Philip Bett, Nicola Martin, Adam Scaife, Nick Dunstone, Gill Martin, Nicola Golding, Joanne Camp, Peiqun Zhang, Chris Hewitt, Leon Hermanson

Rainfall forecasts for the summer monsoon season in the Yangtze River Basin allow decision-makers to plan for possible flooding, which can affect the lives and livelihoods of millions of people.
A trial climate service was developed in 2016, producing a prototype seasonal forecast product for use by stakeholders in the region, based on forecasting rainfall directly using a dynamical model. Here we describe an improved service, based on a simple statistical downscaling approach: using a dynamical forecast of an East Asian Summer Monsoon index, seasonal-mean rainfall for the Upper and Middle/Lower Reaches of the basin can be forecast separately, with significant skill for lead times of up to at least 3 months. The skill in the different sub-basin regions depends on the target season: Rainfall skill in the Middle/Lower Reaches is significant in May–June–July (MJJ), and there is significant skill for the Upper Reaches in June–July–August (JJA). The mean rainfall for the basin as a whole can be skillfully forecast in both seasons. The forecasts issued in 2019 gave good guidance for the enhanced rainfall in the MJJ period, and the near-average conditions in JJA. Initial feedback from users in the Yangtze River Basin suggests that the improved forecast better meets their needs and will enable more robust decision-making.