hits them>u can google pics from microscopes lol...makes for super weak base layer

Leftover summer snow is generally as solid as it gets, I guess it might create a good bed surface for early season new snow though.

[Greydon Clark]

Interesting to read the perspective of people from continental snowpacks. Having done most of my skiing in maritime climates, I tend to think of “good” avy cycles as rain on snow events that kill lots of trees and reset the snowpack.

[whipski]

Not positive but thinking maritime and Colorado snowpack can be two very different creatures

[sfotex]

Just wanted to bump this old thread back up, I'm hearing a lot of 'flushing the facets out' talk this season in Utah and was digging around for info:
And here is the older TGR thread too: https://www.tetongravity.com/forums/...g+the+snowpack

And the TAR article: https://americanavalanche.squarespac...inal_LoRes.pdf

[Core Shot]

Thx for reposting the link. The original one is broken.

Wow. 2010. Rip big Wally.

And yeah. It’s complicated. My personal lay skier opinion (non industry scientist unlike salsa and so many others that do this for a living) is if you have particular knowledge of what has slid and at what layer and what weather has been happening since then might make one area more stable, but general cycles mean nothing if you haven’t been paying attention to what ripped and why and what happened after.

[sfotex]

<p>
I think that&#39;s how it works in general with a PWL though, you need first hand observations of a slope and it&#39;s history to know if it&#39;s safe. And I can see a slide taking out buried surface hoar/exposing it to melt our, etc</p>
<p>
But we have had a nasty buried facet layer from December out here in the Wasatch, and I have poked around the bed surfaces after a few slides and just thought, geeze, there&#39;s some much spacial varibility on this one &#39;small&#39; slope I can&#39;t figure it out.&nbsp;</p>
<p>
I have though about skiing these slopes before a new slab forms on top again though... that seems safe-ish.. I do feel ok observing with my eyes what slid and where the old snow was removed.&nbsp;</p>
<p>
I guess where I&#39;m going with this is there are a lot of &#39;experts&#39; making bad choices down here on repeater slopes, so how useful is first hand knowledge with some PWLs?</p>
<p>
I keep going back to structure trumps all and lemons on the snowpack though..</p>

[MagnificentUnicorn]

The answer is it depends