The fire season for most of Australia's east coast extends from spring to mid-summer, with summer and autumn the most dangerous times of year in southeast Australia. The northern Australian fire season occurs during the warm, dry and sunny winter and spring.
The Victorian bushfires of 7 February 2009 occurred during very dangerous fire weather conditions. Exceptional heatwave conditions had developed over inland South-eastern Australia in the last week of January and continued until 8 February, peaking on 7 February.
On Saturday 7 February strong northerly winds, ahead of an approaching cooler south-westerly change, brought this hot air to southern Victoria. The combination of strong and gusty winds, low humidity and record high temperatures led to very dangerous fire conditions ahead of the change.
The most dangerous weather conditions were observed in the afternoon before the wind change. Maximum temperatures were up to 23 degrees above the February average, and for many places it was the hottest day on record, including:
The wind change passed over the coastal regions of the Western District during the early afternoon, through Melbourne at 5:00 pm, through Kilmore Gap and the Latrobe Valley around 6.30pm and reached Eildon about 8:00 pm. The temperature in Melbourne dropped at this time to around 30 °C in just fifteen minutes and some of the fires merged into larger fires, whilst others changed direction.
Wind gusts to 115 km/h were reported at Mt William and Mt Gellibrand, while gusts over 90 km/h were recorded at a number of places including Port Fairy, Aireys Inlet, Kilmore Gap, Dunns Hill and Mt Hotham. After the change wind speeds in excess of 50 km/h were observed for some hours.
Figure 1: Analysis chart showing the Mean Sea Level Pressure (MSLP) pattern at 06 UTC (5 pm EDT) on Saturday 7 February 2009
Summer and autumn are the most dangerous times of year in southeast Australia. The highest temperatures occur during these seasons and in most years the grass and forests have dried out by mid-summer.
A typical dangerous fire situation occurs in south eastern Australia when a vigorous cold front approaches a slow-moving high in the Tasman Sea, causing very hot, dry, north westerly winds. Figure 2 shows the situation associated with the Victorian Ash Wednesday fires of 16 February 1983. The passage of the cold front can cause the winds to suddenly change direction, shifting fire direction abruptly.
Fires driven by a strong, steady wind are usually long and narrow. When the wind changes with the passage of a cold front, the long side of the fire can suddenly become the fire front.
Figure 2: Historic Chart Analysis at 11am EDT on "Ash Wednesday", Wed 16th February 1983, showing areas affected by dangerous fire weather.
The fire season for most of Australia's east coast extends from spring to mid-summer. The greatest danger occurs after the dry winter/spring period, before the onset of the rainy weather common in summer. The worst conditions occur when deep low-pressure systems near Tasmania bring strong, dry, westerly winds to the coast, as occurred in the major New South Wales fires in January 1994 (Figure 3).
Figure 3: Historic Chart Analysis at 11am EDT on 7 January 1994, showing areas affected by dangerous fire weather that resulted in serious bushfires in NSW and southern Queensland
The northern Australian fire season occurs during the warm, dry and sunny winter and spring, when the grasses are dead and the fuels have dried. In summer, a strong high pressure system over South Australia can bring strong southeast to northeast winds that increase the fire danger in the southern parts of western Australia (Figure 4).
Figure 4: Historic Chart Analysis at 8am WST on 4 April 1978 showing areas by dangerous fire weather associated with the gales in the aftermath of Tropical Cyclone Alby.