Avalanche Forecasting 101
Avalanche Forecasting 101
Avalanche forecasting boils down to understanding how weather, the snowpack and terrain will interact. Then you add the human element to it. It’s really that simple.
https://www.powderhighwaybackcountry...IMG_4561-2.jpgAvalanche Forecasting 101
Avalanche forecasting is a fundamental part of backcountry skiing and splitboarding. It is a skill every backcountry enthusiast should strive to develop. Avalanche forecasting boils down to understanding how weather, the snowpack and terrain will interact. Then you add the human element to it. It’s really that simple.
Of course in reality, the avalanche phenomenon is not that simple to forecast, due to spacial variability of the distribution of weak layers, the variability of the terrain on which the snowpack lies and how local weather differs from valley to valley in the mountains. This article aims to lay out the key components of avalanche forecasting and how to incorporate them into a daily operational plan.
The Human Element
Avalanche forecasting is further complicated by the human element at risk. We wouldn’t care about avalanches if we didn’t interact with them, but we do; thus our daily avalanche forecasts need to take the human element into account. That is why the avalanche danger scale talks about the likelihood of human triggered avalanches.
https://avalanche.ca/glossary/terms/avalanche-danger-scale
https://www.powderhighwaybackcountry...33177498C.jpegThe Building Blocks of an Avalanche Forecast
Certainty and Confidence
The interface of natural avalanche activity and the human factor is where certainty and confidence comes into play. Clearly when there is a human element at risk, we need elevated levels of certainty and confidence in our avalanche forecast to choose to expose ourselves to avalanche terrain.
During periods of high avalanche hazard, we can also have high levels of certainty. For example, there could be an omega block ridge of high pressure that persists for a couple of weeks, where a wide spread surface condition such as surface hoar at tree line and below and facets in the alpine develop. Then a huge storm comes in and deposits over a meter of snow in a short period of time with lots of wind. In that case, forecasters will have high levels of certainty/ confidence that stability will be poor and the avalanche hazard will be high. In this case, it is possible to have a high level of certainty and confidence and still need to employ a conservative operational plan.
Certainty relates to the forecaster’s level of confidence in their understanding of the relationship between weather, the snowpack and terrain. For example, if you spent the entire winter skiing the same planar slope every day and you paid attention to the weather and snowpack, you would have a really good idea of what layers were present in the snowpack, how deep they were buried and how the weather was affecting them. Your level of certainty and confidence as to how they would behave would be quite high. However if that same slope had gullies, micro changes in aspect, variations in pitch, thin to thick snowpack areas and there was a deep persistent weak layer, your level of certainty would be greatly reduced.
Ultimately, the less confidence and certainty you have in your avalanche forecast, the greater margin of error you need to give yourself in your daily operational plan. A case where you probably hear forecasters talk most about low confidence/ certainty is a Deep Persistent Slab. Often times the probability of triggering a deep persistent slab is low, but the consequence of triggering it is high. Therefore caution and conservative decision making must be employed.
Here’s an article that elaborates on this subject:
https://avalanche.ca/blogs/dps-management-and-mindset
Mindset
So how do we take certainty and confidence and apply them into real world decision making? That is where the mindset part of the avalanche forecasting process comes into play. Professionals use the mindset component to help guide their daily operations.
It gives us a means of ground truthing our behaviour in avalanche terrain. For example, if I am guiding a group on a basecamp trip in a remote zone I haven’t been in all year, my operational mindset for the first day will be “Assessment”. I will try and gather as much data as I can about the snowpack as I travel through the terrain. With an “assessment mode” mindset, I will choose terrain that gives me more options and make conservative decisions. I will look to confirm or disprove theories I had about the snowpack before I arrived.
If I gather evidence that demonstrates "very good" snowpack stability and low avalanche hazard, my mindset would shift to “Stepping Out” over the ensuing days. If I observed the presence of instability and elevated avalanche hazard my mindset would shift to “Stepping Back”.
During our daily operations, we can quickly reflect on our morning avalanche forecast and reference whether our decisions and actions fall within the respective mindset and if not, we need to ask ourselves why and what evidence have we gathered to support that change in operational mindset.
The Avalanche Triangle
Weather
Weather is one one third of the avalanche phenomenon, it is what forms the various types of surface forms that create instability within the snowpack. It also brings wind, which shapes and redistributes the snowpack which in turn forms wind or storm slabs, cornices or scours slopes.
Snowpack and Layers of Concern
The second component of the avalanche phenomenon is the snowpack; shaped by time, temperature, wind, sky conditions, precipitation and vapor transport, it evolves over the course of the winter and goes through periods of stability and instability.
Identifying the principal layers of concern on a daily basis, their likelihood of being triggered and their sensitivity to natural and human triggers plus the magnitude of an avalanche produced by the respective layer is an integral component of building an avalanche forecast.
Terrain
Terrain is the final third of the avalanche phenomenon; without it, avalanches don’t happen.
Generally, slopes over 30 degrees are required for avalanches to occur. Furthermore, terrain is what complicates avalanche forecasting, as it creates spatial variability in the distribution of weak layers and on a macro-scale it affects weather, wind and precipitation values, through orographic lift, catabatic winds and slope aspect etc.
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