Here we investigate the factors that lead to downstream anti-cyclogenesis during two recent western Pacific ET events using a 90 member Weather Research and Forecasting (WRF) model EnKF. An analysis ensemble is generated every six hours by assimilating in-situ observations from surface stations, rawinsondes, aircraft and cloud motion vectors. The statistics of this ensemble are used to understand the role of the TC and mid-latitude flow in amplifying the downstream ridge. For the transition of Typhoon Tokage (2004), there are two main regions of precipitation near the storm, one associated with the TC itself and another associated with developing warm front on the poleward side of the TC. Ensemble statistics indicate that the building ridge is sensitive to the amount of precipitation along the warm front, but is relatively insensitive to the precipitation in the TC core. Moreover, these results show that the amplitude of the ridge is inversely proportional to the poleward moisture flux to the east of the TC. This research suggests that observations of lower-tropospheric moisture and winds during ET may improve forecasts of Rossby wave genesis in the western Pacific.