An updated EAWS matrix to determine the avalanche danger level: derivation, usage, and consistency

Abstract. Avalanche forecasting plays a crucial role in mitigating risks associated with snow avalanches in mountainous regions. Standards for regional avalanche forecasting were initially developed at national levels. Therefore, the introduction of the European Avalanche Danger Scale (EADS) in 1993, still in use today, represented a milestone in harmonizing the assessment and communication of avalanche danger. However, standards, concepts and definitions have evolved since then. Here, we reflect on the current standards and definitions used in regional avalanche forecasting, with a focus on the updated European Avalanche Warning Services (EAWS) Matrix, a look-up table intended to ensure consistency among avalanche forecasters when assigning a danger level. The EAWS Matrix links the factors determining avalanche danger – snowpack stability, the frequency of snowpack stability, and avalanche size – to avalanche danger levels. Here, we describe the methodology to obtain a consensus-based EAWS Matrix. Finally, by analyzing the operational use of the EAWS Matrix following its introduction, we gain insights into its implementation across European avalanche warning services and obtain an understanding on challenges and short-comings related to its operational use. As a reliable estimation of the factors determining avalanche danger is a prerequisite for consistency in assigning the danger levels using the Matrix, we also explored the consistency of estimating the factors by comparing forecasts prepared by individual forecasters. Noting considerable variations in the assignment of factor classes, we provide recommendations for practice and ways forward, such as refining the definitions of the classes describing the factors, implementing training sessions, and exploring different matrix layouts. Additionally, the discrepancies between the EADS and current standards and definitions underscore the need for an updated avalanche danger scale. In conclusion, the updated EAWS Matrix represents a next step towards harmonizing avalanche forecasting practices in Europe even though the analysis revealed areas for improvement. Clearly, further efforts are required to develop and implement regional avalanche forecasting standards to reach the goal of avalanche forecasts being a reliable, credible, and timely source of information of expected avalanche conditions, regardless of the forecaster or warning service behind the product.