*3.1. Characteristics of the Unusual Rainfall over the Yangtze River Valley*

Figure 1 reveals the temporal and spatial distribution characteristics of the precipitation anomalies during the summer of 1954. Compared with the precipitable water data in the NCEP/NCAR Reanalysis dataset the ground station data is more accurate (Figures S1 and S2). Figure 1a depicts the national precipitation distribution during the flood period (April–July) in 1954, during which abundant rainfall was observed along the mid- and low-Yangtze River. In particular, the area at the juncture of the provinces of Hubei, Anhui, and Jiangxi received the highest precipitation. The mid- and low-Yangtze areas between 26◦N and 32◦N mostly had rainfall exceeding 1500 mm during April–July, signifying extremely widespread heavy rainbands. The maximum precipitation recorded by weather stations reached 2212.23 mm, whereas the rainy-season precipitation data collected by weather stations across the Yangtze mid- and low-Yangtze areas averaged 1044.4 mm. Severe floods affected the provinces of Hubei, Anhui, Jiangxi, and Jiangsu. The levees at Poyang Lake in Jiangxi Province broke and flooded most of the streets in Jiujiang City. The farmlands in Jiangsu Province alongside the Yangtze—with a total area of 10 million mu (equivalent to 666,666.67 hectares)—were flooded, and the Yangtze water level in Nanjing exceeded the warning limit for 117 days [28].

**Figure 1.** (**a**) Precipitation across the Yangtze River valley during April–July of 1954; (**b**) rainfall anomaly rates across the Yangtze River valley during April–July of 1954; (**c**) precipitation percentile across the Yangtze River valley during April–July of 1954—the 90th percentile indicates that the precipitation in that year exceeded 90% of the years from 1951 to 1980—which can be used as the threshold of extreme precipitation events; (**d**) daily precipitation in Wuhan in 1954; (**e**) monthly precipitation anomalies in Wuhan in 1954. The data used in the figures are ground station precipitation data from 1951 to 1980. We converted the ground station data into contours based on the kriging method.

Figure 1b presents the rainfall anomalies that occurred during the 1954 rainy season. The figure indicates that the total precipitation between April and July in most of the mid- and low-Yangtze areas was twice the normal precipitation. The distribution of areas with high anomalies was mostly consistent with that of heavy rainbands. The rainfall anomaly rates in upstream areas ranged between 20% and 50% in 1954, and precipitation at most stations was greater than the 90th percentile (Figure 1c), suggesting a precipitation anomaly across the entire Yangtze River valley with large outliers centralized in the midand low-Yangtze areas.

#### *3.2. Characteristics of Precipitation Anomalies in Wuhan*

In 1954, the disaster of rainfall and flooding in Wuhan city reached the highest in history; typical damage hit by the 1954 floods was among the cities in the mid- and low-Yangtze areas. This study, using data compiled by the Wuhan weather station, reconstructed the precipitation anomaly process in chronological order. In 1954, most of the precipitation in Wuhan occurred between April and July, whereas the precipitation in other months was relatively low. As illustrated in Figure 1d, the precipitation surged after April with increased daily peak rainfall, and it peaked in July. Figure 1e indicates that the precipitation starting in April was higher compared with that of previous years (i.e., normal conditions). This anomaly continued for 4 months. In August, the continuous rainfall eventually ended, and the Yangtze River valley began to witness high temperatures along with low precipitation. In particular, the monthly total precipitation in Wuhan in August 1954 was only 46 mm, which was 61.28% less than normal conditions.

The cloudburst frequency in 1954 also exhibited a similar trend. Between April and July (122 days), Wuhan saw 58 days of rain, of which 11 days had rainfall, revealing a notable anomaly. Rainfall occurred mostly between mid-June and the end of July. During this period, 3 days had intense rainfall, namely: 13 June (105 mm), 25 June (130.3 mm), and 29 July (142.2 mm—the highest on record). The precipitation between mid-June and the end of July was continuous for most of the time, indicating an extended period of rainfall. The precipitation during April–July of 1954 totaled 1620.1 mm, which was more than twice the precipitation under normal conditions. The heavy rain prompted the water level and discharge of the Yangtze to sharply rise. In 1954, the water level and peak discharge measured at the Hankou (a district of Wuhan) weather station hit historic highs of 29.73 m, and 76,100 m3/s, respectively. During the flood period that year, the water level of the city's levee exceeded its warning limit for more than 100 days, with a total of 21,523 major and minor events reported [29].
