The emergence of highly pathogenic avian influenza A H5N1 in dairy cattle raises many questions related to animal health and changes to the risk of an epidemic in humans. We synthesized information currently published to fit a compartment model of H5N1 transmission within a dairy herd. An accompanying web application allows users to run simulations for specific outbreak scenarios. We estimated R0 near 1.2 with a short duration of infectiousness and fast time course of an epidemic within a farm, which we discuss in the context of possible on-farm control strategies. The web application allows users to simulate consequences of an epidemic using herd-specific information, a tool we propose will help inform stakeholders about potential consequences of uncontrolled H5N1 spread. Our modeling work has identified several key information gaps that would strengthen our understanding and control of this emerging infectious disease.