The Fosberg Fire Weather Index (FWI) was originally designed to provide a means of highlighting the spatial and temporal variability of fire danger due to varying weather conditions (temperature, relative humidity and wind speed). This index is based on Byram’s flame length formulation assuming a constant idealized fuel bed. This formulation combines a moisture damping term with a term for the forward rate of spread of a fire. To include the effects of precipitation and drought in this index the moisture damping term is modified to include rainfall, evaporation and the Keetch-Byram drought index (KBDI). This new parameterization of the moisture damping term allows the FWI to more accurately reflect variations in fire danger.