Cheung (2003, to appear in Journal of Climate) examined six large-scale environmental parameters associated with 405 tropical cyclone (TC) formations in the western North Pacific during 1990-2001 computed from the NCEP/NCAR reanalyses with 2.5o latitude/longitude resolution. These parameters included the sea-surface temperature, 200-850-hPa zonal and meridional vertical shear, 500-700-hPa average relative humidity, convective available potential energy, and 200-hPa divergence. These parameters had quite consistent values with near-Gaussian distributions in almost all the cases examined. This study is an extension of Cheung (2003) in two aspects. One is to overcome the limitation of the coarse resolution of the NCEP/NCAR reanalyses by extracting characteristic mesoscale feactures during TC formations. Besides the six parameters mentioned above, the existence (or not) of mesoscale convective systems, and their interactions will also be considered. The second extension is to examine the temporal evolution of the environmental fields and mesoscale features during formations, instead of examining only those features at a specific formation time that often may be ambiguous. Unlike previous individual case studies, objective analysis techniques will be used to produce high-resolution reanalyses for the TC formation cases that incorporate additional observations and satellite products so that mesoscale fields are available for examination. Then the characteristics associated with TC formations will be extracted using linear and nonlinear principal component analysis. The analysis procedure is performed separately for formation cases in different basins to examine different initial disturbances and formation dynamics.