The meadow moth, Loxostege sticticalis (L.) is an important migratory pest in North China. It makes its long range migrations by flying on several consecutive nights. Radar observations have shown that the adults fly throughout the night and that both take-off and landing process are active rather than passive processes. But as with other aerial migrants, their migrations will be significantly influenced by weather conditions. Some synoptic systems may facilitate massive takeoff, concentration in the atmosphere, and involuntary landing to form an outbreak population. Previous researches have confirmed that synoptic systems affect meadow-moth migration, but we still need to know more details. In this paper, a migration that occurred over the plateau in northern Hebei province during 24-25 May 2004 is analyzed. A set of atmospheric dynamical fields were simulated by using the PSU-NCAR mesoscale non-hydrostatic numerical forecast model MM5. The high spatial and temporal resolution results from the model output were used to diagnose some physical variables and their effects on the migrating process. First, we analyzed the migration dynamics. A large proportion of the population of meadow moths in the permanent source areas emigrated on the night of May 24; most of them flew northeast and landed involuntarily in the northern part of Fengning county. Second, we show that the massive takeoff of the meadow moth on the night of May 24 was associated with the passage of a cold front. The involuntary landing was caused by subsidence rather than precipitation or low temperature. Subsidence exceeded 30 cm/s at the flying level and the area of maximum subsidence speed coincided exactly with the landing location. Finally, we analyzed the motion of the airflow in three dimensions to determine the cause of this exceptional subsidence. It appears to have arisen from the obstruction effect of the plateau′s topography to the cold front and the airflow near the ground.