Using the Met Office's Unified Model, cutting-edge 4.4-km, limited area, convection-permitting, 5-day ensemble forecasts were produced for both storms along with 5-day global ensemble forecasts. Each ensemble (both convection-permitting and global) includes 12 members and forecasts are initialised 12 hours apart over a 6 day period. Consistent with the operational forecasts produced at the time of the storms, the spread of tracks is greater in the forecasts produced for Hagupit than Haiyan. The reasons for these differences are investigated using several techniques: (i) Comparisons of the three-dimensional structures of the storms, their intensities and their environments in different ensemble members; (ii) Steering flow calculations in the simulations both with the storm and following storm removal; (iii) Three-dimensional Lagrangian trajectories, to investigate the dynamical interactions between the storm and other large-scale weather systems, such as the outflow of convective systems over the Maritime Continent during Typhoon Hagupit. In particular, it is shown that prior to making landfall Hagupit slowed down as it entered an area between two high pressure systems, where the environmental flow advecting the storm was weak. It is at this point that the track forecasts begin to diverge.