The chances of a wetter-than-average summer for eastern Australia are increasing, with the Bureau of Meteorology set to declare a La Nina weather pattern is likely to take hold in the Pacific.
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The bureau's shift from neutral conditions to a "La Nina watch" will be formally stated in Tuesday's fortnightly update, Rob Webb, the head of the bureau's national forecast services, told Senate estimates on Monday.
Andrew Watkins, manager of climate prediction services at BOM, told Fairfax Media the declaration of a "watch" indicated at least a 50 per cent chance of a La Nina, although shy of the 70 per cent level that would prompt the declaration of an event as virtually certain.
"We're not quite up to the 70 per cent yet," Dr Watkins said.
Even so, the shift towards conditions favouring wetter conditions has become more evident in recent weeks.
During La Nina years, the easterly trade winds across the equatorial Pacific strengthen, shifting rainfall westwards to places such as northern and eastern Australia, and Indonesia.
Lately, unusual cooling of waters off South America - a signal for the La Nina's formation - had become clearer. That pattern typically shifts Australia's outlook "to the wet side of our cycle", Mr Webb said.
In the most recent assessment of conditions in the Pacific, the bureau also noted the areas of unusually warm waters east of Papua New Guinea. (See map below).
Dr Watkins said La Ninas are usually evident from autumn or winter, unlike this year: "It's quite unusual to be so late in the year."
While La Ninas can bring heavy rain and floods to eastern Australia, the "seasonal outlooks are not as roaring wet as they were in 2010", Dr Watkins said. The back-to-back La Ninas of 2010-11 and 2011-12 included major flooding in Queensland, NSW and Victoria.
Indeed, Indian Ocean conditions, another driver of Australia's climate, "have, if anything, been unfavourable for rainfall" over Australia, he said.
The bureau's nod to a likely La Nina follows similar calls by other international agencies, such by the US National Oceanic and Atmospheric Administration.
NOAA earlier this month rated the chances of a La Nina as a 55-65 per cent chance during the southern spring or summer.
Fire authorities are hoping a tilt towards a La Nina will bring extra rain to take the edge off a potentially severe bushfire season.
Mr Webb told Senate estimates, eastern Australia had had "an incredibly dry winter...[with] deep drying of the soils".
"Any rain that falls will drain away, and as those temperatures heat up, we are concerned [about] a busy fire season over most parts," he said.
While Sydney just had its best rainfall in about four months, the best chance for follow-up falls will be 3-10 millimetres on Thursday, the bureau said.
The mercury, though, will start to climb, with Sunday and Monday likely to see Sydney reaching 32 degrees, or 10 degrees above the October average.
La Ninas, while bringing above-average rain to northern parts of the country, can also trigger more cyclones in Australia's region. For now, though, the bureau's forecast is for a typical season.
One other impact of La Nina years, though, tends to be a moderation of global surface temperatures as the Pacific takes up more of the atmospheric heat.
So far this year global temperatures are running at about the second equal-warmest on record for the January-September period - marginally ahead of 2015 and behind only 2016. (See NOAA chart below.)
According to NOAA, this year will almost certainly be the third warmest on record behind only the two previous years, based on data that goes back to the 1880s.
