This website from the U.S. Geological Survey [USGS] provides "several products for understanding and forecasting the probability of large wildland fires on all land in the conterminous U.S.," The Expected Number of Large Fires per Predictive Service Area (PSA) map "takes into account both probability of ignition and conditional probability of spread (to 500+ acres in East and 1000+ acres in West*) given ignition. The map shows the total number of fires to expect in a given Predictive Service Area (PSA) and the degree of confidence for each expected value." The Large Fire Probability map "'translates' the Fire Potential Index (FPI) map into a probability map. It shows how often we can expect a fire to burn more than 100 acres if a 1+ acre ignition occurred in a given km2 grid cell with vegetation flammability conditions similar to those indicated by the FPI map." The Fire Potential Index 7-day forecast "indicates the estimated proportion (percentage) of the vegetation that is dry enough to burn, thus the FPI is highest when dead fuel moistures and vegetation greenness are low. 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 USGS Fire Danger Forecast Viewer "provides a dynamic online map interface that can be used to view USGS datasets."
U.S. Geological Survey: http://www.usgs.gov/