Avalanche Discussion

It's kinda looking like this upside-down snowpack is on it's way to being rightside-up given the recent weather patterns, and forecasted dry period over the next weeks... Cleansing period? Any thoughts/testimonials?

With the freezing level hovering about 10-12,000 ft. the snowpack has consolidated over the past number of days.

In the future we should only have to worry about the new snow on top and wind slab build up.

Update (my comments are for Central Washington to Southern Oregon)...with the understanding that conditions may be different at elevations above 8-9,000 ft. As soon as it snows (any amount) and the wind starts to blow surface slabs with once again be an issue.

(I keep forgetting their are others outside of Oregon and Washington that read TAY...)

It really depends on what area you're going to be skiing. Down here in Oregon, the the snowpack has defitinetly consolidated and the avy danger has gone down, but we haven't seen as much rain as up north. I'm still concerned about the buried depth hoar, especially at the higher elevations, and will be keeping an eye out as spring time approaches.

Also, I'm headed to the Whistler area next week, and the danger is still very high in that area due to the continental like snowpack. This winter's upside down snowpack has made me appreciate living in the northwest (I wouldn't like to see this stuff every stinking year...........)

We should really be preparing ourselves for the likely eventuality of a bad persistent weak layer once there is sufficient load and cohesion overlying the existing surface structure. One of the surfaces that exists is an amazingly slick ice glaze. I saw this surface near Paradise in a few isolated spots but also noted that one TAY trip report mentioned an ice glaze near Baker. If it is the same surface layer at Baker it will definitely be one to watch for in the weeks ahead. In general the bond of eventual new snow is not likely to be very good even absent this ice glaze.

I disagree with some earlier posts regarding the application of NWAC or other avalanche forecasts on a slope scale. Avalanche forecasts are used in the planning phase of a trip and NWAC (or other forecasts) combined with telemetry obs are the primary sources I use for planning a trip down to the slope scale. I then use the power of observation once at a destination to modify the original trip plan if necessary or desireable. Initially, most of this modification is done visually with wind effects and visible avalanches (if any) playing a major role. The keys in being responsive to the potential for trip changes are to remain alert and to pay attention to observations and secondly to make sure to ski with partners who are open to communication. Any time unexpected observations can be made and also at any terrain-critical decision point it is the best practice to regroup and to communicate.

The problem with observations are really twofold: 1) Visual observations (primarily of wind effects with respect to dry snow decisionmaking) are limited by visibility. By definition then, decisionmaking and route scale (perhaps meso is a good word here) choices are limited by poor feedback from observations with poor visibility. 2)Snowpack observations based on test results, particularly in the case of persistent weak layers, may be unreliable for anything greater than the precise location of the snowpack test. New information on the ECT may provide some greater confidence but the test itself is very new. With persistent weakness I would personally rely to a greater extent on the avalanche forecast (which is based in part on multiple observations like the professional INFO-EX in Canada). It's worth noting that CMH in Canada ruled out by fiat 2/3 of it's heli-skiing terrain in the winter of 2002-3; a winter dominated by unpredictable and multiple persistent weaknesses. Even just two weeks ago both skiers near Washington Pass and Canadian forecasters warned of unpredictable and highly variable snowpacks, to the extent that at Glacier National Park forecasters maintained a high rating for nearly a ten day period due to such weaknesses. Natural slides were still coming down and results from control work were definitive.

In snowpacks with sufficiently buried weak layers I think it wisest not to try to forecast stability on most slopes (perhaps with a few exceptions) but rather to turn one's attention to the consequences of triggering a slide. Persistent weak layers are easier to trigger when deeply buried than are most storm snow layers and triggering a deep weak layer obviously opens the door for the worst of potential consequences. The ultimate key to safety is wise use of terrain not trying to be a snowpack guru, for, as La Chapelle said, "Don't try to outguess the snow". This is especially so with snowpacks dominated by persistent weakness. Everyone over time will make mistakes in evaluating snowpack stability on all scales, slope or otherwise.

Nice treatise on our current snowpack, Gary.
Thanks.

1. The bulletin is an outstanding, high-quality resource that is simply not applicable at the slope scale. It is dangerous to use a synoptic scale or mesoscale forecast product as the sole source of information for go/no go decisions. From trip reports, it seems most TAY members pay attention and evaluate instability in the field - regardless of the bulletin. This is definitely the right course of action. Relying on the bulletin is foolish.

2. The three principle difficulties in forecasting are a.) spatial and temporal variability, b.) incremental changes to weather and snowpack, and c.) variations in human perception. The NWAC forecast specifically discusses how to use their forecasts on their web site. I don't think NWAC wants anyone to use their forecast at the slope scale.

For Gary_Brill. I'm sure you know all of this already but I'll explain my thinking.

1. Your post disagrees with my thoughts on using the bulletin at the slope scale. I'll explain my reasoning.

The bulletin is a great tool for choosing which mountains to go ski ( or whether to go out at all ) but the scale of the information ( synoptic scale and sometimes mesoscale ) doesn't match the scale of an individual slope ( the slope scale ).

It is unwise to forecast stability for an individual slope with Class III information. Class I and Class II information more closely match the scale of the problem. I understand that information from specific slopes, and even reports on TAY, is integrated into the bulletin but even then, correct forecasting procedure says that information from each data class must be weighed according to the degree to which the information reveals instability.

For slopeside forecasts, this information must be relevant to the current place and time and must be derived from the conditions at hand. Not the bulletin.

2. Avalanche forecasting is a Bayesian activity. This requires the prior ( data about the history of the terrain, weather, and snowpack ) and the likelihood ( data about the current terrain, weather, and snowpack. ) Finally, the posterior ( the prediction ) is assembled.
One important characteristic of Bayesian revision is that a single piece of data ( cracking in the snowcover ) has the power to completely revise the entire forecast.

According to The Avalanche Handbook, you can't form an accurate prediction without the prior and the likelihood. The bulletin does not include information on instability for specific slopes ( the likelihood ). Furthermore, The Avalanche Handbook specifically states that over-reliance on the bulletin is probably the source of many backcountry accidents.

3. Your post doesn't actually address the key problem with observations.

Observations and data sampling, especially snowpits, form much of the basis for perception of instability. Problems arise when these observations foster "false stable perception". It's like the proverb: "absence of evidence is not evidence of absence".

This is especially important during conditional instability - which is the prevailing state of the winter snowpack. Perception is always poorer during conditional instability than absolute instability and low instability. Data sampling can be a helpful, or harmful, part of the search for instability.

Anyway, poor visibility and the well-known difficulties of extrapolating slopeside instability tests across a wider area, aren't nearly as important as how the results of observations alter the recreationist's perception of instability.

The "human factors" of course come into play if you start to examine what the recreationst "wants" to perceive about instability. That's really the key issue with respect to observations: how observations affect perception of instability.

The correct procedure during poor visibility, or any other high uncertainty situation, is to implement a very wide margin of safety.

4. Quote: "Persistent weak layers are easier to trigger when deeply buried than are most storm snow layers and triggering a deep weak layer obviously opens the door for the worst of potential consequences."

I profoundly disagree with your statement. Skier triggering of instabilities below 1 meter is rare.
If you want to talk about deeply buried weaknesses, you have to discuss the problem of perception. The persistent forms, when deeply buried, present a very serious perception problem, regardless of the energy required to release the slope.

Deeply buried persistent forms are not easy to trigger when skiing. Shallowly buried persistent forms can be skier-triggered and the fracture can propagate to areas where the snowpack is very deep.

I'm surprised you would even say that "deeply buried weaknesses are easier to trigger" because it's not true. Storm snow instabilities are much easier to release than deep instabilities. Soft slab avalanches that run in storm snow are responsible for many fatalities.

Once again, winter 2003 is discussed. Those famous accidents occurred because a large number of people were in harm's way. I'm not sure I see the parallel between the snowpack that is hundreds of miles away ( and in a different snow climate ) and the snowpack that is "right here, right now"?

I've heard forecasters in Revelstoke compare the current snowpack in their area to the winter of 2003 but I haven't heard any forecasters in Washington compare the snowpack here to the snowpack in the southern interior of British Columbia. I'm not sure how/why the avalanche bulletin up at Rogers Pass ( I take it that's the Glacier Park to which you refer ) has any relevance to conditions here in the Cascades?

But all this aside, it is true that deep instabilities have a well-deserved reputation for producing very dangerous avalanches. However, I'd hate to have someone read this thread and think that storm snow instabilities are harder to trigger than deep instabilities.

6. Quote: "We should really be preparing ourselves for the likely eventuality of a bad persistent weak layer once there is sufficient load and cohesion overlying the existing surface structure."

What does this mean? Did you mean to say "We should prepare ourselves for another avalanche cycle once there is a cohesive slab overlaying a persistent weak layer."?

Otherwise, can you clarify your statement?

Load and cohesion are good for stability in the long term. In fact, the persistent forms are dangerous because they resist strength gains from load ( due to their anisotropy ) which means the weak layers often have very poor cohesion ( which would be evidenced by hand tests and a low number of bonds per unit volume ).

7. Ice glaze.

Sure an ice glaze exists in places. However the temperature of the snow that falls onto the ice glaze, and the temperature difference between the ice glaze and the new snow crystals, is what determines how the new snow bonds to the ice glaze.

Does it seem reasonable to expect a nice soft slab cycle to run on this glaze? Sure. In some places. Does it seem reasonable to expect a nice soft slab cycle to run in places where this glaze isn't found? Sure. In some places.

Is this ice glaze the layer to watch? Possibly. But with the general powder hound culture in these parts, I'd probably be more concerned about instabilities in new snow ( soft slab avalanches ).

8. Quote: "New information on the ECT may provide some greater confidence but the test itself is very new."

Did you mean propagation saw test? Because as far as I'm aware, the ECT has been validated. Doesn't the ECT result in fewer "false stable" results ... and more "false unstable" results? The ECT is specifically designed to balance fracture propensity with propagation propensity - very helpful for instabilities in new snow.

9. Quote: "With persistent weakness I would personally rely to a greater extent on the avalanche forecast (which is based in part on multiple observations like the professional INFO-EX in Canada)."

My thoughts are that persistent weaknesses call for conservative terrain choices. I bet most avalanche professionals would agree. Relying on the bulletin is not the same as making conservative terrain choices.