Philip Bett, Gill Martin, Nick Dunstone, Adam Scaife, Hazel Thornton, Chaofan Li

Seasonal forecasts for Yangtze River basin rainfall in June, May–June–July (MJJ) and June–July–August (JJA) 2020 are presented, following successful forecasts in previous years. The 3-month forecasts are based on dynamical predictions of an East Asian Summer Monsoon (EASM) index, which is transformed into regional-mean rainfall through linear regression. The June rainfall forecasts for the middle/lower Yangtze River basin are based on linear regression of precipitation. The forecasts verify well in terms of giving strong, consistent predictions of above-average rainfall at lead times of at least 3 months. However, the Yangtze region was subject to exceptionally heavy rainfall throughout the summer period, leading to observed values that lie outside the 95% prediction intervals of the 3-month forecasts. Our forecasts are consistent with other studies of the 2020 EASM rainfall, whereby the enhanced Meiyu front in early summer is skilfully forecast, but the impact of mid-latitude drivers enhancing the rainfall in later summer is not captured. This case study demonstrates both the utility of probabilistic seasonal forecasts for the Yangtze region, but also potential limitations in anticipating complex extreme events driven by a combination of coincident factors.