The 1.5D transport code JETTO has been applied to model the transition from the Low (L) to the high confinement mode (H-mode) in the JET tokamak. Computed values of the critical power, Pth, required for the L-H transition on JET are directly compared with experiment across line averaged density and magnetic field scans. Reasonable agreement is found between computations and experiment across all densities considered, including low density discharges, where Pth increases with decreasing density. The minimum of Pth(ne) dependence is explained by the enhanced contribution of the particle convection to heat losses at the edge. Higher convective losses result in lower temperature and its gradient, and therefore more power is required for the L-H transition. Computations performed for JET discharges with varied magnetic field show a rough agreement with the experiment, nonetheless both computed and experimental power threshold are substantially higher than the inter-machine scaling predictions.