CalWater is a multiyear program of field campaigns, numerical modeling experiments, and scientific analyses focused on phenomena that are key to the water supply and associated extremes (drought, flood) in the U.S. West Coast region. A multi-institutional team of researchers, including a scientist at Pacific Northwest National Laboratory, summarized the major milestones in the development of the CalWater program of field studies and science, the specific science questions on atmospheric rivers and their hydrometeorological impacts and aerosols and their impacts on cloud and precipitation, key results and accomplishments to date, and future directions. Key science questions addressed by CalWater include: (1) how will precipitation characteristics change in the future? (2) How are water supplies associated with precipitation likely to change? (3) How can better information on extremes enable climate adaptation for drought resilience, flood control, hydropower, ecosystems, and coastal inundation? (4) How accurate are atmospheric rivers predictions and how can they be improved? (5) What roles do aerosols from long-range transport and local sources play in short-term regional weather or climate predictions? (6) What information can be gained from subseasonal-to-seasonal predictions? Using a coupled modeling-observational strategy in CalWater-1 (2008 – 2014) and CalWater-2 (2015 – 2018), researchers advanced key findings regarding the water vapor budget, dynamics, and predictions and climate projections of atmospheric rivers and the extent of different types of aerosols and their impacts on cloud microphysical processes influence precipitation efficiency. A key result is the determination that roughly 40%–50% of annual precipitation in northern California is associated with landfalling atmospheric rivers, revealing that they are critical to the water supply in the region. The research also revealed the key role of long-range transport of dust at high altitudes from Asia, the Middle East, and Africa in affecting precipita­tion processes under atmospheric river conditions and in systems responsible for the balance of winter precipitation. A highlight of CalWater-2 is the CalWater-2015 field operations that occurred during January-March 2015 over California and offshore, and involved a wide array of sensors and platforms sponsored by NOAA, DOE, NSF, NASA, the California Department of Water Resources, and the Naval Research Laboratory. CalWater’s findings have relevancy to regions beyond the U.S. West Coast, especially in the west coasts of midlatitude continents. Most importantly, our society needs this key information now to manage and plan for risks, espe­cially in a landscape of increasing pressure on water resources, as well as those from a changing climate.