Fire managers in the Northwestern United States are often confronted by the problem of determining when precipitation might stop an ongoing fire. The possibility that a useful probability for fire-stopping precipitation could be developed from historical weather records was investigated. Persons familiar with weather and fire behavior suggested a fire-stopping precipitation criterion of 'at least 0.5 inches of precipitation in 5 days or less.' Daily precipitation data from four fire weather stations for the period 1970 to 1985 were used to estimate the chance that the criterion would be satisfied by a specified date in the fire season. The Weibull probability distribution was found to be an excellent fit for the problem. The method of fitting the distribution to the data is described and results given for four weather stations in the Northern Rocky Mountains. This method can be applied to other 'fire-stopping' criteria using the same technique.