The availability of data collected during the Convective and Orographically-induced Precipitation Study (COPS) provides a unique opportunity to study the role of orography in determining the predictability of convection. Accurate prediction of the location of convective initiation requires high-resolution grids which can represent the fine-scale terrain details. In this study, we conduct high-resolution simulations using the UK Met Office Unified Model over the COPS region to quantify the predictability of severe convective storms encountered during COPS. An ensemble approach is taken using initial and boundary data from the Met Office Global and Regional Ensemble Prediction System (MOGREPS). Results are presented for IOP 9c (20th July 2007) where convection was triggered by a squall line produced in the outflow boundary of a weak Mesoscale Convective System (MCS). In this case, the high resolution ensemble is shown to be sensitive to errors on the low-resolution grid, which propagate in through the lateral boundaries. Ensemble sensitivity analysis is used to link the mesoscale response of the high-resolution ensemble members to the differences in the large-scale driving conditions.