The key climate influence is from the Indian Ocean, where conditions off north-western Australia favour reduced moisture flows. Such a pattern typically produces a drier than normal spring for south-eastern Australia.
The odds also suggest day-time temperatures for most of the continent will be warmer than average for the September-November stint.
Coastal showers have bumped up Sydney’s winter rainfall tallies but these have virtually dried up in July and August. Inland regions, such as the city’s catchments, have been drier still.
The last time Sydney’s dams dropped below the 50 per cent was in May 2004, during the Millennium drought of the mid-2000s.
According to a WaterNSW spokesman, the rate of the present slide in storage levels continues to exceed the pace of that dry spell more than a decade ago.
The decline continues even though Sydney’s desalination plant began producing water for the city’s users in March and reached full capacity of about 15 per cent of total demand at the start of August, a spokesman said.
“Preliminary expansion planning” has now begun on doubling the plant’s capacity of 250 million litres of water a day, he said.
Sydneysiders have responded to first-stage restrictions, with usage about 7 per cent lower since July than forecast, a Sydney Water spokesman said. Total demand is about 100 million litres per day less than a year ago.
“From August 2018 to July 2019, Sydney used 563.5 gigalitres of water compared with 602.5 gigalitres for the same period the year before,” he said.
Warmer conditions and longer days will likely see evaporation rates increase. So far this month, Sydney’s Observatory Hill has collected 3.2 millimetres of rain while evaporation was about 20 times that.
Sydney can expect to reach 25 degrees on Friday, about 7 degrees above the August norm. Inland temperatures will rise further – ahead of a front moving through early next week – with Bourke to hit 30 degrees on Sunday, Jordan Notara, a bureau forecaster said.
The best chance for rain for Sydney in the next week may be on Monday but even then it is just a 15 per cent probability, he said.
Alpine regions could fare better, with another 20-30 centimetres of snow possible early next week.
The generally dry conditions have prompted the Rural Fire Service to add another nine districts to the list of areas where the official fire season is under way. Those regions – including Byron, Bellingen and Lismore – will enter the fire restriction period from August 17.
Authorities are considering whether to bring Sydney’s official fire season start to September 1 rather than the typical October 1 commencement.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.