Abstract  [Objectives]  To identify the origin of the mass irruption of rice leaf folder (RLF; Cnaphalocrocis medinalis) moths that appeared in late season (late August and early-mid September) rice crops in the Lower Yangtse River Valley (LYRV) in 2021. [Methods]  Monitoring data, including systematic field investigations, sex pheromone trap results, and female ovarian dissection, were used to identify the population characteristics of moths collected during the Nanjing outbreak. A trajectory analytical approach, and the effect of weather on likely migration routes was also analyzed. [Results]  The moths originated from two locations: (i) From 24 August to 2 September most moths were immigrants, and came from southern Anhui, northern Jiangxi, northern Hunan, and eastern Hubei. (ii) During 3-14 September, most moths were local and emigrated towards southern Anhui, northern Jiangxi, and northern Zhejiang. Compared to normal years, the western Pacific subtropical high-pressure system (WPSH) was much stronger, and its orientation was more southerly, covering a large area of Southern China to the Yangtze River. As result, strong southwesterly winds (towards the northeast) prevailed over much of Hunan, Jiangxi, Anhui, and Jiangsu. The wind speed was also stronger than in other years. 87.29% (419/480) of moths were caught by sex pheromone traps when the downward airstream was predominant, suggesting that the downward airstream induced by the WPSH was a key factor forcing RLF moths to concentrate and land. [Conclusion]  Special weather conditions caused high numbers of RLF moths to immigrate into the LYRV in later August 2021. These findings can help develop better integrated pest management strategies for this pest.