The Joint Probability Method (JPM) is a statistical approach to determine extreme water levels based on a large number of combinations of storm parameters. This is the primary method used to predict the 1-%-annual-chance storm surge elevation for a coastal flood study. The JPM describes tropical storms using five or six parameters (central pressure, storm size, forward speed, direction, point of landfall or exit or closest approach, and wind speed variation with distance from storm center). Probability distributions are developed for each of the parameters based on local climatology. All possible parameter combinations are simulated using a hydrodynamic model constructed to faithfully represent the bathymetry, topography, and ground cover of the study area.
The JPM was developed in 1975 and has been modified since that time. The most recent modification was to optimize the parameterization of storm characteristics and reduce the number of storm simulations that must be performed to estimate 1-%-annual-chance flood levels; this method is known as JPM-OS (Joint Probability Method - Optimal Sampling).