A model is presented to compute the transport and deposition of volcanic ash particles. Calculations use variable winds along the particle trajectories. The model was exercised by performing numerical experiments with data from the May 18, 1980, eruption of Mount St. Helens, Washington. Ashfall arrival times, location of grounded ash, a particle size distribution, and particle size sorting across the grounded ash lobes were calculated and compared to observations. Agreement was generally found, except for the particle size distribution. The calculated size distribution contained particles that were larger than measured particle sizes. This result supports the speculation made by others that falling aggregates break during landing and subsequent measurement.