We present a study of the spatial variation of forecast conditions at La Palma, Canary Islands. Previsions are obtained with the Weather Research and Forecasting (WRF) model in combination with the Trinquet-Vernin (TV) model, which allows us to compute the optical turbulence from the vertical gradient of the temperature and the wind velocity. The first aim of this study is to demonstrate the ability of the WRF model to reproduce the spatial evolution of the weather and especially the optical conditions, even using low-resolution input data. The second purpose is the definition of a quality parameter Q, which can be used to determine and discriminate between astronomical sites. More than 4500 h of simulations were calculated in 2009 and we present an average of these predictions over a 2D grid. The results show that the WRF model is able to simulate the spatial evolution of the weather at a better resolution than the coarse input data and it adapts the dynamic flow to the terrain orography. The quality parameter is applied to the Observatorio del Roque de los Muchachos at La Palma and shows that it is, within 2 km precision, installed at the best possible location on the island.