In its most recent forecast, the bureau noted that all major international climate models were predicting La Nina thresholds would be met in October, with the event likely to last into early 2021.
A bureau spokesman declined to confirm it will declare a La Nina on Tuesday but other specialists are confident such an event is highly likely.
“It’s quite a distinct La Nina pattern [in the Pacific],” Ben Domensino, a senior forecaster at Weatherzone, said. “It’s quite likely that the bureau will follow suit [and match NOAA’s declaration].”
Agus Santoso, a climate scientist at CSIRO and the University of NSW studying La Ninas and their opposite, El Ninos, said a La Nina would most likely take hold for the rest of the year and into next although it remains unclear how strong the event will be.
La Ninas typically follow El Nino years. The latter are marked by drier than usual conditions for Australia, with more heatwaves and extreme bushfire weather – as happened last year.
Also unlike this time last year, conditions in the Indian Ocean off north-western Australia are favouring above-average moisture flows across the continent.
The so-called Indian Ocean Dipole, which tracks the relative warmth of sea-surface temperatures between the western and eastern Indian Ocean, is close to its negative phase. During such phases, the eastern two-thirds of Australia typically receive above-average rain during spring.
The climate influences from the Indian and Pacific oceans contributed to the recent heavy rains across inland Australia, with more to come, Mr Domensino said.
“It’s very unlikely we’ll see anything like last year’s bushfires,” he said.
Still, fires will still be a threat in places, particularly where there is strong grass growth that dries off during warm spells.
Not all places will record higher than usual rainfall either, Mr Domensino said.
Sydney, for instance, has had only about a third of its usual September rainfall, although it only takes one big burst to shift the totals.
Dr Santoso said current projections indicate the more extreme versions of El Ninos and La Ninas will become more frequent as the world continues to warm from climate change.
Even with 2020 likely to end with a La Nina event it will still come close to being the hottest year on record, he said.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.