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In 2020 summer, the excessive rainfall over the Yangtze and Huaihe River valley (YHRV) broke the record held since 1961, and the induced flood disasters caused huge losses. The record-breaking rainfall was primarily attributed to the abnormally strong western Pacific subtropical high (WPSH), characterized by a westward expansion to southern China. The extreme northern India Ocean (NIO) warming and the rapid La Niña development were responsible for the abnormally strong WPSH via enhancing its two leading modes (EOF1 and EOF2), with the normalized principal components (PC1 and PC2) of +1.07 and +1.30 in 2020, respectively. Compared to the observations, the excessive rainfall over the YHRV and the abnormally strong WPSH was successfully predicted by the National Centers for Environmental Prediction Climate Forecast System version 2 (CFSv2) initialed in May. Although the CFSv2 can capture the influence mechanisms of two SST forcings on the WPSH, only the La Niña development provided a primary predictability source for the extreme 2020 summer. First, the above-normal PC2 was accurately predicted (+1.60) beneficial from the accurate simulation of the La Niña development, whereas the predicted PC1 (+0.43) was much weak due to the underestimation of the NIO warming. Second, owing to a stronger (weaker) linkage between the YHRV rainfall and the PC2 (PC1) in the CFSv2, the predicted YHRV rainfall exhibited a stronger (weaker) response to the La Niña development (NIO warming) in 2020 summer. Future model improvements need to more emphasize the role of the NIO SST in the summer YHRV rainfall. • The record-breaking rainfall over the Yangtze and Huaihe River valleyoccurred in 2020 summer. • The northern India Ocean warming and La Niña development were jointly responsible for the extreme 2020 summer. • The predictability of 2020 summer by the NCEP CFSv2 was dominantly originated from the La Niña development. [ABSTRACT FROM AUTHOR]