Is Korea’s ‘once-in-a-century’ rain now happening every year? Experts say yes

People wade through chest-deep muddy water on a street in Yongbong-dong, Buk District, Gwangju, Thursday, as heavy rain drenches the city. Yonhap

What experts describe as a once-in-100-to-200-year rainfall event has left parts of Korea reeling, with record-breaking precipitation turning the unusual into the new normal.

On July 17 alone, the city of Seosan in South Chungcheong Province was hit with 438.9 millimeters of rain, while Gwangju recorded 426.4 millimeters — far exceeding the region’s average rainfall for the entire month of July. In Seosan and the nearby city of Hongseong, around 100 millimeters fell within just one hour during the early morning, shattering the 30-millimeter-per-hour threshold typically used to define a heavy downpour.

Experts say climate change is behind the intensifying and increasingly frequent rainstorms affecting the Korean Peninsula, with once-rare extreme weather events now occurring annually in many regions.

Narrow cloud bands, warm seas fuel the deluge

A resident looks over the wreckage inside his home in Yesan, South Chungcheong Province, Friday morning, following severe flood damage. Yonhap

The latest flooding was caused by a phenomenon known as a "training convective system," in which a narrow band of cumulonimbus clouds forms between two different air masses. This long, thin band hovered over parts of the country, dumping vast amounts of moisture. Southwesterly winds traveling along the edge of the North Pacific high-pressure system acted as a conveyor belt, feeding moisture into the rain clouds.

While such formations are not unusual during the summer, recent conditions have made them more dangerous. Elevated sea temperatures off the southern coast of Korea have created an environment rich in hot, humid air. When this moist air encounters an unstable atmosphere, it fuels towering vertical clouds capable of intense rainfall.

This dynamic led to highly localized extremes: over a three-day period starting July 16, Gwangju and nearby Naju saw over 400 millimeters of rain, while adjacent counties such as Yeongam and Jangheung received as little as 25 to 90 millimeters.

Forecasting challenges and nocturnal storms

The scale and rapid onset of these narrow rainbands also make them harder to forecast. “Because they’re so small and short-lived, predicting these types of intense local rainfalls is extremely difficult,” Kim said.

Adding to the risk, Korea often experiences “nocturnal rainstorms” during the summer rainy season, when rainfall intensifies during the night. The July 17 storm that struck South Chungcheong Province occurred largely between the evening and early morning hours.

This phenomenon is linked to the low-level jet stream — fast-moving air at altitudes of around 3 kilometers. During the day, rising warm air from the ground can disrupt this stream. But at night, the absence of this interference allows the jet stream to efficiently transport tropical moisture into the region, resulting in heavier rain.

This article from the Hankook Ilbo, the sister publication of The Korea Times, is translated by a generative AI system and edited by The Korea Times.