Given the length of the instrumental record in most of the United States, this is typically a once in 50 to 80 year condition.
Watershed area is a factor.
Streamflow in the Orinoco River in Venezuela can be forecast accurately for several months based on its current watershed state. If watershed conditions from a historic year that was much wetter than average were used, future simulated flows would exceed historic flows. The yield of a reservoir is a maximum if future drawdown period reservoir inflows are known. If forecasts were ideal, if future weather and streamflows were precisely known at now, the forecast pdfs in Figures 5 and 6 would be single valued. A Tool for Water Resources Management in the Seattle Area, National Conference on Hydraulic Engineering and International Symposium on Engineering Hydrology, ASCE, July 1993.