The Japan Meteorological Agency has issued heavy-rain and landslide-related warnings for parts of the region, with authorities urging residents to remain alert for rapidly changing conditions.
Some of the most persistent rain has affected Tsushima, an island city in Nagasaki Prefecture between Kyushu and the Korean Peninsula. More than 68 mm (2.7 in) of rain had fallen there during the 24 hours through Monday afternoon.
Radar imagery showed areas of strong rainfall repeatedly developing around the Tsushima Strait, with intermittent heavy rain continuing through the day.
No major casualties or large-scale evacuations had been confirmed in Nagasaki at the time of publication, making this primarily a warning-led weather event. However, western Kyushu’s steep terrain means localized rainfall can quickly create dangerous conditions.
Why is Western Kyushu so Vulnerable to Landslides?
The main concern is not simply how much rain falls across the region as a whole, but where the heaviest downpours repeatedly occur.
Moist air is feeding into an unstable atmosphere around western Japan, while a low-pressure system moving through the East China Sea has helped support rainfall across Nagasaki Prefecture.
As moisture-rich air reaches Kyushu and its surrounding islands, hills and mountains force the air upward. This process, known as orographic lift, causes the air to cool and condense, enhancing rainfall along exposed slopes.
That means mountainous locations can receive substantially more rain than nearby lower terrain.
Nagasaki’s geography adds to the risk. Much of the prefecture consists of steep hills, narrow valleys, islands and densely developed coastal areas. Intense rain can therefore run rapidly downhill rather than draining gradually into the ground.
Once soils become saturated, the risk of slope failure increases considerably.
Flash Floods Can Develop Quickly
Short periods of intense rainfall can also overwhelm streams, drainage channels and urban drainage systems.
This makes flash flooding possible even where rainfall totals do not initially appear exceptional compared with larger regional rain events.
JMA’s warning system includes information covering heavy rainfall, flooding and landslide hazards, allowing residents to monitor how risk changes as individual rain bands move through the region.
The atmospheric instability affecting Nagasaki is also part of a broader wet pattern around the Tsushima Strait. Exceptionally heavy rain has fallen on the opposite side of the strait in southern South Korea, illustrating the enormous amount of moisture available within the regional atmosphere.
For Nagasaki and western Kyushu, the immediate concern is whether additional rain repeatedly crosses the same locations.
Even if rainfall becomes more intermittent, saturated slopes can remain unstable after the heaviest downpours have ended. Residents in areas near steep terrain, swollen streams and locations with a history of landslides should therefore continue following official warnings as conditions evolve.
