Best-track and NCEP/NCAR Reanalysis data are used to study multiple cyclone events (MCEs), in which one tropical cyclone (the "daughter") forms to the east of another (the "mother") during the mother's lifetime, in the western north Pacific. It is found that approximately 30% of all tropical cyclones become mothers. A statistical analysis of the differences in large-scale conditions between MCEs and events in which a daughter does not form is carried out to test the hypotheses that a) the

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... ence of MCEs is a result of Rossby wave radiation from the mother, and b) that Rossby wave radiation is more likely to occur in easterly vertical shear and cyclonic low-level horizontal shear, as in the theory proposed earlier by Krouse et al. (2008) . It is found that MCEs are indeed favored by easterly vertical shear and more cyclonic low-level horizontal shear. However, the sign of the total vertical shear is on average westerly in MCEs, which contradicts the theory of Krouse et al. (2008) (at least to the extent that the 850-200 hPa zonal wind difference is the correct measure of the shear to employ in that theory, which is ambiguous). The theory is also unsuccessful in predicting the variation in zonal distance between mother and daughter from event to event based on large-scale shear differences, although it does successfully predict the mean distance.