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The Once‐in‐a‐Century extreme rainstorm event caused severe floods over Henan province during July 18–21, 2021, which resulted in large casualty and property losses. Although the rainstorm event occurred in Henan after July 18, the excessive rainfall had occurred to the east of Henan before July 18, with the 4‐day accumulated rainfall exceeding +130 mm during July 14–17, 2021. How the rainfall evolving westward and intensifying after July 18 remained a puzzle, which is the focus of this study. The prerainstorm stage (July 14–17) was related to the South Asian High (SAH) extending eastward and the western Pacific subtropical high (WPSH) extending northwestward, and a low vortex between the SAH and WPSH caused above‐normal rainfall to the east of Henan. The rainstorm stage (July 18–21) was associated with an inverted trough and excessive southerly and southeasterly water vapor transportation above Henan, which resulted from the combined effects of a deep trough in the upper troposphere and typhoon activities. Additionally, three subseasonal forecasting systems predicted this rainstorm event 3 days in advance, with the European Center for Medium Range Weather Forecasts (ECMWF) performing the best, which was related to a better prediction of the inverted trough and the water vapor transportation in the middle‐lower troposphere. These results advance our understanding of the extreme rainstorm event in July 2021 in Henan. [ABSTRACT FROM AUTHOR]

By using the hindcast and forecast data from the National Centers for Environmental Prediction Climate Forecast System version 2 (NCEP CFSv2) for the 1982–2018 period, we investigate the forecasting skills of the mid‐summer (July and August) surface air temperature (SAT) at interannual timescales in this study. Although CFSv2 predictions show a warm bias for the climatological mean SAT over the Yangtze River valley (25°–32°N, 105°–122°E), they show a consistent and great performance in predicting the interannual variability of the mid‐summer SAT over this region until 4 months in advance, where the linear correlation coefficient between the predicted and observed time series reaches +0.65, +0.51 and + 0.68 for 4, 2 and 0 months in advance, respectively. The CFSv2 predictions well simulate the linkage between the SAT anomalies over the Yangtze River valley and the anomalous atmospheric circulation aloft, including the circumglobal teleconnection and zonal extension of the Western Pacific Subtropical High. However, CFSv2 has trouble in simulating the associated vertical velocity, cloud cover and solar radiation anomalies, except for 0 months in advance. The persistent forecasting skills result from the accurate response of the circumglobal teleconnection and Western Pacific Subtropical High to the El Niño/Southern Oscillation and sea surface temperature (SST) anomalies over the mid‐latitude North Atlantic. Correspondingly, the forecasting skill, signal and signal‐to‐noise ratio are effectively improved in the years with strong mid‐summer SST anomalies over the tropical central‐eastern Pacific or mid‐latitude North Atlantic. These results are useful for understanding the predictability of the mid‐summer SAT over the Yangtze River valley. [ABSTRACT FROM AUTHOR]

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]