When Hans struck Norway: Forecasting, communication and municipal preparedness during an extreme weather event

Extreme weather events pose significant challenges to communities worldwide, necessitating proactive measures for mitigation and adaptation. We will present a case study of the extreme weather event Hans, which struck Norway with remarkable intensity in August 2023 and led to large-scale flooding and landslides. Two characteristics set Hans apart from typical extreme rainfall events hitting Norway: 1) its unusual trajectory from the southeast, contrasting with the customary storms from the west, and 2) the large area of heavy rainfall, with return periods exceeding at least 100 years. The event caused heavy rainfall that lasted over several days, thus affecting a widespread area.Through accurate forecasting, successful communication of risks, and well-organized collaboration between key actors, significant societal costs were saved and no lives were lost. We will present the perspectives of the forecasters in the onset of Hans, and on the preparedness efforts undertaken by municipalities in response to the forecasted threat. We will also discuss the value of climate adaptation to such events in an area where extreme precipitation is one of the main direct physical climate risks, including the importance of understanding Hans in a communication and outreach perspective.The event itself is examined by its temporal evolution, spatial distribution, and associated meteorological phenomena. By analyzing observational data, we can interpret the key atmospheric dynamics driving the extreme weather event, thereby enhancing understanding of its underlying mechanisms. To assess the events rarity, we will also present the associated return periods for the observed precipitation amounts.The presentation will highlight our experience from the event, emphasizing the role of communicating forecasts and the forecasters' close collaboration with preparedness and response services, while also highlighting the significance of climate adaptation efforts in regions vulnerable to extreme precipitation.