What are the top priorities for (co-) creating hydrological forecast systems that add value across spatial scales and time horizons? Outcomes from the 2023 HEPEX workshop

Creating forecast systems that add value across spatial scales and time horizons is crucial for a variety of fields, from meteorology and climate science to business and public policy. The priorities for developing such systems may vary depending on the specific domain and objectives. The international community of practice of the Hydrological Ensemble Prediction Experiment (HEPEX) has been seeking to advance the science and practice of hydrological ensemble prediction and its use in impact- and risk-based decision-making. Over the years, HEPEX has been promoting knowledge utilising cutting-edge techniques and data to innovate hydrological forecasting methods, products and systems, and improve services for users in the water-related sectors. During the 2023 HEPEX workshop in Norrköping, Sweden, (https://hepex.org.au/hepex-workshop-2023-forecasting-across-spatial-scales-and-time-horizons/), the community proposed and elaborated on the key priorities for (co-)creating hydrological forecast systems that are broadly applicable and can add value for local/regional decision-making. The notes from five breakout groups (about 10 participants in each group) were collected and analysed, while the proposed efforts and ways forward were classified and prioritised. Here, we present the outcomes from these breakout discussions, while we support the identified priorities providing backgrounds on the science needed, the HEPEX contribution towards these priorities, and the path forward for contributing to the United Nations Early Warnings for All (EW4All) initiative.