As of 10 a.m. Tuesday, some parts of the region had already seen rain due to a stationary front, the Korea Meteorological Administration said.
"A precipitation zone is expected to form at night as the front is forecast to clash with dry air approaching from the west," the agency said.
As a consequence, 30-50 mm of rain per hour is expected to batter the central area covering Seoul and Gyeonggi, Gangwon and Chungcheong Provinces between 9 p.m. Tuesday and 9 a.m. Wednesday, the agency said, warning of possible landslides and flooding.
A northwest wind is then forecast to spread the rain to the southern region on Wednesday afternoon.
It will be cloudy all over the country on Thursday, the eve of Chuseok, Korea's autumn harvest celebration, with rain expected on the west coast, according to the agency.
But people will be able to enjoy a full moon across the nation on one of the country's two biggest traditional holidays, it added. (Yonhap)