The "Improving Atmospheric River Forecasts Act" establishes an Atmospheric River Forecast Improvement Program under the Under Secretary, in collaboration with the weather enterprise and higher education institutions. This program's primary goal is to reduce the loss of life, property, and economic losses from atmospheric rivers by developing and extending accurate, effective, and actionable forecasts and warnings. It seeks to achieve this through establishing quantitative forecast skill metrics and developing a unified atmospheric river forecast system capable of providing seasonal to short-range predictions, including snow accumulation and other hydrologic components.

The program will advance scientific understanding of atmospheric rivers and improve tools for predicting landfall locations and inland penetration, while also enhancing the transition of research to operations. It emphasizes incorporating social, behavioral, risk, communication, and economic sciences to better understand public response. The Under Secretary will also periodically examine and evaluate innovative observations, data assimilation, and modeling techniques, such as radar, aircraft, satellite, and ocean buoy data, to further refine analyses and predictions.

The bill authorizes improved modeling for precipitation forecasts, particularly in complex terrain, to support enhanced water resource management and resilience to extreme water-related events. It mandates acquiring and sustaining adequate crewed and uncrewed aircraft, scientific equipment, and personnel for annual atmospheric river reconnaissance missions from November through March, ensuring data availability for research and operations. Additionally, the program focuses on improved hazard communication by developing methods to categorize atmospheric river intensity, establishing best practices, and gathering information on public preparedness. Within 270 days of enactment, the Under Secretary must develop and make public a comprehensive plan detailing the program's research, development, and technology transfer activities.