Up to 150 mm of Rain Fell Overnight in the Capital Region... More Than 200 mm Still to Come

[Financial News]  Rainfall approaching 150 mm poured across parts of the Capital Region overnight, with 148.5 mm recorded in Gimpo, Gyeonggi. Heavy rain warnings were issued for all of Seoul, while heavy rain advisories were in effect for Gangwon Province and South Chungcheong Province. The Korea Meteorological Administration (KMA) said more than 200 mm of additional rain could fall in Gangwon and the Capital Region through the 19th.
According to the KMA on the 18th, strong rain of 20 to 40 mm per hour continued as of 6 a.m. across the Capital Region, Gangwon Province and South Chungcheong Province. The downpour was driven by a mesoscale low-pressure system developing along a stationary front, along with a surge of hot, humid air from the southwest. In particular, moist air with precipitable water content reaching 70 mm kept flowing in, preventing the intensity of the rain from easing.
The center of the rain clouds shifted from the south to the central part of the country overnight. The first emergency disaster alert for extreme rainfall was sent at around 10:10 p.m. the previous day to Jisan-dong, Suseong-gu, Daegu, where rain had been concentrated in North Gyeongsang Province. But the main rain band moved to central regions early on the 18th. Emergency alerts were then issued in succession, starting at 3:18 a.m. for Bol-eumdo in Ganghwa County, Incheon, followed by 4:29 a.m. for Deungchon-dong, Gangseo District, Seoul, and 4:47 a.m. for Gugi-dong in Jongno-gu. These alerts are sent when hourly rainfall exceeds 50 mm or three-hour rainfall tops 90 mm.
From midnight to 6 a.m. that day, rainfall exceeded 60 mm per hour at times in Seodaemun-gu and Yangcheon-gu. Over the same period, cumulative rainfall also topped 120 mm in several parts of the Capital Region, including 138.0 mm in Tanhyeon-myeon, Paju, Gyeonggi, 135.5 mm in Gangseo District, Seoul, 128.5 mm in Eunpyeong District, 121.1 mm in Dongducheon, Gyeonggi, and 120.5 mm in Goyang.
The bigger challenge still lies ahead. As of 5 a.m., the KMA forecast an additional 80 to 150 mm of rain in the Capital Region through the 19th, with more than 200 mm possible in some areas, and 100 to 150 mm in Gangwon Province, with more than 250 mm possible in inland and mountainous areas. Chungcheong and central and northern North Gyeongsang Province are expected to receive 50 to 100 mm, while some areas of Sejong, northern South Chungcheong Province, central and northern North Chungcheong Province, and northern North Gyeongsang Province could see more than 200 mm. Some parts of Daejeon, southern South Chungcheong Province, southern North Chungcheong Province and northern North Gyeongsang Province may receive more than 150 mm.
Elsewhere, 30 to 80 mm is forecast for Gwangju and Jeollanam-do, 20 to 60 mm for Busan, Ulsan and Gyeongsangnam-do, 30 to 100 mm for the Five West Sea Islands, Jeollabuk-do, Daegu, southern North Gyeongsang Province, Ulleungdo and Liancourt Rocks, and 5 to 30 mm for Jeju. Through the 19th, very intense rain of 20 to 80 mm per hour, accompanied by gusts and thunder and lightning, is expected to repeatedly hit central regions, Jeollabuk-do and North Gyeongsang Province.
The monsoon rain will not let up over the weekend. On the 20th and 21st, the entire country except Jeju will remain under the influence of the stationary front. Expected rainfall on the 20th includes 50 to 100 mm in Chungcheong, 30 to 80 mm in southern Gyeonggi, 20 to 60 mm in the Capital Region excluding southern Gyeonggi, Gangwon, Daegu and North Gyeongsang Province, 5 to 40 mm in Honam, Busan, Ulsan and Gyeongsangnam-do, 5 to 20 mm in the Five West Sea Islands, and 5 to 10 mm in Ulleungdo and Liancourt Rocks.
Temperatures are expected to remain close to or slightly above seasonal norms. Rain and cloudy skies will limit daytime heat, but the continued inflow of hot, humid air will keep nighttime temperatures from falling much. As of 6 a.m. that day, major city temperatures stood at 27.1 degrees Celsius in Gwangju, 25.5 in Busan, 24.7 in Daejeon, 24.1 in Ulsan, 22.9 in Daegu, 20.7 in Incheon and 20.3 in Seoul. Daytime highs are forecast at 24 to 33 degrees Celsius, while on the 19th morning lows are expected to range from 21 to 26 degrees and afternoon highs from 25 to 33 degrees. From that day through the 20th, gusty winds and thunder and lightning are expected over most seas, and sea fog over the East Sea and the South Sea will require caution for navigation.
The KMA classifies hourly rainfall of 30 mm or more as "very heavy rain," a level at which visibility is poor and storm drains and drainage channels can no longer handle the water. For reference, 15 to 30 mm is considered "heavy rain," while 3 to 15 mm is classified as "moderate rain." Torrential rain that falls overnight, as in this case, makes it difficult to see and increases the risk that evacuation will be delayed. The KMA and the Ministry of the Interior and Safety advise the public to avoid underground spaces such as underpasses and underground parking lots, riverbanks and steep slopes during heavy rain, and to move early to safer places if they live in areas at risk of flooding, such as semi-basement homes.

ahnman@fnnews.com Ahn Seung-hyun Reporter