Tropical cyclone formation in the NE Pacific Ocean is poorly understood. We show that by dividing the NE Pacific Ocean into two regions, east and west of 116 ¿W, our understanding is enhanced in terms of which climatological factors are involved. We show that for the period 1972--1997, in our western region, significant relationships exist between several environmental parameters and tropical cyclone numbers on a seasonal timescale, and there is also a significant increasing trend with time. Important parameters common to all tropical cyclone indices examined include relative humidity, sea surface temperature, pressure vertical velocity and precipitable water. The potential for predictability of tropical cyclones in this region is noted. ¿ 2000 American Geophysical Union