The study tested how accurate and reliable water supply forecasts are for Sierra Nevada watersheds in California. They used a new forecasting system and found that forecasts are more accurate closer to the forecast period. The most likely forecasts tend to underestimate water supply, while the average forecast is more accurate. Wet forecasts are generally reliable, but low flows are over-predicted and medium flows are under-predicted. Extreme conditions have a big impact on forecast accuracy. Using different data sources or longer records didn't consistently improve forecasts. This study helps improve future water supply forecasts and gives water managers useful information to make decisions.