Introduction

Hydrometeorological forecasts are used in a wide range of applications such as early warning systems, reservoir operations, managing pollution incidents and river basin management. Typically, meteorological observations and forecasts are used as inputs to hydrological models, whose outputs are then processed into a range of products tailored to operational needs. Increasingly this includes estimates for the uncertainty in forecasts, based on probabilistic techniques. This chapter presents a general introduction to operational meteorological and hydrological forecasting techniques, and introduces concepts such as climate services, value chains and risk-based approaches to decision-making. Other topics include the increasing roles of citizen science and artificial intelligence. Flood, drought and other early warning systems are considered too, with a particular focus on end-to-end warning systems, including the WMO/UNDRR Early Warnings for All (EW4All) initiative.