A Climatology of Major Avalanche Winters in Western Canada

A nearly continuous record of avalanches on the Canadian Pacific Railway for the 70 years from 1910 is used to identify four major avalanche winters (1919-20, 1932-33, 1934-35 and 1971-72). The selection is based on the frequency and mass of avalanche snow, and the length of rail line affected near Rogers Pass, British Columbia. Daily weather data are compiled for these winters, and surface synoptic charts are examined for the most catastrophic avalanche events. Their climatology is further analysed by applying a weather-typing scheme and by constructing monthly sea-level pressure anomaly maps. More than one climatology is needed to explain these winters and the daily sea-level map types are not very discriminating. Major avalanche winters tend to be cold in December or January or both. Some show large monthly anomalies in the average pressure distribution over the province. Large avalanches occur when there is a change in the atmospheric circulation, with a Pacific frontal system and warm sector replacing an Arctic high. These aspects of synoptic climatology are related to the mechanisms for producing big avalanches.