Extreme precipitation information is of interest for a variety of purposes, including public safety, water supply, dam design and operation, and transportation planning. Two common parameters calculated for extreme precipitation purposes are probable maximum precipitation (PMP) and intensity-duration-frequency (IDF). The definition of PMP is “theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given area at a particular geographical location at a certain time of the year.” PMP estimates are used to calculate the probable maximum flood (PMF), which in turn is used to evaluate the adequacy of hydraulic structures.

Historically, computations of these parameters have been accomplished using mostly manual techniques (e.g., hand-drawn maps). Advances in spatial climate mapping and geographical information systems (GIS) technology have created new opportunities for mapping extreme precipitation. Using GIS software and climate grids created using PRISM, new approaches to extreme precipitation mapping have been applied to specific locations with good success. In addition to automating analytical processes, thereby saving time, the new techniques have been shown to produce more consistent and defensible coverages than those of previous analyses.