The “historic” simulation line represents the long-term climate response a river might have according to the hydrologic model. If the hydrologic forecast is for near-normal conditions, the “conditional” simulation curve will be similar to the historic simulation curve. If the streamflow forecast is for wetter than normal conditions, the conditional simulation curve will be above the historic curve. Likewise, if the streamflow forecast is for drier than normal conditions, the conditional simulation curve will be below the historic curve.
The slope of the simulation curve indicate the amount of uncertainty associated with a forecast. The flatter the line, the more confident the forecast. The steeper the line, the less confident the forecast.
NOAA’s National Weather Service produces such plots as part of the Advanced Hydrologic Prediction Service.