Ensuring a safe water supply for society and the sectors that rely on water for economic and environmental purposes is a difficult problem that needs significant investment in R&D across public and private organizations. Climate change accelerates and exacerbates this challenge, causing a breakdown in some traditional approaches that use the past to predict the future streamflow on a seasonal (10 day to one year) horizon. Water managers planning on this seasonal timescale need help advancing models, tools, and transforming model results into actionable information for making decisions. In this Phase I effort, Upstream Tech proposes to expand its HydroForecast Seasonal prototype by achieving three technical objectives: 1) improve methodology and data used for generating weather forecast inputs, 2) integrate a new weather forecasting source to improve accuracy at longer lead times, and 3) benchmark performance in critical basins that benefit customers and society. At the conclusion of Phase I, we will have created a set of seasonal models in key basins of interest to managers, hydropower, state agencies etc., and performed a robust benchmark evaluation of HydroForecast Seasonal and how it complements with NOAA models.