A numerical model of the atmosphere has been employed to evaluate the assumptions used in the simple two-parameter model that is utilized for many probable maximum precipitation (PMP) calculations. These assumptions are (1) the precipitation is linearly related to the precipitable water; (2) the precipitation efficiency of the storm does not change as the moisture available to the storm increases; and (3) terrain modulates the distribution of the precipitation but does not affect the synoptic-scale dynamics of the storm. A single case study is used to illustrate the techniques employed and to describe the results that were common to four case studies. We show that long-lived, moderate rainfall processes are important contributors to the total precipitation produced by the storm. Increases in the moisture availability result in the heavy rainfall beginning earlier, lasting longer, and being more continuous. As the moisture availability changes, the spatial distribution of the area over which more than 50% of the total rainfall falls as heavy rainfall changes. As the moisture availability is increased, the precipitation efficiency of the storm does not change significantly. Terrain effects are shown to have an effect on the amount of rainfall that occurs over the higher terrain as well as on the distribution of the rainfall due to the convergence component of the storm. Despite these deficiencies in the assumptions used to estimate PMP, improvements in the estimation of PMP may soon be possible if increased effort is placed on (amongst other things) the numerical modeling of extreme rainfall events. These improvements are only possible if the results of these efforts are communicated to the hydrological community. ¿ 1999 American Geophysical Union