An arc of eastern Australia stretching from western Victoria to central Queensland faces a more active bushfire season following another unusually mild and dry winter.
The seasonal bushfire outlook for southern Australia, released on Tuesday by the Bushfire & Natural Hazards CRC, highlighted the potential for the fire season to get underway earlier than usual in NSW, Queensland and parts of Victoria.
Hobart, Melbourne, Canberra, Sydney, and Brisbane were all located in the regions with above-average fire potential for this year. Above average fire potential was also predicted for the Illawarra and South Coast.
"[T]he dry conditions now mean that vegetation is already dry with very low greenness evident in satellite data," the report said. "The dry state of vegetation means that warm, windy conditions are likely to see more elevated fire risk than is normal for the time of year."
Rob Rogers, NSW Rural Fire Service deputy commissioner, said the state had already had 2320 fires so far this season amid "extremely dry conditions". The service had already begun rotating staff to help give active areas a breather.
"What we're seeing is fires further down [south in] the state than normal," Mr Rogers said. "I can't recall over the past 10-15 years it being as dry as it is going into spring."
NSW had about 100 fires burning on Tuesday including about 40 that were uncontained, RFS said.
Craig Lapsley, Victoria's Emergency Management Commissioner, told a media briefing that the highest risk area in his state coincided with key population centres: "it's in the outer metropolitan area, it extends through central Victoria, [and into] eastern Victoria".
Even though Victoria's winter was closer to average rainfall and temperatures than some other states, "the rain isn't significant to enough to see the forecast change from what will be a serious fire season", he said.
The CRC's annual report adds to evidence that fuel moisture levels are low for this time of year, including around the Sydney region.
While the Bureau of Meteorology predicts a swing towards more average rainfall levels in spring, that shift may not occur until well into the season. Temperatures are likely to remain above average, continuing a pattern that saw Australia just post its warmest ever winter for daytime conditions by some margin.
The relatively poor recent rains over the Murray-Darling Basin - winter rainfall was about half the norm, and the lowest in 15 years - mean grass growth is likely to be stunted. Instead, this year's above-average fire activity appears centred on forests along the Great Dividing Range. (see chart below.)
The Southern Australia Seasonal Bushfire Outlook 2017 is compiled at an annual workshop run by the bushfires CRC and the Australasian Fire and Emegency Service Authorities Council (AFAC), and is used by fire authorities to plan for the season ahead.
For NSW, which had its third-warmest winter for average maximums and roughly half the normal rain, ongoing dry and mild conditions are resulting in fuel loads drying out.
"In the case of forests, this dries the fine fuels, and in the case of grasslands, frosts cause grass to cure early," the report said. "Both maximum and minimum temperature outlooks through to November indicate conditions are likely to be much warmer than average."
In the latest season outlook, the bureau forecast much of the state's east would have odds favouring average to better-than-normal rainfall for spring. (See chart below). The shift, though, would likely come after September.
Of NSW's 80 million hectares, about 20-30 million were bush-fire prone, Shane Fitzsimmons, RFS commissioner, said. Those 20-30 million hectares had "very much above normal potential" for fires this year, and overlapped with large population centres, he said.
The dry conditions also meant authorities would soon be reaching "threshold points" when prescribed burning efforts would have to be reduced or deferred because they would be "too dangerous or too risky", Mr Fitzsimmons said.
Victoria, which had better August rains than NSW, is also expected to have an early and above-normal season for bushfires.
"Severe rainfall deficits persist along the Great Dividing Range in Victoria," the report said. "Forests in these areas may experience sudden changes in fire activity with the onset of warmer or windier weather."
Bursts of heat, combined with forecast average rainfall and temperatures to the end of October, "may be enough to move drought indices in some tall forests up to 100 by the commencement of summer", it said. "This is a critical threshold for these forests, allowing them to support fire growth."
The report highlighted the long-term warming and drying climate across southern Australia, in particular, which was increasing the likelihood of more active and early bushfire seasons.
For instance, south-west Western Australia just posted its 11th consecutive below-average cool season for rainfall. For Victoria, that run now counts 17 of the past 20 cool seasons as having sub-par rain.
"Very long-term deficiencies like these are not matched in the historical record, and have been associated with a marked increase in fire weather severity in the past decade," it said, adding "climate change now means that Australian temperatures are usually above average" for any time of the year.
David Jones, head of climate monitoring at the bureau, said south-west WA had been enduring relatively dry winters for the past 40 years and a similar trend was evident in the continent's south-east for two decades.
The warmer temperatures meant the fire season was starting earlier and was becoming more intense, Dr Jones said.
"Certainly climate change is a key factor here," he said. "If you look at Victoria, for example, we've seen about a 50 per cent increase in the fire-season severity."
The near-term outlook suggests NSW in particular will continue to be dry, especially along the coast, although Victoria should get some more falls over its ranges.
(See bureau chart below of forecast rain for the next eight days.)