Return of the 'big dry' points to active and early fire season across the south

Flammagenitus forms from bushfire smoke. The type of cloud was formerly known as a pyro-cumulus. Photo: Alex Ellinghausen

Flammagenitus forms from bushfire smoke. The type of cloud was formerly known as a pyro-cumulus. Photo: Alex Ellinghausen

Weather patterns delivering an exceptionally dry winter across south-eastern Australia show little sign of breaking up, heightening concerns the coming fire season will be an early and active one.

Fire authorities won't formally release predictions for spring and summer until September. Wet periods in autumn, though, curbed hazard-reduction burning to half the year-earlier totals in NSW and curtailed it in Victoria.

"The next four to six weeks are going to be very important," said Ben Shepherd, an inspector and head of media at the NSW Rural Fire Service.

"Many of the agencies are looking for that opportunity to get out there and start those [hazard-reduction] burns again in earnest" before conditions turn too windy and warm, he said.

Authorities stress a significant rain event could yet push fire risks for the coming season back towards more typical levels but note that there is little sign of such a development.

High-pressure systems have dominated southern Australia, nudging rain-bearing systems south of the continent.

Weather patterns delivering an exceptionally dry winter across south-eastern Australia show little sign of breaking up, heightening concerns the coming fire season will be an early and active one.

Fire authorities won't formally release predictions for spring and summer until September. Wet periods in autumn, though, curbed hazard-reduction burning to half the year-earlier totals in NSW and curtailed it in Victoria.

"The next four to six weeks are going to be very important," said Ben Shepherd, an inspector and head of media at the NSW Rural Fire Service.

"Many of the agencies are looking for that opportunity to get out there and start those [hazard-reduction] burns again in earnest" before conditions turn too windy and warm, he said.

Authorities stress a significant rain event could yet push fire risks for the coming season back towards more typical levels but note that there is little sign of such a development.

High-pressure systems have dominated southern Australia, nudging rain-bearing systems south of the continent.

Saturday offered a clear illustration of the set-up. The bureau predicted almost the entire mainland of Australia would be dry. (See chart below.)

The skies have been clear with an unusually wide diurnal temperature range over many regions. Nights were marked by heavy frosts or cold making way for milder than average days.

Big dry

Barring some unexpected instability, July will join last month as among Australia's driest.

Nationally, last month was the second-driest June since standard data began in 1910 with rainfall barely a third of average, the Bureau of Meteorology said. 

Victoria had its driest June, NSW was the driest in 15 years and the Murray Darling Basin posted its lowest rainfall totals since 1986.

"It very much seems to be centring on the border between Victoria and NSW," Richard Thornton, chief executive of the Bushfire and Natural Hazards Cooperative Research Centre, said of the rainfall deficits. 

Winter-to-date rainfall levels have been "very much below average" for most of southern Australia, bureau data shows. (See chart below).

'Back foot'

Fire authorities are keen to take advantage of the cool and dry conditions to ramp up controlled burns before windy and warmer days make such activities more risky.

Cyclone Debbie's aftermath brought heavy rain to much of the east coast in April, reducing opportunities for hazard-reduction burning. "That really put us on the back foot," Mr Shepherd said.

In a typical year about 56 per cent of controlled burns in NSW are conducted in autumn and 23 per cent in spring. 

During autumn, the RFS treated 93,116 hectares to protect 23,171 properties. By area, the tally was less than half the previous autumn's 191,684 hectares involving 27,867 properties.

Forest Fire Management Victoria places less emphasis on the size of area burnt in its hazard-reduction program than the risk. During the year to June 2017, FFMVic crews conducted 351 burns over 113,172 hectares statewide.

"The wet winter and spring conditions proved difficult, however our prioritisation of burns based on risk has resulted in the statewide risk being maintained at 2015-16 levels," said Stephanie Rotarangi, the agency's chief fire officer.

Longer-term trends

The key influence behind this year's relatively dry winter is far to the south. A positive phase in the so-called Southern Annular Mode marks a poleward contraction of the westerly winds belt towards Antarctica.

As the bureau notes, such a phase marked the Millennium drought during the "big dry" from 1997-2010. High-pressure systems that would normally sit further north track across more southerly parts of Australia blocking rain-bearing cold fronts. 

In June, average mean sea level pressure readings were at record highs for many parts of southern Australia, including all but one site in South Australia and all but three in Victoria.

As the bureau noted: "Mean sea level pressure over southern Australia has been increasing in winter over recent decades. This pattern is consistent with future climate change projections for the Australian region."

Dr Thornton of the Hazards CRC said that along with trends towards drier winters for southern Australia, warmer-than-average temperatures during July to September are also the new norm.

"It's almost locked in to have above-average temperatures from climate change," he said.

The bureau will update its three-month outlook this coming week. At its most recent projection, the agency said odds were strongly pointing to a drier-than-usual July to September for much of the south of the country. (See chart below.)

The story ‘Return of the 'big dry' points to active and early fire season’ first appeared on the Sydney Morning Herald.

Smartphone
Tablet - Narrow
Tablet - Wide
Desktop