The operational calculation of Forest Fire Danger Index (FFDI) in the Australian Bureau of Meteorology requires the calculation of a ‘Drought Factor’ to represent fuel dryness These calculations have hitherto only been calculated at a network of observing sites, where the data to both calculate and to verify forecasts of the FFDI are available.With the advent of increasingly more accurate and higher resolution numerical weather prediction models, forecasting the spatial distribution of the FFDI is desirable. Therefore, in order to duplicate operational forecast practice, but in a spatially distributed way, the spatial distribution of soil moisture deficit over Australia, using either the Keetch-Byram Drought Index (KBDI) or Mount’s Soil Dryness Index (SDI) formulations, has been calculated at daily intervals on 0.25° (25 km) grids over Australia, and 0.1° (10 km) grids over southeastern Australia.

In addition, the ‘Drought Factors’ based on the two soil moisture deficits and recent rainfalls are also computed on the same grids. The comparison of the two soil moisture deficit schemes shows that the main difference between the two is due to the treatment of evapotranspiration. SDI generally leads to higher soil moisture deficits, which in turn leads to a higher drought factor. This is especially the case at warmer inland locations, where the SDI is significantly higher than the KBDI due to their increased evapotranspiration.The analyses described in this paper make it possible to provide a range of fire weather related products to forecasters and clients on a national scale.

Examples of such products are presented in this paper. In addition, using the National Climate Centre national daily rainfall and maximum temperature analyses since 1965, a 40-year archive of national soil moisture deficit and drought factor analyses has been generated and examples of the way this climatology can be used in operations
are presented.