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Chance doubles for rain-causing La Nina

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Australia has double the normal chance of a La Nina weather pattern forming this year, increasing the risk of heavy rainfall, the Bureau of Meteorology says in its latest update.

The bureau has raised its El Nino-Southern Oscillation status to “watch” following changes in sea surface temperatures and says there is now around a 50% chance of an event occurring, twice the normal likelihood.

The most recent La Nina to have a significant impact in Australia lasted from 2010-2012, covering the period when the Brisbane floods and torrential rainfall in nearby regions generated insurance claims topping $1.5 billion.

Manager of long-range forecasting Andrew Watkins says tropical Pacific Ocean sea surface temperatures have cooled in recent weeks and models suggest this will continue through winter and into spring.

“Other indicators of La Nina, such as stronger trade winds and lower air pressure over Australia compared with the central Pacific, have yet to appear,” he said.

“Once we start to see a change in these weather patterns, the likelihood of a La Nina event will increase much more. And it also increases the chance of a wetter second half of the year for Australia.”

Typically, the event brings above-average winter-spring rainfall, particularly across eastern, central and northern regions, cooler days, more tropical cyclones, more snow and an earlier onset of the tropical wet season.

Contributors to Australian weather patterns also include temperatures in the eastern Indian Ocean, where warmer conditions have favoured more cloud and rainfall across the country.

Dr Watkins says some warmth remains in the eastern Indian Ocean, despite recent influences such as Tropical Cyclone Mangga.

“For the July to September outlook, it is looking wetter than normal over much of southern Australia, with the exception being down in Tasmania,” he said in a separate monthly update.