Record low sea levels in the Bothnian Bay in November 2024

On 21 November 2024, there was an extreme event within the Bothnian Bay, a sub-basin of the Baltic Sea, when a record low sea level of −153 cm was measured at Kemi, at the northern end of the basin. This minimum was 34 cm lower than the previous minimum measured in 2022. We calculate that the estimated return period for this event is several thousands of years, based on the Kemi tide gauge time series from the 1970s to the day preceding the storm. When the new record low is included in the analysis, the estimated return period for this event is reduced to just 151 years. In this study we also investigate the accuracy of the Baltic Sea Monitoring and Forecasting Centre (BAL MFC) Near Real Time (NRT) physical system in predicting the storm event, and show that the accuracy of the forecast was good up to 3–4 d before the event. Longer lead-time forecasts failed to predict the record low sea level at Kemi, although they did indicate a decrease in sea level during the storm. During the 2024 event, north-easterly winds were measured at coastal weather stations with a maximum 10 min average speed of ∼ 28 m s −1 (with gusts of 32 m s −1 ). However, we find that the storm intensity alone likely does not explain the magnitude of the sea level extreme, with the exceptionally long duration and unusual track of the storm also playing large roles. This highlights the sensitivity of coastal risk assessment to natural variations in storm tracks, which have the potential to produce rare but high-impact events.