Abstract: The Fifth-Generation Penn State/NCAR Mesoscale Model (MM5) was used to investigate the extra-area effects of silver iodide (AgI) seeding on stratiform clouds at the supercooled layer. A bulk two-moment microphysical scheme and the new software package for AgI were incorporated into MM5. Extra conservation equations were applied to trace the seeding agent, which was transported along the flow field and interacted with the supercooled cloud fields. In this study, the model was run for widespread precipitation caused by a stratiform cloud system using three nested grids, with a 3.3 km × 3.3 km horizontal resolution in the finest grid. The model results showed that seeding with AgI at the -5℃ to -10℃ levels had microphysical effects on the simulated clouds and that the simulation produced a seeding effect that lasted 3 h longer because of transport of the seeding agent by upper-level winds. Most of the AgI particles acted as deposition nuclei, and the deposition nucleation process contributed most to additional cloud ice formation. The results showed that more precipitation results from seeded than unseeded clouds, and the precipitation was redistributed downwind of the target. Augmented precipitation (varying from 10% to 30% downwind) was confined in space to within 240 km of the seeding target and in time to the 3-h period after initial seeding.