Located in the southwestern part of Japan, and sandwiched by two warm ocean currents, Kyushu is the wettest and warmest area of the country. It holds the national records for the annual and hourly rainfall. (8,670mm, 153mm respectively).

The Southern Kyushu region just notched one of the wettest Junes on record, and they were pounded by even more heavy rainfall on Saturday (7/4). This time it was unprecedented.

Authorities issued an emergency warning for rain in Kumamoto and Kagoshima Prefectures for the first time for these regions. An emergency warning is the country’s highest level of alert and is only issued when life-threatening disasters are happening or imminent.



24HR Rainfall. The lines indicate all-time record. (Courtesy: JMA)
24HR Rainfall. The lines indicate all-time record. (Courtesy: JMA)

Kumamoto Prefecture, known for an active volcano, was hit the hardest. Though the area is frequently hit by drenching downpours, Saturday’s rain easily surpassed any of the records in the past.

The observed rainfall was as below:

[1 hour]

Amakusa 98mm

[3 hours]

Ashikita 190.5mm

[6 hours]

Ashikita 325.5mm

[24 hours]

Yunomae 489.5mm

Minamata 474.5mm

Amakusa 428mm

(All broke the previous all-time records)



The rain triggered the Kuma River to overflow. The water led to deadly floods and landslides. Water reached the second-floor of houses, inundated numerous vehicles and destroyed iron bridges. Many people waited for help on top of roofs.

According toKYODO, at least 40 people have been confirmed or presumed dead, including 14 elderly who were in a nursing home when the floodwater hit. 11 others are still missing.



Surface analysis on July 4 (Courtesy: JMA)
Surface analysis on July 4 (Courtesy: JMA)

The weather map above is for early Saturday morning when the disaster happened.

The sea surface temperatures around Kyushu are 2-3 degrees Celsius higher than normal, allowing them to bring more rainfall.

The extremely moist air was flowing from the south, magnifying the rainy season front. A particularly developed line of rain, known as "training", formed over Southern Kyushu. Training is a succession of storms which continues to develop and causes rain over the same area for hours.

On July 5, 2017, the training phenomenon caused record rain in Northern Kyushu, killing at least 40 people.

(↑Mechanism of the training phenomenon)



The timing and geography are thought to have made the situation worse.

Rain developed from Friday night to early Saturday morning when most people were asleep. Although heavy rain was warned of beforehand, the actual rainfall amounts exceeded the forecast.

In addition, due to the fear of the coronavirus infections in shelters, most people stayed at home.

The Kuma River is the biggest river in Kumamoto Prefecture, and is registered as one of the three steepest rivers in Japan. Water from upstream quickly gushes downstream. And the river width suddenly narrows like a bottleneck. Gushing water became clogged and caused deadly floods.

On top of the geographical factors, it was almost a full moon, meaning the river mouths had more water from the connecting ocean. Rain which fell in high elevations could not flow and ended up breaching levees.



Rainfall forecast (Courtesy: JMA)
Rainfall forecast (Courtesy: JMA)

The rain stopped on Saturday evening, bringing a brief chance for search and rescue operations.

However, heavy rain will likely develop again into Wednesday(7/8) with the expected 48-hour rainfall of over 300mm. The ground has already absorbed plenty of rainfall, and additional rain could trigger more floods and landslides.