An unusual combination of climate conditions is confounding climate experts, with the possibility that Australia could post a warmer and drier than average summer even with a La Nina event being declared in the Pacific.
The Bureau of Meteorology on Thursday released its summer outlook, indicating the odds have shifted back from favouring cooler and wetter than average weather in the coming three months.
Summer daytime and overnight temperatures are now leaning towards a warmer than average December-February for eastern Australia, with the odds strongest for the Top End and Tasmania.
Rainfall forecasting is less clear, with no strong tendency except for a patch of relatively dry conditions expected for north-eastern NSW and parts of northern Australia, the bureau said.
"It's not an easy year to be a climatologist," said Andrew Watkins, the bureau's manager of climate prediction services. "We've got some competing climate drivers."
In the Pacific, a "classic but weak" La Nina appears likely to form, Dr Watkins said, a pattern that would typically produce cloudier than normal conditions - with more rain and cooler than usual temperatures - for eastern Australia.
But the Indian Ocean is not playing its usual role during a La Nina, and instead is favouring relatively sunny and dry weather for much of the continent.
Drying off
After a wet and mild spell in October, eastern Australia has lately turned dry again, with only a fifth of the usual November rainfall up to the half-way point of the month, Dr Watkins said.
Southern Australia has had some record-breaking heat, including the most November days in a row at Perth Airport above 30 degrees.
Adelaide, meanwhile, has had six consecutive days above 30 degrees, a feat observed in only 10 Novembers in the past century or more of records.
Melbourne, too, has had six days of 30-plus days before the official start of summer, compared with an average of two such days, Dr Watkins said.
Sydney and Brisbane have lately bucked the trend, with slightly cooler than normal conditions that may run for another week or more.
"There's been a big high pressure system drawing air in off the ocean, keeping it a bit cooler for Sydney," Dr Watkins said.
Tilting warmer
For the December-February period, though, most of Australia can expect warmer than average daytime temperatures, the bureau says.
(See bureau chart below showing the chance of warmer than usual maximum temperatures.)
La Ninas typically mean easterly trade winds blowing along the equatorial Pacific strengthen, fostering more rain than usual for eastern Australia. More cyclones than normal also develop in the Australian region.
But if a La Nina is declared - as it may be within weeks - it is likely to be only a weak and short-lived one, Dr Watkins said.
Rainfall view
According to the rainfall outlook for summer, the odds point to lower-than-average rainfall across eastern Australia, in contrast to what a La Nina would typically deliver.
(See bureau chart below, showing the chance of exceeding median rainfall for the December-February period.)
The expectations for summer contrast with the recent conditions, at least for temperatures, for east coast cities such as Sydney.
Halfway through November, Sydney's average maximum temperature is running a degree lower than average for the whole month.
The blocking high pressure in the Tasman has steered easterly winds over the city, keeping the build-up of inland heat from reaching coastal suburbs, Rob Sharpe, a meteorologist with Weatherzone said.
With the absence of any strong frontal system likely to move through - drawing in warm winds ahead of it - Sydney's recent run of mild conditions could last for another week or more, he said.
Storm chance
One benefit of the cooler conditions and more rainfall has been an easing of the bushfire threat, at least for the Sydney area.
"We've pressed pause on the bushfire risk," Mr Sharpe said.
Sydney's main chance for less stable conditions could come in the form of thunderstorms, with a chance of them developing over the western suburbs on Thursday afternoon.
Storms, though, are more likely on Friday and may be a widespread, with shower activity also likely, Mr Sharpe said.
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