Australian rainfall deciles for January to August 2019. Image Credit: BOM
The Bureau Of Meteorology has detailed the state of the climate, which led to one of our most severe starts to the bushfire season yet.
From 6 September, areas of northeast New South Wales and southeast Queensland had warmer than average daytime temperatures, very low humidity, and gusty winds leading to dangerous fire weather conditions according to the BOM'S Special Climate Statement.
The fire risk was exacerbated by below average rainfall on a range of timescales, leading to a prolonged and severe drought and very high dryness factors for fuels:
- January to August rainfall totals 50 per cent below average for many locations.
- Driest January to August on record, since at least 1900, for some locations.
- Lowest rainfall on record for the 20 months starting January 2018, and 32 months starting January 2017 for many areas on and west of the ranges in northern New South Wales and southern Queensland.
- Maximum temperatures on 5 and 6 September were more than 10 °C above average in some areas.
In addition to large year-to-date rainfall deficits in 2019, below average rainfall totals in 2017 and 2018 saw large multi-year rainfall deficits accumulate in areas on and west of the ranges in northern New South Wales and southern Queensland.
Rainfall over much of this area had been lowest on record for the 20 months starting January 2018, and the 32 months starting January 2017.
It is likely that we will have to endure a longer drawn out fire season in parts of Australia, with the fire season typically starting earlier in the year in southern Queensland, inland and southern New South Wales, and Victoria.
Low rainfall Fire weather reflects a combination of factors including longer-term rainfall and temperature patterns, and shorter term weather on a given day, including temperature, humidity and wind.
The Fire risk is driven by a number of factors including fire weather and fuel availability. Rainfall influences the dryness of fuels and is a key factor in calculating indices for determining fire weather risk such as the Forest Fire Danger Index (FFDI).
Rainfall prior to September 2019 had been below average on a range of timescales from months to years, leading to a prolonged and severe meteorological and hydrological drought in southern Queensland and northern New South Wales.
Figures 2, 3, and 4 show the rainfall deciles across eastern Australia for January to August, winter, and August 2019 respectively, with large areas of very much below average rainfall at each timescale.
Rainfall for January to August 2019 was lowest on record in the Southern Downs (Queensland) and Northern Tablelands (New South Wales).
At Tewantin on the Sunshine Coast, the year-to-date rainfall total was around 35–40% below average and ninthlowest on record. Most of that deficit was in January and February.
March to June had near-average rainfall, but July and August were about 60% below average.
Tewantin had only received 6.4 mm in the 30 days before the start of the fires.
Tragically, earlier this month we saw a home claimed at Peregian on the Sunshine Coast. A couple of children were accused of deliberately lighting that fire which scorched almost 1,000 hectares of land.
On September 5, one home was lost in northern Queensland near Mareeba in the Atherton Tablelands.
By September 7, more than 50 fires were burning across Queensland and this had increased to 80 by 9 September.
By 11 September, there were 70 fires still burning across Queensland, and Queensland Fire and Emergency Services (QFES) confirmed there had been 17 homes lost across the region.
Bushfires had blackened more than 33,000 hectares statewide, including 10,600 hectares at Hawkwood.
• Late on the 9th, a fast-moving fire at Peregian Beach in the Sunshine Coast was travelling in an eastnortheast direction from Emu Mountain Road towards Pitta Street and Lorikeet Drive, and was heading towards Coolum State High School on Havan Road East.
Peregian Beach residents were asked to evacuate either south towards Coolum or north towards Noosa. By 8pm, the fire was still travelling eastnortheast towards Peregian Beach and Marcus Beach. Overnight on the 9th, at least one home was destroyed at Peregian Beach.
A southerly wind change around midday on the 10th caused the fire to spread northwards, but the moist sea-breeze assisted with retarding the movement of the fire. By 11
September, the fire was under control, and residents returned to their homes.
Dry soil was also detected over much of southeast Queensland and northeast New South Wales, with dust clouds affecting many regions.
The low soil moisture reflected both the recent intense dry conditions, as well as longer-term below average rainfall stretching back to the start of 2017.
Low soil moisture means high stress in the vegetation and will mean fuels that might not normally burn become available to bushfires.
The low rainfall and dry soils meant runoff had also been below average to locally lowest on record across most of southeast Queensland and northeast New South Wales (see Figure 6).
This highlighted that the long meteorological drought (lack of rainfall) was also reflected in an intense and long-lived hydrological drought (depleted surface and groundwater storages).
These factors set in place the preconditions for dangerous fire weather.
Special Climate Statement 71—severe fire weather southeast Queensland and northeast New South Wales in September 2019 11
Figure 6: Runoff deciles for 8 September 2019 from the Australian Water Resources Assessment Landscape model (AWRA-L) 6.0 (based on all years since 1911).
The drought factor3 , which is an input into the calculation of the FFDI, was above 9 at sites throughout northern and central Australia, including almost all sites in southeast Queensland and northeast New South Wales.
This means the dryness of fuels was at or near the highest value used for the FFDI calculation.
On 4 and 5 September, a ridge of high pressure over eastern Australia combined with a dry, stable air mass resulted in sunny conditions throughout New South Wales and much of Queensland, with warmer than average maximum temperatures.
The high pressure system weakened and drifted northwards over Queensland and by the 6th, extended a ridge along the Queensland coast. On the 6th, a cold front with a pre-frontal trough approached from the west, bringing strong northwesterly winds ahead of the change (Figure 8).
The strong winds generated areas of raised dust as well as elevated fire dangers through southeast Queensland, northeast New South Wales, and parts of northern inland New South Wales.
This early in the season, the high daytime temperatures in southeast Queensland and northeast New South Wales on 5 and 6 September 2019 were generally second only to the exceptional winter heat of 23–24 August 2009, meaning this would have been a record-breaking early season heatwave were it not for 2009.
Some locations surpassed the observed temperatures from August 2009, particularly around Brisbane and further north along the Queensland coast, and in the Central Western Slopes and Plains of New South Wales where early season heat records were set.
Figures 9 and 10 show the daily maximum temperature anomalies for 5 and 6 September respectively, with large areas of northern New South Wales and southern Queensland having daytime temperatures more than 8 °C warmer than average (based on the 1961–1990 average).
The high fire dangers during this event followed on from very much above average accumulated FFDI values during winter 2019 over Australia.
The only areas of the country that had average to below average accumulated FFDI values for winter (FFDI values summed across the season) were those areas of southern and western Victoria, western Tasmania, and northern inland Queensland that had average to the above average rainfall during the season (see Figure 15).
Special Climate Statement 71—severe fire weather southeast Queensland and northeast New South Wales in September 2019 20
Figure 15: Accumulated FFDI deciles for winter (June to August) 2019, based on all years since from 1950.
In August 2019, the Bushfire and Natural Hazards Cooperative Research Centre (BNHCRC) issued the Australia Seasonal Bushfire Outlook: August 2019, reporting that northeast New South Wales and southeast Queensland were among those areas of Australia that faced above normal fire potential for the 2019–20 fire season.
From 6 September, high temperatures, low humidity, and strong winds, coupled with the dry conditions, led to elevated fire danger across southeast Queensland and northeast New South Wales.
The measured FFDI values were in the extreme category (over 75) across large areas, reaching the catastrophic category (FFDI values of 100 or above) at some locations in New South Wales.
Figure 17: Highest daily FFDI values on 6 September 2019 compared with the previous highest September daily FFDI (1950–2018). Shaded areas indicate places where it was the highest FFDI on record for September.
Special Climate Statement 71—severe fire weather southeast Queensland and northeast New South Wales in September 2019
In most districts of northeast New South Wales, 6 September 2019 had the highest regionally averaged daily FFDI in September, based on all years since 1950 (see Figure 18). East of the ranges though, the daily averaged FFDI for the North Coast was the third-highest in September, behind 27 September 1994 and 14 September 2017.
For each of the first eight months of 2019, the highest regionally averaged daily FFDI was above average in all regions.