Traditionally, tephra transport and dispersal models have evolved decoupled (off-line) from numerical weather prediction models. There is a concern that inconsistencies and shortcomings associated to this coupling strategy might lead to errors in the ash cloud forecast. Despite this concern, and the significant progress to improve the accuracy of tephra dispersal models in the aftermath of the 2010 Eyjafjallajökull and 2011 Cordón Caulle eruptions, to date, no operational on-line dispersal model is available to forecast volcanic ash. Here, we describe and evaluate NMMB/BSC-ASH, a new on-line multiscale meteorological and transport model that attempts to pioneer the forecast of volcanic aerosols at operational level. The model predicts volcanic ash cloud trajectories, concentration of ash at relevant flight levels, and the expected deposit thickness for both regional and global configurations. Its on-line coupling approach improves the current state-of-the-art of tephra dispersal models, especially in situations where meteorological conditions are changing rapidly in time, two-way feedbacks are significant, or distal ash cloud dispersal simulations are required. This work presents the model application for the first phases of the 2011 Cordón Caulle and 2001 Mt. Etna eruptions. The computational efficiency of NMMB/BSC-ASH and its application results compare favorably with other long-range tephra dispersal models, supporting its operational implementation.