A meteotsunami hit southwest Kyushu on 25 February 2009, with an estimated maximum amplitude of 290 cm, which was higher than that recorded for the 1979 Nagasaki event. This study investigated mesoscale meteorological systems over the East China Sea during the time leading up to the February 2009 event using a Weather Research and Forecast model. The disturbance in the sea-level pressure originated from a gravity wave over southeastern China. The sea-level pressure disturbance observed and modelled over the East China Sea had its source over the southeastern China mountains and was then propagated by a jet stream toward western Japan with the help of both wave-duct and wave-CISK (conditional instability of the second kind) mechanisms. Two synoptic systems supported the momentum convergence and the formation of band-shaped unstable layers in the mid-troposphere. The high-latitude trough extended from eastern Siberia and there was subtropical high pressure over the western Pacific Ocean. The phase speed of the atmospheric wave was as high as 25–30 m s⁻¹, corresponding to the phase speed of long ocean waves on the East China Sea. Improvements in determining the amplitude and timing of the disturbance remain for future work.