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What is the ideal stable snow base scenario?

post #1 of 21
Thread Starter 

Now that the snow is flying and the natural bases are being laid down in the avalanche vulnerable areas, I was wondering what the ideal snowfall scenario is for a season long stable base and what the nightmare scenario is for a season long unstable base.

post #2 of 21

I think you want to start with the snow landing on cold ground. Warmth coming up from beneath is not good at all.

post #3 of 21

The last 2 seasons in the Wasatch have not been good as far as early season instability.  October snows turned to sugar & were stubborn to heal.

 

It was looking like the same problem may repeat itself this season.  A field report this morning from the Alta periphery sounds encouraging:

 

Quote:
 

(By 4pm) About 12" or so of thick powder on either old melt freeze crusts or some raggedy faceted snow from last week.  The new snow came in warm and bonded well to the old snow surfaces.  Shears were found a couple inches into the new snow on a graupel layer.  In the high northerly aspects, hard shears were also evident on the facets.  Overall, I feel it's a pretty good right-side-up foundation.  The 'touch of grey' (ie-the couple cms of diurnal faceted snow) I feel should now be insulated by the warm new snow and heal quickly.  A powerful lens indicated that they were already rounding and starting to 'neck', or bond with one another.

 

Only time will tell .

 

JF

post #4 of 21

The best situation is when you start with a ground surface that is large boulders, heavily shrubbed or has other good anchors. Cold ground is better than warm ground, but a caveat to that is that the ground should not have frost, nor should there be any cold dry periods that allow surface hoar to form. Storms that start wet, even to the point of starting as rain, then cooling through the storm allow good bonding between layers. Storms that start cold and dry and turn warmer are considered "upside down" and slide easily. Snowfall that is consistent in temperature and snowfall will generally be better than temperatures that go up and down. High moisture content in the snowfall is better for bonding than drier snow.

 

post #5 of 21

a surface of faceted snow caused by wind buff is a poor layer for stability. keep track of wind buff (ha, love it then leave it)

post #6 of 21

Faceted snow isn't caused by wind buff.
 

Quote:
Originally Posted by davluri View Post

a surface of faceted snow caused by wind buff is a poor layer for stability. keep track of wind buff (ha, love it then leave it)

post #7 of 21
Quote:
Originally Posted by davluri View Post

a surface of faceted snow caused by wind buff is a poor layer for stability. keep track of wind buff (ha, love it then leave it)



Do SAR a favor and stay inbounds BRO

post #8 of 21
Quote:
Originally Posted by davluri View Post

a surface of faceted snow caused by wind buff is a poor layer for stability. keep track of wind buff (ha, love it then leave it)


although wind loading can cause a layer. sometimes in itselfs a danger. Its not facets at all like you say in your post. facets form below the surface from temp gradient in the snow. Its anglar and pretty easily spotted in any pits if you know what your looking for. Which you dont yet.....

 

 

 

 

 

post #9 of 21

When it starts dumping and continues 30"-50" a week fo ra month or so with out any ridiculous wind or deep freeze conditions.

post #10 of 21

Basic snow theory: Rockies.

Temperature gradient through the snow pack = Temperature changes move vapour and change snow crystal shapes from flakes to cups or ball bearings.

Typical Rockies snow packs are shallow = 1 - 2 meters. The ground stays around freezing 0*C, 32*F,  and the air only a meter or two away can be - 20 or -30 C or F.  (these scales cross at about -35* )...

So you have a shallow snow pack early season and big temp differences promoting Destructive Metamorphosis at ground level and then you get wind loading ( really big factor and produces very local traps ) or new snow that is sitting on depth hoar... boom.  big badda boom.

Coastal snow is better because temp ranges are narrower due to warmer air and take place over a 2-5 meter snow pack, from ground to air = Not as much depth hoar... it is buried way deep and usually waiting for spring Isothermal snow packs. Still,  on top of the snow pack, frost layers form due to generally moist snow with cold nights. Flakes are great glide for that new 30 or 40 cms fresh. Later its slabs and then cornices.

This is general theory and I am rusty. I have no idea how to "make the call" and usually wait for a nice boot pack to be formed by many dozen avalanche guinea powder pigs. Read the avi reports and ask the the patrol if they have had any results lately.

post #11 of 21

you don't die by getting a word wrong, guys, and the concept is close to accurate: certain types of snow cause the subsequent layer not to bond. what is snow called after movement in the wind knocks off the (tendrils?) or the star shaped arms? It's been a while since I read the books on the subject. Wind buff would be an example of a layer that doesn't neck well to anything and creates bad bonding? Storms that don't come in "upside down" are the best, start warm and wet and cool throughout the storm. I think. Correct?

 

In Tahoe, one of the worst slides we had was a deep heavy layer on top of wind buff, so I mention it for safety. Just saying, if it snows a foot the day after you were skiing epic wind buff, be aware. That's helpful information for resort skiers. (I understand the need to nip bad information in the bud, my bad....bud)

 

just a point on learning, which is the OPs intent with the question as I read it. I don't guess piling on and not offering the correct term or explanation teaches anyone anything (except that I'm not the consumate expert, ha). (that was like a pirhana frenzy)

 

and no, I don't ski back country (and have never wasted SAR's time), and for me, to each their own way of enjoying the mountain: mountaineers, campers, alpine tourers, hikers, racers, huckers, and many more. want to cop an attitude about placing an emphasis on hiking? it's an easy pose, see it every day, means nothing. 

 

post #12 of 21

The thing about snow thats been pulverized by wind is that the particle size is small so it tends to pack well (sinter) to itself as there is a lot of grain to grain contact.  Also the crystals are "heated" up a bit as they are being pulverized so that they are inherently more reactive and form bonds quickly when they stop moving.  This leads to slab formation through wind loading.  The fact that a slab is cohesive is what makes it dangerous.  How dangerous depends on a lot of other factors.  Slabs and layers formed by one wind or precipitation event may or may not bond well to each other at the interface even if they are well bonded internally.  It can get fairly complex, but is somewhat easy to evaluate stability wise if you understand the science and can apply the right modeling to your test results.  It's also critical to understand the role of terrain in slope stability for choosing safe pit locations that are representative of the slopes in question.  IMO terrain choice is the second most important variable after the human factor for safe backcountry travel.  

 

BTW an "upside down storm" is one that comes in cold and warms up.  You are right that what we want for stability is a storm that comes in warm and gets colder.

 

AND I wasn't trying to rip on you, just pointing out that what you said was a misapplication of terms.
 

Quote:
Originally Posted by davluri View Post

you don't die by getting a word wrong, guys, and the concept is close to accurate: certain types of snow cause the subsequent layer not to bond. what is snow called after movement in the wind knocks off the (tendrils?) or the star shaped arms? It's been a while since I read the books on the subject. Wind buff would be an example of a layer that doesn't neck well to anything and creates bad bonding? Storms that don't come in "upside down" are the best, start warm and wet and cool throughout the storm. I think. Correct?

 

In Tahoe, one of the worst slides we had was a deep heavy layer on top of wind buff, so I mention it for safety. Just saying, if it snows a foot the day after you were skiing epic wind buff, be aware. That's helpful information for resort skiers. (I understand the need to nip bad information in the bud, my bad....bud)

 

just a point on learning, which is the OPs intent with the question as I read it. I don't guess piling on and not offering the correct term or explanation teaches anyone anything (except that I'm not the consumate expert, ha). (that was like a pirhana frenzy)

 

and no, I don't ski back country (and have never wasted SAR's time), and for me, to each their own way of enjoying the mountain: mountaineers, campers, alpine tourers, hikers, racers, huckers, and many more. want to cop an attitude about placing an emphasis on hiking? it's an easy pose, see it every day, means nothing. 

 

post #13 of 21

awesome, it's cool. always learning, and even learning to accept correction. this stuff is too important to goof it up.

 

we all ski some huge sloughs every year, and it's an eye opener how helpless you are all of a suddent.

 

another comment: wet slides; they look so slow and innocent and easy to just ski across; I learned a lesson: NOT. and once you're stuck, point em in the direction of the slide.

 

Tahoe just got a rightside up storm, not much in accumulation, but it was like plaster and stuck to everything. I'm optimistic for the season. still praying for (wet!) snow.

 

there, any truth to any of that?

post #14 of 21
Thread Starter 

So it sounds like what you all are saying, is the worst case base snow stability would be a very cold dry first snowfall that was then windbuffed to create a solid slab base, than have a period of warm weather with no snow followed by a hard freeze.  This would make a dense cold slab base that the next layer would not easily bond with therefore building your entire seasons snowfall on a very unstable foundation.

 

Do I have that correct?

post #15 of 21

That's close.  It would be much worse if instead of a period of warm weather you had a period of very cold weather.  Then have a storm come in very cold with wind and finish warm with heavy accumulations. It's also worse when the original snow layer is shallow.  A deeper snow layer insulates the warm ground from the cold atmosphere and slows the formation of facets through vapor transport as g-force pointed out.  The windbuffed slab is not good, but buried surface hoar and layers of faceted snow and depth hoar can be harder to quantify.  Also the slab surface is more prone to positive metamorphosis than the hoar and the facets which tend to be very resistant to rounding and neck formation (strengthening).  

 

Quote:
Originally Posted by Roadrash View Post

So it sounds like what you all are saying, is the worst case base snow stability would be a very cold dry first snowfall that was then windbuffed to create a solid slab base, than have a period of warm weather with no snow followed by a hard freeze.  This would make a dense cold slab base that the next layer would not easily bond with therefore building your entire seasons snowfall on a very unstable foundation.

 

Do I have that correct?

post #16 of 21

You need to be cautious with thinking you can get a snowpack that's identifiably "stable" for a season.  You can get, and often do get, unstable and highly varied early season conditions, but if you have that clean off down to dirt you can start over and get a nice snowpack later.  You can also have a snowpack heal over time in a variety of ways -- one reason why heli-guide "stability testing" with explosives really doesn't make much sense in the overall scheme of things in some places -- and can have snowpacks start off stable and then get very unstable in one storm.

 

While that may seem confusing, in practical terms if you are aware of conditions, both regionally and for your specific choice of terrain, it doesn't need to be super complex. 

 

The internet is not the place to sort that out, though.  For someone in GA, you could probably work with a guide in a specific area to get some combo of avy education and specific terrain education in a way that could leave you, at the end of a week (or two, depending on where you're coming from in some other areas), able to deal with good conditions stability-wise on your own, in a reasonable way, maybe even better than many locals who may be inclined to push things a bit.  That would be expensive to do, but far cheaper than trying to suss this out on the internet with all its pitfalls and huge sloughs.  And, you'd probably get some great skiing in at the same time.


Edited by CTKook - 11/14/10 at 8:10am
post #17 of 21
Thread Starter 

My interest is purely academic.  I'm too old and out of shape to be venturing into avalanche prone areas.  I was just curious why some years (like last year or the year before) in Colorado were so bad from the get go with avalanches and some years aren't at all.  
 

Quote:
Originally Posted by CTKook View Post

You need to be cautious with thinking you can get a snowpack that's identifiably "stable" for a season.  You can get, and often do get, unstable and highly varied early season conditions, but if you have that clean off down to dirt you can start over and get a nice snowpack later.  You can also have a snowpack heal over time in a variety of ways -- one reason why heli-guide "stability testing" with explosives really doesn't make much sense in the overall scheme of things in some places -- and can have snowpacks start off stable and then get very unstable in one storm.

 

While that may seem confusing, in practical terms if you are aware of conditions, both regionally and for your specific choice of terrain, it doesn't need to be super complex. 

 

The internet is not the place to sort that out, though.  For someone in GA, you could probably work with a guide in a specific area to get some combo of avy education and specific terrain education in a way that could leave you, at the end of a week (or two, depending on where you're coming from in some other areas), able to deal with good conditions stability-wise on your own, in a reasonable way, maybe even better than many locals who may be inclined to push things a bit.  That would be expensive to do, but far cheaper than trying to suss this out on the internet with all its pitfalls and huge sloughs.  And, you'd probably get some great skiing in at the same time.

post #18 of 21
Quote:
Originally Posted by Roadrash View Post

My interest is purely academic.... I was just curious why some years (like last year or the year before) in Colorado were so bad from the get go with avalanches and some years aren't at all. 
 

 

Most of the regional forecasts carry at least a year's worth of archives.  Also http://www.telemarktalk.com/phpBB/viewforum.php?f=2 and www.wildsnow.com have good discussion, and http://www.wowasatch.com/ is great for seeing how this may play out when actuallyskiing.  Aside from the general books on the topic, between those sources you could actually read how this can play out over time, for multiple regions.
 

post #19 of 21

I am reading Staying alive in avalanche terrain. One of the themes he discusses so far is that stability conditions tend to be  highly variable from location to location even if its just 1 ridge over or just on the other aspect from where I am now. Also that conditions tend to have both positive and negative impacts on stability depending on where you are and what else is happening. E.G. Cold temps can help to firm up surface slabs and bridge over instabilities making conditions less risky in the short term. But it can also lead to formation of depth hoar, sugary snow, or surface facets which will later get buried and cause problems.

post #20 of 21

Staying Alive in Avalanche Terrain is one of my favorite avalanche books.  I had the opportunity to do some training with Bruce Tremper several years ago.  The biggest influence he had on me was on my pit technique.  I had stopped using pits so much due to the spatial variability problem he talks about in his book.  I still don't think too highly of pit data as a primary tool.  It is however the best way to identify certain structural deficiencies.  Bruce showed me how to get a pit done in 15 minutes collecting the data, recording it, interpreting it in a clean go/no go way and moving on.  Being fast and decisive makes pits more appealing.  IMO terrain is still king.  You can't dig pits everywhere, you have to be able to identify the correct place to collect relevant data and you have to do it safely.  It's really important to be able to recognize subtle terrain variations that affect spatial variability.  Finally you need to be able to separate yourself from your decisions so that you can actually act on the information that is in front of you.  Bridging is real for instance, but IMO can often be a rationalization for justifying a decision that was already made.  Around here people start talking a lot about bridging when conditions are unstable.  Bridging becomes far less important as the size of the slab increases and the harder the slab is, the more destructive it will be if it runs.   

post #21 of 21
Quote:
Originally Posted by Roadrash View Post

My interest is purely academic.  I'm too old and out of shape to be venturing into avalanche prone areas.  I was just curious why some years (like last year or the year before) in Colorado were so bad from the get go with avalanches and some years aren't at all.  
 

 


 


Take a look at the primer on this subject from NWAC: http://www.nwac.us/resources/snowpackinfo/

 

I won't speak to Colorado conditions, as I rarely venture that far east. In the Cascades, every season sees thousands of avalanches, and every season has stable, safe skiing and riding conditions. Usually, what makes the news as far as a "bad" season or a "not so bad" season are the decisions made by people.

 

That said, the stability of a snowpack on a seasonal basis has quite a bit to do with the climatic conditions prevalent in an area. The sunshine (and accompanying cold temperatures) that make the Rockies so attractive also reduce the average stability of slope compared to the typical conditions found out here. The early season around here is also the peak of the hydrological calendar. Throughout November we get snowfall at or near FL, accompanied at times by pack consolidating rainfall. This builds a deep, early base which is consistent in temperature from the ground up, and can often form a truly "bomber" base layer.

 

The stability of the snowpack will still vary from day to day, but -on average- a coastal pack is more stable than a continental pack.

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