Major avalanches occurrence at regional scale and related atmospheric circulation patterns in the Eastern Pyrenees

Abstract

Abstract The occurrence of major avalanche episodes in the Eastern Pyrenees (NE Spain) is estimated by means of the Poisson and the negative binomial distributions, assuming that they are rare events. Occurrence fits better the Poisson distribution provided that the avalanche episodes are independent. The differences observed in the spatial distribution of major avalanche episodes, within the seven avalanche forecasting regions in which Eastern Pyrenees were divided, suggest us to look for spatial variations in the relationship between major avalanche episodes and synoptic-scale atmospheric circulation patterns. By applying principal-component analysis procedure, six atmospheric patterns leading to major avalanches were obtained at 500 hPa geopotential height. Next, the atmospheric pressure deviation of each component is calculated with respect to the average atmospheric circulation to assess the abnormality of atmospheric patterns leading major avalanches in the Pyrenees. The pressure anomaly maps give also an idea of the role that the North Atlantic Oscillation index (NAOi) plays in the occurrence of major avalanche episodes in the Eastern Pyrenees. The interest of connecting major avalanche episodes with NAOi lies in the fact of this low-frequency pattern seems to be predictable at medium term for the next winter with a reasonable level of confidence. The accuracy of regional avalanche warning is expected to increase by means of the proposed methodology. It should reduce the trend to overestimate the avalanche danger level when forecasting is based on expected critical snow depth. This procedure is especially suitable for regions where long term snow-climate databases do not exist and avalanche warning is strongly supported on meteorological forecasting models.