A warning system for the Laseyer wind storm in a narrow and deep valley of northeastern Switzerland

The Laseyer is a very local and uncommon windstorm in a narrow and steep valley in northeastern Switzerland. Whereas the ambient wind is from west to northwest, the strong surface wind in the valley is from the east, leading to gust speeds that become dangerous for the local train running into the valley to the Wasserauen station. To minimize the risk of derailment and to improve passenger comfort, the train service provider Appenzeller Bahnen (AB) has developed a new warning algorithm in close collaboration with academia (ETH Zurich) and the Swiss national weather service (MeteoSwiss). The aim is not only to accurately predict the Laseyer windstorm several hours in advance but also to reduce the number of false alarms. The new warning system is based on the MeteoSwiss operational ensemble prediction system at 1.1-km horizontal mesh size, which is then used in combination with an observation-based machine learning approach to probabilistically forecast Laseyer events up to 30 h in advance. A particular challenge in developing the new system was to introduce the customer, AB, to the modern concept of probabilistic numerical weather prediction, which requires a careful risk assessment by the customer. Hence, the development of the warning system is a process in which the customer and the warning provider closely collaborate and specify the final warning products to be delivered operationally. The operation of the new warning system during the 2021/22 Laseyer season shows that it is working successfully and also indicates that the warning thresholds in the warning algorithm can be adjusted in the future to minimize false alarms without increasing the number of missed events.