Abstract Significant progress has been made in the short-range (1–2 days) prediction of east coast cyclogenesis over the past decade. This is the result of improved model resolution, physical parameterization, and good analysis of the upstream conditions often well sampled by the high-density North America observing network. The prediction of cyclogenesis over the eastern Pacific Ocean, or a longer-range forecast of east coast cyclones, does not share the same degree of success, largely due to the fact that the upstream conditions fall over the data-void regions of the Pacific Ocean. In this paper, the authors study the prediction of the ERICA IOP-4 storm using a 120-km hemispheric version of the Penn State/NCAR Mesoscale Model MM5, with forecast duration ranging from 36 to 120 h. Specifically, the impact of uncertainties in the initial conditions on the 5-day forecastof this cyclogenesis event was examined. Initial perturbations were then introduced to the original analysis at initial time based on the 1...