CLIMATE - SEPTEMBER 2021

HEADLINES


  • Rainfall chances are generally more biased on the wetter side than normal for many areas of the nation heading into September.

  • Temperatures are expected be mixed over the nation, with a near normal season for much of the south, though the month likely warmer than normal over much of the north of the nation.

The pattern is largely unchanged from the last few data runs for September. We are looking at the rainfall increasing again, wetter as we go for the east and north as the moisture increases with the early build up.


But the one element that I am concerned about is the Indian Ocean Dipole. It is not performing as it should be at this time, and therefore I am pulling back my forecast rainfall and temperature outlooks for the month and indeed spring to reflect this.


HOWEVER - it does not mean a drought is developing or many areas will see months of below average rainfall, it is more likely to normal spring time for many locations, which mitigates the flood risk. From where the modelling was to where it is now, suggests that the influence of the IOD is less than where it was earlier in winter.


CLIMATE DRIVERS


INDIAN OCEAN - NEGATIVE

The Indian Ocean remains negative however recent cooling of the IOD region, has now seen the strength of the event in my opinion decrease down the line. This will see a reduced impact from the IOD through spring time, but this does not mean that some locations will not see heavier rainfall and increased cloudiness through September. Based off current observations, the modelling from Autumn and Winter suggesting a strong event, is now less likely but not impossible. Some agencies are going for a very wet spring based off this, but again, looking at the lack of influence in the last month supports that perhaps we need to review the influence of the IOD moving forward.

SOUTHERN ANNULAR MODE - SAM - NEUTRAL/POSITIVE

Higher chance of the SAM staying in a neutral trending positive phase through the coming fortnight, this driver is playing havoc with the rainfall and temperature outlooks in the short, medium and longer term, as what many are experiencing watching the modelling from day to day. We are seeing that positive trend appearing in modelling so in the short to medium term, expect the bias of rainfall to be over the eastern half of the nation for the coming 2 weeks and lighter falls over southern Australia.

ENSO - NEUTRAL

The ENSO remains neutral for the coming month - though some agencies are suggesting a La Nina will be present through the coming Summer with others agencies keeping the signal in the neutral zone for the period which I think it is right. Though from now through to the end of the month, this climate driver will be continuing to evolve - and all climate agencies will have an outlook by the end of the month into October.

Rainfall Outlook - September 2021

% chance of exceeding the median rainfall totals for September 2021

Temperature Outlook - September 2021

% chance of exceeding the median temperature values for September 2021

Here is the climate update video for September 2021 for a breakdown around the forecast.

And for those wanting to see the change in the forecast I made previously, you can see the charts below, and note the bias shifting further east and relaxing a little bit in the wet bias across the nation, with a lesser impact from the IOD this month. This forecast remains a low confidence forecast. These were the forecasts I made back in early August.

Less cloud and rainfall opportunities across all the data you see in the video means the previous forecast turns warmer over much of the nation.

I will have an Indian Ocean Update during Wednesday.




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