USGS Fire Danger Forecast
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The Fire Potential Index (FPI) is a moisture-based vegetation flammability indicator. It is a function of current living vegetation greenness as a proportion of maximum greenness, and current 10-h dead fuel moisture as a proportion of the moisture of extinction. Thus the FPI is high when the living vegetation is mostly or completely cured and the 10-h dead fuel moisture is low. The FPI is low when the living vegetation is near fully green and the 10-h moisture is high i.e., near the extinction moisture. In any case, if the 10-h moisture is equal to or greater than the extinction moisture, the FPI will be zero. Wind is not included in the calculation because of its spatial variability and transitory nature.
The FPI is calculated once daily for the continental U.S. at a resolution of 1 square kilometer. Although these maps provide a relative measure of fuel flammability across the U.S., on a scale of 0 to 100, they do not indicate the chance that a large fire will occur. The same can be said for the National Fire Danger Rating Indexes as they too lack the capability required to deal with the probabilistic nature of fire danger across a large spatial area. The Large Fire Probability and Predictive Service Area maps accomplish that.