Avi conditions near the tipping point?

Something everyone should read before heading out into the backcountry - this is from Karl Klassen at the CAC:

Heads up in the mountains. I suspect we reached critical loading and slab property in some areas yesterday while many other regions in the province are at or very near the tipping point. Check the CAC forecaster blog for a new posting on this:

http://www.avalanche.ca/cac/bu.....aster-blog

The Tipping Point, February 19 2012 – Karl Klassen

After extended periods of no snow in winter, weak layers almost always develop on the surface of the snowpack. The extent and characteristics of these weak layers depend on the nature of the dry spell: temperatures, winds, humidity, etc. and the nature of the pre-existing snowpack before the dry spell started. Sometimes the weak layers are non-persistent (they react when loaded by new snow then settle and bond relatively quickly). Sometimes the weak layers become persistent and react not only when initially buried for weeks and sometimes months after.

Regardless of the weak layer characteristics, how and when avalanche activity on them is a not a matter of “if” but “when.” When activity will begin depends on a variety of factors. How weak is the weak layer? What is it sitting on top of? How much new snow accumulates and how rapidly does it accumulate? How dense is the new snow when it falls and how rapidly does it settle? How much wind and from what direction? What are the temperatures during and after the loading cycle and does the sun come out after or not? You get the picture—it’s not always simple, and each weak layer/loading cycle and subsequent avalanche cycle is unique.

When we are approaching the point where a significant change in stability from good to poor is expected, you will hear forecasters talking about “critical” factors such as load (how much weight has been added/is being added) and slab property (how stiff the layer of snow over the weak layer is). When the snowpack becomes unstable, we often say we have reached “criticality” or the “tipping point.” When a warm, windy storm rapidly dumps large amounts of new snow the tipping point generally comes quickly—sometimes in a matter of hours; this scenario is usually obvious and relatively easy to forecast. When weak layers are incrementally loaded (small, cool, calm storms drop minor snowfalls) and the weak layers are complex mixes of different grain types, the tipping point arrives slowly and it’s much harder to predict when and where criticality will be attained and how the avalanches associated with the tipping point will look.

The latter scenario I describe above should sound familiar; it’s what we are experiencing now. The recent dry spell left a complex sandwich of weak layers on the surface. I will not go into detail—read the recent and current forecasts for your region and other posts in this blog and you’ll get an idea of what I mean. In the last week or so, these upper layers have been incrementally loaded by small snowfalls of low density snow with little wind and cool temperatures. This new snow is now settling fairly rapidly and is very susceptible to transport by wind.

Looking at field data with a critical eye Friday, I could see a very gradual increase in avalanche activity—mostly human triggered, mostly pretty small. In my opinion, as of Friday afternoon, the snowpack in most areas was in a state of precarious balance—the weak layers were holding on by a fingernail as accumulations of new snow gradually piled up on top of them.

Yesterday’s data indicated this trend was continuing: a few more slides, a little bigger, still mostly human triggered. Preliminary reports are trickling in this morning and it looks like some regions got significantly more snow than forecast in the last 24 hours. On the west side of the Monashees, east of Vernon for example, there’s a report of 60cm of new snow—far more than was forecast. Places that got more snow and/or wind than was discussed in yesterday’s forecasts are likely now over the tipping point and local danger ratings in those areas are almost certainly higher than what was forecast yesterday in areas where this occurred.

I think there’s also a human tipping point. After a long drought, people get frustrated and fed up with the poor riding conditions and eagerly await the arrival of new snow that will improve their back-country experience. However, the arrival of new snow and the improvement in riding conditions in these situations is almost always associated with the arrival of the tipping point. The slower the tipping point comes, the more people get lulled into a sense of false security and the more they underestimate the potential consequences that result when the tipping point is reached. For example, the current situation: complex weak layers + 10 cms new snow = great fun. + another 10 cms = great fun and a few small avalanches that get ignored. + another 10cms = surprise! avalanches catch people and incidents/accidents occur but everyone’s too embarrassed to say anything so others don’t hear about it and they get caught in larger avalanches until eventually there’s a serious wreck. This is what we are seeing in some areas now, notably the South Coast Mountains and the North Shore: people got surprised and caught, some lost gear and were partially buried, and they all slunk away embarrassed and didn’t tell anyone. This is probably also occurring or about to occur in the west-central Monashees and any other areas that went over forecast for snow, temperatures, or winds yesterday—we’re just not hearing about it.

Areas that have not gone over the tipping point yet, will probably be there in the next few days when a bit more snow, wind, and/or warmer temperatures arrive. PLEASE: check with knowledgeable locals about what’s happening, look at the avalanche forecasts for your region every day, critically examine the weather factors that are driving the danger ratings and avalanche problems, then constantly observe what’s happening around you as you go into the mountains: if the avalanche forecaster rates danger Moderate in the Alpine and that’s based on a weather forecast of 10cm new snow, -5.0, and light SE winds while you are seeing 40cms of new, +1.0, and a SW wind, you have to adjust your trip plans and terrain choices or you will get caught by surprise. If your personal tipping point isn’t under control when the snowpack stability/avalanche activity tipping point arrives you are just asking for trouble.