The weather forecast is forecast in advance. Why didn't Miyun Reservoir release water in advance?

The weather forecast predicts that Miyun Reservoir will not release water in advance because the weather forecast can predict the future weather changes, but the drainage of Miyun Reservoir needs to consider more factors, such as the water level, the capacity of the reservoir, the condition of drainage pipes, the local topography and climate. Therefore, it is not possible to decide whether it is necessary to drain water in advance only according to the weather forecast.