South Korea Experiences Second-Hottest September on Record, October Outlook Remains Warm
South Korea faced an unusually warm September 2025, marking the second-hottest September on record. The month was also characterized by frequent rainfall and thunderstorms, departing from the typical crisp fall weather. The Korea Meteorological Administration (KMA) attributes the high temperatures to the lingering effects of the North Pacific high-pressure system.
Record-Breaking Temperatures and Rainfall
The average nationwide temperature in September reached 23 degrees Celsius, a 2.5-degree increase from previous years. While September 2024 remains the warmest on record at 24.7 degrees Celsius, this past September marked the third consecutive year within the top three warmest since nationwide records began in 1973. Seoul experienced a rare September heatwave on September 5th, and Jeju Island recorded a heatwave and tropical night on September 25th, the latest occurrence on record.
Precipitation levels also exceeded expectations. The nationwide rainfall averaged 228.8mm, 1.5 times higher than the average of previous years. Furthermore, the month saw an average of 15 rainy days, the second-highest since weather records began.
Torrential Rains and Sea Surface Temperatures
Between September 6th and 7th, torrential rains impacted areas in South Chungcheong Province and North Jeolla Province, causing significant disruptions and dumping nearly 300mm of rain in certain areas. Gunsan in North Jeolla Province experienced over 100mm of rainfall per hour. The KMA reported that sea surface temperatures around the Korean Peninsula averaged 26 degrees Celsius in September, which is 1.5 degrees Celsius higher than the 10-year average and the second-highest in a decade. This rise in sea surface temperature raises concern about stronger typhoons.
October Weather Outlook
The KMA forecasts that higher-than-average temperatures are likely to continue into October. Their one-month outlook indicates a 46.6 percent chance that temperatures will exceed those of previous years. While a gradual cooling trend is expected to begin around October 20th, there is a 60 percent chance that temperatures between October 13th and 19th will still be higher than in previous years. From October 20th to November 2nd, the probability of temperatures being higher or similar to previous years is 40 percent.
