26.06.2023
The most prominent information in the avalanche bulletin is the (avalanche) danger level. They are divided into a five-level scale and indicate the degree of avalanche danger. Parameters for the output of the danger levels are the stability of the snow cover snowpack, the distribution of danger spots and the size of the expected avalanches. This information relates to a specific area and a defined period of time.
The five avalanche danger levels are given according to the following classification: 5-very large, 4-large, 3-substantial, 2-moderate, 1-low.
Most avalanche warning services have agreed that the area for which a danger level is issued should be at least 100 square meters in size. Many avalanche warning services prepare this assessment as a forecast for the following day, which is why it is referred to as an avalanche forecast and no longer as an avalanche situation report.
A danger level always applies to an entire region and therefore cannot reflect the particularities of a specific individual slope. In addition, the danger level described in the avalanche forecast is largely a forecast and should always be checked on site.
Even though the European avalanche warning services describe the avalanche danger on the basis of five danger levels, it is important to know that the avalanche danger changes continuously in nature (see Figure 1). The curve in Figure 1 (see Harvey et al., 2012) shows the possible course of the avalanche danger, the three dots describe different situations in any given winter:
Point A could classically represent the situation directly after the last heavy snowfall with lots of wind: A number of self-triggering avalanches and other alarm signals, high triggering readiness by winter sports enthusiasts and even remote triggering are conceivable. The danger is easily recognizable in the terrain and is often widespread.
At point B, we do not recognize any spontaneous avalanche activity, alarm signs are also rare and often only certain parts of the terrain (e.g. hollows and gullies) are affected.
For winter sports enthusiasts, the avalanche danger for points A and B is very different, but the danger level is the same. It has become commonplace to speak of a “tense three” or a 3+ in a situation like point A, or to read “delicate situation for winter sports enthusiasts” in the headline of the avalanche situation report. With these descriptions, the avalanche warnings try to adapt the danger level to the existing differences in avalanche danger within danger level 3-Substantial.
Point C and B are different: both have more or less the same avalanche danger, but the danger level differs by one level, which has significant consequences for tour planning. Points C and B are representative of the protracted periods with old snow problems: In terms of weather, nothing dramatic has been happening for a while, there is a dormant weak layer somewhere in the snow cover snowpack, but both natural triggering and triggering by winter sports enthusiasts* are rarely if ever observed. Then suddenly an avalanche is triggered by people with dramatic consequences, huge avalanches and often accidents with serious consequences. This is referred to as a situation with “low probability but high consequences” (with a low probability of occurrence but enormous consequences).
The three parameters for assessing the danger level were defined by representatives of the European avalanche warning services and illustrated using examples (see also https://www.avalanches.org/standards/snowpack-stability-frequency/).
We have very weak snowpack stability mainly in periods in which avalanches release spontaneously, i.e. without human intervention. We find many places with a weak snow cover when more than 20 percent of the potential avalanche terrain can be linked to signs that indicate this weak snow cover. This can be information from observations, webcams, snowpack models or stability tests.
The avalanche sizes are also defined so that every avalanche warning officer in Europe can communicate in a standardized language and assess what avalanche size is expected for the coming day. The largest number of locations with the weakest snow cover snowpack and the resulting potential for avalanche size then determine the danger level.
The EAWS matrix was introduced to ensure that this assessment is not arbitrary and is harmonized across Europe. In this matrix, each combination of the three parameters mentioned above leads to a hazard level. For some fields, there is an additional number in brackets. In these fields, the avalanche forecasters in Europe were not entirely in agreement as to which danger level should be assigned. A larger minority had a further suggestion.
Forecasters first assess snowpack stability and how often a particular class of snowpack stability (very poor, poor or moderate) is given by choosing a column and row (many, some or few) that best describes the combination of snowpack stability and frequency distribution for a particular avalanche problem. Finally, forecasters estimate how large avalanches can become and select the appropriate field within the current stability column and frequency row.