From midnight until 10:23 a.m. on the 17th, 438.5 mm of rain fell in Seosan, South Chungcheong Province. This is the maximum daily precipitation for July. The Korea Meteorological Administration stated that it is a rain level that can occur once every 200 years. The history of meteorological observations in Seosan began in 1968, compiling records of heavy rain for 57 years. However, how can it be said that this is a level that occurs once every 200 years?
In response, a representative from the Korea Meteorological Administration explained, "This is based on existing observation data applied to the concept of 'frequency precipitation' estimated through probabilistic methods."
The Korea Meteorological Administration calculates the average and distribution based on the July observation data collected over 57 years from 1968 to 2024, stating, "If it rains this much, that represents a certain percentage (%) based on the probability distribution."
If the calculated occurrence probability is 1%, it is seen as a probability that might occur once during a 100-year observation period. A representative from the Korea Meteorological Administration noted, "The recorded daily rainfall in Seosan on this day can be interpreted as 'a level that might occur once every 200 years for July.'"
It is said that regional characteristics, such as whether the area typically receives a lot of rain, are also taken into consideration. Even if the observation figures for A and B regions over 10 years are exactly the same, the 100-year or 200-year frequencies can differ based on regional characteristics.
On the same day, in addition to Seosan, the maximum daily rainfall for July recorded once every 200 years was also noted in Dangjin and Asan City, as well as Yesan and Hongseong counties in South Chungcheong Province. Taean in South Chungcheong Province recorded a 100-year frequency. Record-breaking heavy rain fell in several areas of the Chungcheong region.
The Korea Meteorological Administration began providing such frequency rainfall data starting last year. Since it is difficult for the public to perceive how much rainfall is indicated by a certain number of millimeters, the aim was to intuitively inform the public about the amount of rainfall that may occur once every several years.
The Korea Meteorological Administration collects and announces these values when abnormal weather events like heavy rainfall occur. Last year, it reported ▲ one point at a 50-year frequency, ▲ one point at an 80-year frequency, ▲ one point at a 100-year frequency, and ▲ nine points at a 200-year frequency.
Frequency rainfall was originally a concept used by the Ministry of Environment and the Ministry of the Interior and Safety, relating to design frequencies. It is aimed at ensuring that facilities related to water management, such as dams and sewage systems, are designed to withstand rainfall events of '200-year frequency' or '300-year frequency' without issues; these standard values are publicly disclosed for construction applications. However, the Korea Meteorological Administration compiles its own data separately using data from nationwide weather observation sites.
Recently, as extreme weather events like 'intense rainfall' have become more frequent, there are forecasts that the frequency of rainfall reports may increase each year.
Meanwhile, intense rain is expected to continue until the 19th, which falls on a Saturday. The Korea Meteorological Administration forecasted that heavy rain is likely to pour down in the Chungcheong region on the night of the 17th and in the southern regions on the night of the 18th.
The Ministry of the Interior and Safety raised the flood response alert to 'serious' level at 3:30 p.m. on the same day and activated the Central Disaster Management Headquarters at Level 3. The 'serious' level and Level 3 are the highest within the crisis and emergency phases.