Forecasting the fatal Longyearbyen avalanche. Possibilities and challenges
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One of the main questions after the fatal avalanche in Longyearbyen in December 2015, where two persons perished and eleven houses were destroyed, was whether an active avalanche warning system would have identified the upcoming danger. At the time of the catastrophe, no avalanche warning was in operation. Observation routines established in the early 90ties were discontinued. The forecasted and observed weather is compared with 42 years of meteorological data from Longyearbyen airport. Similar weather conditions are identified and corresponding records of avalanche events clearly states that alarm bells should have sounded. The analysis showed that heavy snowstorms usually occur with westerly winds, while avalanches on the hillside leading to the fatal avalanche are associated with winds from Southeast. Minor precipitation and critical wind causes minor avalanches almost annually, while the fatal event in 2015 occurred due to a rare combination of intense precipitation and wind from Southeast. After the fatal avalanche on Saturday morning, larger parts of the settlements below west facing mountain sides were evacuated. Norwegian Geotechnical Institute managed to establish a fully manned avalanche warning service within Tuesday evening. A month later replaced by regional forecasts. The lessons learned from this work will be helpful in future local avalanche warning for settlements.