The Bureau of Meteorology’s ENSO Outlook has confirmed we are now in a LA NIÑA phase.
Climate models suggest this La Niña will be weak and short-lived, persisting until early southern autumn 2018.
Signs of La Niña in the equatorial Pacific have increased during spring.
The central to eastern tropical Pacific Ocean has cooled steadily since late winter, and is now at La Niña thresholds (0.8 °C below average).
Atmospheric indicators, including the Southern Oscillation Index (SOI), trade winds and cloud patterns, also show clear La Niña signals.
In order for 2017–18 to be classed as a La Niña year, the event needs to last for at least three months.
Climate models suggest that while this event is likely to persist over the southern summer, it will be weaker than the strong La Nina of 2010–12.
La Niña typically brings above-average rainfall to eastern Australia during late spring and summer.
However, sea surface temperature patterns in the Indian Ocean and closer to Australia are not typical of a La Niña event, reducing the likelihood of widespread above-average summer rainfall.
La Niña can also increase the chance of prolonged warm spells for southeast Australia.
The Indian Ocean Dipole (IOD) is currently neutral. IOD events are typically unable to form between December and April.
The BOM has reported that, as of Tuesday, a surface trough was sitting over far western Queensland and was expected to shift further west into the Northern Territory later in the day.
Storms are expected over western parts of the state near this feature, particularly about the Gulf Country.
An upper trough lies near the Wide Bay and central coasts, generating some instability and isolated storms.
This feature and the associated instability is expected to shift further north towards the northeast tropical coast during the day before moving more into the Coral Sea during Wednesday and Thursday.
A high pressure system over southeastern Australia will maintain a firm ridge and a general fresh southeasterly wind flow about the east Queensland coast as it moves slowly east into the Tasman Sea during Tuesday and Wednesday.
The ridge will then gradually relax from late Wednesday as the high shifts further east over the Tasman Sea.
Heatwave breaking cool start to summer
Summer started out on a cool note for much of southern Australia, although a countrywide heat wave has sent temperatures soaring this week.
Many areas in southern and southeastern Australia experienced a notably mild start to summer this year but the first heatwave of summer has swept across southern and eastern Australia this week.
A pool of hot air sitting over Western Australia started spreading over South Australia on Sunday and has continued to extend further east throughout the week.
Severe heatwave conditions were forecast to affect parts of NSW, northeastern SA and south west Queensland from Tuesday onwards, where some outback will experience oppressive heat.
Thargomindah is forecast to average at least 35 degrees for three consecutive days and nights from Thursday.
A cool change will reach SA on Wednesday and parts of southeastern Australia by Thursday, although it won't penetrate into central and northern NSW and Queensland until early next week.
- additional reporting, Ben Domensino, Weatherzone