SYNOPTIC weather patterns in the Australian region have changed significantly in the past one to two weeks.
No longer are the westerlies dominant to the south and with trough lines developing in the easterlies over Queensland, some moisture from the tropics is starting to be dragged into the set up.
Although the subtropical high pressure ridge will persist in the south of the country, reducing rainfall potential in the immediate future (one to two weeks), at least the signs are to fall into place, confirming the possibility of a weak La Nina developing towards the end of the year.
The sea surface temperatures in the tropical Pacific are heading in the right direction.
In the eastern tropical Pacific they are between 0.4 and 0.6 degrees below normal – a persistent reading about 0.8 degrees below normal is needed for a La Nina event so we are not quite there yet.
Something similar applies to the Southern Oscillation Index (SOI). In the past one or two weeks, the 30-day running mean has been about +4 to +6.
Persistent values above +7 indicate a La Nina so again we are not quite there yet but heading in the right direction.
In addition the south-east trades in the tropical western Pacific remain a little up on normal while cloudiness around the International Date Line in the central tropical Pacific has been below normal in September and it is generally below normal in a La Nina event.
So all features indicate the potential for a La Nina event to develop.
However, when and if it does develop, it will be a little later than normal.
In the past, late developing La Nina events have had results as far as late spring and summer rainfall as far as NSW has been concerned.
Variability has often been a feature but in more seasons than not, rainfall has been at least average over the north-east two-thirds of the state.