Why are there sometimes discrepancies between Fire Danger Rating maps and BOM Fire Weather forecasts

I've done a bit of digging around to try to sort out some discrepancies we have seen over the last few days between the Forest and Grass Fire Danger Ratings maps which are available on the Queensland Rural Fire Service web site and the Fire Danger Ratings included in Bureau of Meteorology District Forecasts.

BOM Fire Danger Ratings (which apply to quite wide areas compared to the QFRS FDR map zones) have sometimes been higher than the mapped ratings within the same area in Southeast Queensland (and apparently across wider areas of the State).

As I understand it, the QRFS FDR maps are constructed on the basis of BOM data, using a computer program to produce the ratings.  There is no human intervention in the production of these ratings, which is what you would expect because the Fire Danger Ratings are in fact what is called the McArthur Fire Danger Indices (Forest and Grassland), and these indices are calculated using a standard formula.  The purpose of the indices (Ratings) is to provide a relative measure of the difficulty of suppression for a standard fuel type.

While these indices generally work well enough in practice, the reality is that not all variables can be included.  For example the calculation assumes a fuel load arranged as a standard continuous pasture.

The BOM FDRs are apparently produced on the basis of the same data set (I have no confirmation of this), but with a degree of human intervention that takes into account actual fire behaviour as reported by fire fighters in particular districts.  This allows for adjustment based on variables which are not allowed for in the calculation of the McArthur indices, such as for example, continuity of fuel.

This produces a more "refined" FDR, but not currently at the detailed scale that is available in the Fire Danger Rating maps.

So, I guess the "take home message" is that you should check the BOM website - in our case their Southeast Coast Forecasts, via the Updated weather Ipswich & Fire Danger link in the right hand column of this web page, and always ask FireComm whether it is safe to burn when you notify them of your intention to light up a permitted burn.

You can find more information on the McArthur fire danger indices on the web.  For a fairly simple account you can read this presentation from the Bushfire CRC, or for more detail this paper from the Centre for Australian Weather and Climate Research which compares the McArthur Forest Fire Danger Index approach with the Canadian Forest Fire Weather Index.

If you have some "inside knowledge" of how the system works and can improve on my attempt at explanation please feel free to contact me so I can share the information with readers of this blog.