Hydrological forecasters are on the frontlines of water management, disaster preparedness and risk reduction. The accuracy and reliability of these forecasts is therefore paramount.
The World Meteorological Organization's (WMO) has issued 2025 Guidelines on the Verification of Hydrological Forecasts (WMO-No. 1364), offering straightforward, scientifically grounded methods to help assess and improve the accuracy of forecasts, improve systems and better serve communities.
In response to demand from WMO's Members, they showcase practical implementation through case studies and visualization tools, all aimed at supporting more informed decision-making and boosting the Early Warnings for All (EW4All) initiative .
"Verification of forecasts plays a vital role by providing several key benefits: it provides operational insight by highlighting the strengths, weaknesses and uncertainties of the forecasts and their systems; it guides system enhancements; and ultimately it enables more informed and effective decision-making," writes WMO Secretary-General Celeste Saulo in the foreword.
Forecast verification is the process of evaluating how well a forecast corresponds with actual observations. It addresses two primary objectives: improving forecast systems through systematic performance assessment and enhancing end-user decision-making by quantifying forecast uncertainty and skill.
"This topic holds significant interest for members of the hydrological forecasting community as it serves as a benchmarking for evaluating the quality of their forecasts. Though specific to the hydrological sector, these guidelines provide an additional element contributing to the Early Warnings for All (EW4All) initiative, allowing Members to advance in their hydrological forecasting capabilities," writes Ian Lisk, President of WMO's Commission for Weather, Climate, Hydrological, Marine and Related Environmental Services and Applications (SERCOM).
Streamflow and river basins
The publication provides comprehensive guidelines focused on the verification of forecasted streamflow, or discharge, at designated locations. This plays a crucial role in understanding and predicting water movement within river basins. Flood preparedness and emergency response often prioritize the verification of river stage or flow height forecasts, given their direct impact on critical thresholds and timing of flow peaks.
The publication includes real-world applications from various regions, demonstrating both deterministic and ensemble forecast verification. These cases serve as templates for implementing verification under different hydrological and operational settings. There are flood-specific case studies - such as real-time verification systems in Canada and Australia - that show how verification can work even under tight operational constraints.
The guidelines define several key highlights for assessing forecast quality:
- Accuracy: Degree of closeness between forecasts and observations.
- Bias: Systematic deviation in forecasts.
- Reliability: The consistency of forecasts with observed frequencies.
- Resolution: The ability to distinguish among different outcomes.
- Sharpness: Concentration of the forecast distribution, indicating confidence.
Effective verification requires clearly defined objectives, appropriate tools, and stakeholder-specific communication. The guidelines advocate for:
- Clear identification of verification goals.
- Data collection and preprocessing.
- Use of visual analytics for stakeholder engagement.
- Hands-on examples and case studies for practical insight.
Although the subject is technical, the guidelines are designed to introduce verification concepts to beginners and assist practitioners in evaluating their operational system. The publication avoids a one-size-fits-all approach.
"We hope this publication serves as a valuable resource for professionals dedicated to improving the accuracy and reliability of hydrological forecasts, ultimately contributing to more informed decision-making and enhanced flood preparedness and supporting the Early Warnings for All (EW4All) initiative," writes Celeste Saulo.
The guidelines were drawn up over a period of seven years, involving dozens of experts.
The guidelines were developed by the Standing Committee on Hydrological Services (SC-HYD) under the Commission for Weather, Climate, Hydrological, Marine and Related Environmental Services and Applications (SERCOM) , together with Regional Association II (RA II).
