In watershed and master drainage planning studies, information on flood frequency distributions must be available for locations throughout the study area and for conditions with and without planned stormwater control facilities. Design storm modeling and continuous simulation approaches are used to obtain the required information. Analytical probabilistic stormwater models are computationally more efficient alternatives but they were developed originally for urban stormwater management planning and design purposes. In this paper, methods and strategies are developed to expand the capabilities of existing analytical probabilistic stormwater models so that they can also be used in watershed and master drainage planning studies. The case study tests and validates the proposed methods and strategies and demonstrates the versatilities of the new approach.