Late season La Nina begins

Although the sea surface temperatures (SSTs) in the eastern tropical Pacific have cooled to almost the La Nina thresholds, the Southern Oscillation Index (SOI) is above the thresholds and has been so for almost a month now. 

SSTs in the western Pacific and over the Coral and Tasman Seas remain above average while the south east trade winds in the tropical western Pacific are a little stronger than normal for this time of year – all these features indicating that a La Nina is upon us. 

In recent months, however, the Indian Ocean has cooled a little and although its influence is lower in eastern Australia from December to April, nevertheless, this would indicate that widespread above average rainfall that can often be associated with a La Nina is less likely this year. 

Also, the fact that the La Nina is developing late in the year and is probably going to be weak, would support this scenario. 

During October and early November, however, the weather patterns have largely reflected what I have forecast – rainfall events have occurred and given good rains in many parts of NSW and Queensland and the prevalence of severe storms has been greater than normal. 

There is no reason to believe such patterns won’t continue into at least the early part of summer. 

Temperatures have been relatively cool due to local synoptic features. 

High pressure has been more dominant in the Great Australian Bight than in the Tasman resulting in a generally southern air flow in eastern Australia. 

This will vary as the season progresses and therefore it is expected that temperatures will again be above average for summer although not as hot as the last two summers. 

The hottest weather in western and southern NSW, Victoria and South Australia is likely after midseason.