Yoroku: Japan's historical storm warnings meet an age of record rain

On September 11, 2021, Japan experienced record-breaking heavy rain coinciding with two traditionally ominous dates. The 210th day of spring and the ancient lunar calendar marker known as Hassaku fell on the same day this year. These dates have long been linked to heightened risk of natural disasters, particularly typhoons and torrential rain.

In the Edo period, the day Tokugawa Ieyasu entered Edo Castle on Hassaku was marked by formal attire and ceremonies. However, for ordinary farmers, it was a vulnerable time. Two days after Ieyasu's arrival, torrential rain caused Shinobazu Pond and Senzoku Pond to overflow. In response, Ieyasu ordered the construction of embankments and drainage systems, which contributed to the development of urban infrastructure in Edo, including water supply and sewerage networks.

This year, the overlap of the 210th and 220th days posed a rare opportunity for disaster. Since August's torrential rain in Chiba Prefecture, Japan has been hit by record-breaking heavy rain nationwide. In recent years, urban areas have faced rainfall events exceeding sewer system capacity, leading to floods in subway stations and underpasses, resulting in fatalities.

Climate change, including rising global temperatures and the El Nino phenomenon, has increased water vapor in the atmosphere and altered weather patterns. This year, the meandering of westerlies also contributed to linear precipitation bands and typhoons. As abnormal weather becomes more common, society may need to adapt its behavior, considering staying home during heavy rain warnings.