The regional NOAA Hydrometeorologic Reports specify storm characteristics, but they do not specify hydrologic processes within watersheds, the watershed state as the storm begins, or the PMP storm track and speed as it moves over a watershed. Hydrologic simulation, calibrated using the historic records in a watershed, represents the response of the land surface and soils, and the response of the channel network and floodplains to rainfall and flood flows. Simulation does not automatically 'solve' for the appropriate watershed state or storm track characteristics. It does, however, provide information that can be used to make the necessary choices. Alternate storm tracks can change PMF peak flows by 25 to 50 percent in larger watersheds.
PMF analysis requires choices about the simultaneous occurrence of independent rare events.
It is unreasonable to assume a storm track and speed that matches and adds to the flood wave as it moves downstream in a large watershed, if this storm track and speed is unlike anything observed historically. Hydrologic simulation for Probable Maximum conditions, or simulation of Probable Maximum Floods, has been done for many reservoirs in the United States and worldwide. These effects are particularly noticeable in some heavily forested watersheds, and in watersheds where high permeability soils lead to high interflow flowrates. Dynamic simulation of the non-linear hydraulics of flood plain storage and overbank conveyance during the PMF is critical in some watersheds. Linear or unit-hydrograph assumptions for channel flows are inappropriate when stream channels overflow extensively.