People holding umbrellas walk in the rain near City Hall in Seoul on Wednesday. (Yonhap)

With the whole nation in the middle of the annual monsoon season, heavy downpours of up to 150 millimeters are forecast to pound the central region between Wednesday and Thursday morning, the state weather agency said.

The Korea Meteorological Administration (KMA) said up to 100 mm of monsoon rains are expected to soak the Seoul metropolitan area and almost all parts of the central region, including Gangwon Province, until early Thursday.

The precipitation may reach up to 150 mm in some mountainous districts in the central region and the northern part of the capital region, the KMA said.

Also, western parts of the capital area and northern parts of South Chungcheong Province will experience heavy downpours of 30 mm per hour Wednesday morning, the KMA said.

The KMA issued a downpour warning in five Yellow Sea islands close to the North Korean border.

Most of the downpours will have stopped by early Thursday, the KMA said. (Yonhap)