Kanto Enters Rainy Season! Shifting from Dry Spring to Rain Alerts

Entering Tsuyu on Schedule: A 'Blessing' for Dams Facing Water Shortages?

On June 7, 2026, the Japan Meteorological Agency announced that the Kanto-Koshin region has likely entered its rainy season (Tsuyu). Although the timing aligns with average years, this year's rains carry greater expectations and anxieties than usual. The primary reason is the severe lack of rain from the end of last year through early spring.

In the Kanto region, parts experienced a "once-in-30-years" record low rainfall from autumn 2025 to February 2026. Water storage levels in major dams, such as those in the Tone River system, plummeted temporarily, even prompting discussions of water intake restrictions. While recent spring rains have led to a recovery trend, the low winter snowfall means the supply of snowmelt water was cut off earlier than usual. Stable rainfall during the Tsuyu season is key to securing water resources moving forward.

2026 Outlook: Alerts for Above-Average Rainfall

According to the latest 3-month forecast, the 2026 rainy season shows a strong extension of the Pacific High, making it easier for moist air to flow in. Rainfall is predicted to be "average or above average." While this raises hopes for resolving water shortages, it is crucial to remain alert for localized heavy rains caused by an active Baiu front.

In recent years, Tsuyu has transformed from the emotional image of "long, gentle drizzles" into a "concentrated downpour type" characterized by intense rain over short periods. Particularly this season, the atmosphere is expected to become significantly unstable due to rising temperatures, making preparedness for river flooding and landslides more important than ever.

History of 'Tsuyu': Once Known as the 'Moldy Rain'

The origin of the word 'Tsuyu' (or Baiu) carries both practicality and poetic sentiment. Originating from the Yangtze River basin in China, the term initially referred to "the rain around the time plums ripen" (梅雨). However, because it's a highly humid period prone to mold, it was also called "mold rain" (黴雨). A prevailing theory suggests that because "mold" had a negative connotation, the character for "plum" (梅), which shares the same pronunciation (bai), was adopted.

It is said that the term "Tsuyu" became common in Japan around the Edo period. Before that, during the Heian period, it was generally called "Samidare" (fifth-month rain). Historically, this rain was a "blessed rain" supporting rice planting, but simultaneously a harsh season requiring meticulous care to prevent food from rotting.

Era Characteristics	Image of Rain and People's Lives
Heian Period	Called "Samidare," a season for quiet reflection. Popular subject in Waka poetry.
Edo Period	"Tsuyu" takes root. Crucial period for rice planting. Praying for rain to avoid dried wells.
Showa/Heisei	Continuous gentle rain. Hydrangeas and umbrellas become iconic imagery.
Reiwa (Present)	Frequent linear precipitation zones and guerrilla rainstorms. Disaster prevention is top priority.

Japan's Changing Seasons: Preparing for the 'Intensification' of Tsuyu

Due to climate change, the duration and nature of the rainy season are changing dramatically. While it used to rain steadily from early June to mid-July, irregular patterns are now prominent, such as sudden concentrated downpours after a "dry Tsuyu" (Karatsuyu), or intense rain returning after the season was supposedly over.

The Kanto region this year needs sufficient rainfall to compensate for the dry early spring, yet faces disaster risks if that rain falls all at once. While giving thanks for the "blessed rain," we must take self-defense measures suited to the modern "Tsuyu," such as checking hazard maps and inspecting stockpiled supplies.

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