1. A computer system for generating a forecast loss probability distribution for at least a first insurance risk, the system comprising: an electronic data storage device configured to store data describing one or more portfolios of insurance risks, the data for each insurance risk including at least measured characteristics, environmental variables, expert opinion parameters, historical loss data, historical policy coverage, and current policy coverage; and a computer processor configured to compute a forecast loss probability distribution for at least a first insurance risk selected from the one or more portfolios of insurance risks, wherein the forecast loss probability distribution is computed by: constructing a first historical loss likelihood function for the first insurance risk, based on compound Poisson processes describing the historical loss data according to at least a historical loss frequency distribution and a historical loss severity distribution for a first loss type associated with the first insurance risk, wherein each Poisson process is a function of at least the measured characteristics and the environmental variables for the first insurance risk, according to the expert opinion parameters for the first insurance risk; constructing a similar historical loss likelihood function for each of one or more other insurance risks in the same portfolio as the first insurance risk; updating at least the first historical loss likelihood function according to a Bayesian predictive model incorporating the historical loss data for each of the one or more other insurance risks, adjusted by the historical loss likelihood function for each of the one or more other insurance risks; and storing the updated first historical loss likelihood function on the electronic data storage device as the forecast loss probability distribution for the first insurance risk.