2009 alpine fires
The fires that flared up in Victoria on Saturday Feb 7 2009 were not unforecast with warnings starting as early as the Wednesday 4th that it would be a bad day.
|Absolute Extreme Fire Weather spike day.... FFDI and GFDI above 50 will be reached in the morning - from
9am onwards, with ratings above 80 by early afternoon. Poor relative Humidity recovery due to warm overnight temps.
FFDI = Forest fire danger indicator GFDI = Grass fire danger indicator 40 is normally enough for a TFB. So 80 is alarmingly high.
By Friday the 6th the worst fire weather forecast ever released for the state came out.
|The Bureau of Met has issued its estimates for fire weather for
Saturday 7 Feb. They are the worst that I have ever seen. A forest fire danger index of 100 is forecast for almost the whole state. We in Gippsland fare a little better with only 57 for East Sale airport.
Using Tullamarine as an indicator for the worst case for Victoria the estimates are as follows.
Temp 44, Dew point -5, RH 7%, Wind NNW 55 kph gusting to 85 Forest fire danger index (FFDI) 100, Grass fire danger Index (GFDI)l00.
When you use McArthur meters this results in (assuming a fine fuel of 30 to/ha) in a forest rate of spread of 5.6 kph and a flame height of 77meters.
I have found (based of Hobart 1967) that McArthur underestimates in these extreme conditions and if the Drought factor is allowed to expand beyond 10 and an exponential average for wind speed + and - the gusting McArthur then works well.
I have recalculated the FFDI for Saturday using the expanded drought factor of 12 but only using the average wind speed as forecast and not the exponential average which would increase the FFDI some more.
My calculations yield
FFDI 186 Forest Rate of spread 6.7 kph Flame height 90m Spotting 20km
A westerly change of 35 kph i.e. strong but not as intense as Ash wednesday 1983 ( around 60 -70 kph) is forecast for late afternoon or into evening. That wind change of course greatly increases the area burnt as the east flank becomes the fire front for an hour or so and the fuels take 30 minutes or more to build their fuel moisture up again.
The fire intensity calculates at 100.5 megawatt per metre. The max for fire fighting is 2.5Mw/m, crown fires start at about 10Mw/m.
Assuming GFDI of 100, the grassland rate of spread is 13 kph but I would expect that it would reach the J.Noble max of 20 -25 kph.
These predictions are for the fire conditions experienced in Canberra 2003.
I doubt if the State has ever before faced such extreme conditions with fuel levels higher than ever, the prospects for Saturday are horrible.
Friday looks a little difficult and Sunday after the Saturday change is forecast to be very good although unfortunately no rain seems to be forecast.
The saving situation is that the extreme dry air does not encourage lightning and it is not mentioned on the forecasts for the next few days. Thus we are subject only to accidental and arson ignitions. Let us hope that neither of these occur although a realistic assessment would have to expect some. The high risk areas because of the terrible fuel situation are The Yarra catchment, the Otways and the remainder of the Strezlecies.
After checking on my analysis and if you deem it helpful could you let your networks know that this is the situation this fuelled up State is facing at least in my opinion.
You have no idea of how much I hope that I am wrong.
Unfortunately he was spot on.
While the fires ravaged much of the state only one alpine ski resort was actually struck, that was Lake Mountain. Baw Baw was on warning and threatened but only Lake Mountain was actually hit by the fire front.
Almost everything at Lake Mountain bar the main building was burnt down.