Fire Danger Forecast this Thursday

Article from David Packham

The fire weather estimates for Thursday 19 Nov 2009 are looking a little concerning.

The Forest Fire Danger Index (FFDI) for the North West of the State is in the 60 – 90 range, the Grass Fire Danger Index (GFDI) is in the 40 – 60 range.
Both of these indices are such as to cause serious fire behaviour.
The predicted rate of spread would be around 3 kph in the forests with heavy fuels. The predicted flame heights are around 40 metres with spotting up to 3km. The Byram Fire Intensity for the forests about 50 Megawatts per metre some twenty times greater than firefighting capability (2.5 Megawatts per metre being the limit to direct attack).

The very dry areas around Bendigo and Shepparton are somewhat similar but the grass greenness is keeping the GFDI at a manageable level, in other words good firefighting in the grasslands should be successful but not so in the heavy forests and I might add along the neglected roadsides.

The central areas, for example Tullamarine, look serious for forests with FFDI just reaching the 50 mark, the GFDI with an estimated curing of around 50% ( half the grass stalks are still green) is sufficiently low to be initially manageable with good fire fighting and very early response. I would not be surprised if the curing did not rise rapidly during the day as happened during the Lara Fires in 1969.

The West, Northeast and Gippsland both look difficult except for the grasslands in the West where the grass is still apparently all green .

The serious concern for me is the synoptic situation which like Ash Wednesday 1983 is proving difficult for the forecasters. They have gone for a “more pessimistic forecast” thus the analysis above could be worse than what actually occurs.

The problem is our old enemy the Southwest wind change. If it is a prolonged and nasty one like Ash Wednesday the results could be very difficult. The problem being the response times of the fuels which remain dry just after the wind change even though the FFDI and the GFDI show them becoming much more moist. That is because we are using the models beyond their capability.

I would suggest that we watch carefully the developments in the systems over the next few days whilst preparing for a possible big fire event on Thursday (due to the heavy grass and forest fuels and the ignored roadsides). On Thursday the major threat would come in the late afternoon and the timing of the wind change will be critical.

This early in the season normally I would discount any real fire problems but the dryness continues and we could be in for an unpleasant surprise.

With a little luck the forecasts may change as the weather systems develop and the day may pass us with a whimper rather than a shout.

Once again I hope the above analysis turns out to be wrong.

David Packham