Time to look at the Climate Drivers for the coming month looking at main three.
Still lots of attention on the Indian Ocean Dipole, the BoM forecasting a wet and warmer spring, but again I caution you, what I am seeing is a lot of dead dry air over the region and persistent high pressure near the region suppressing convection to build in the region, that means less moisture surging south.
We may have to turn our attention to the Coral Sea and areas over northern Australia to inject moisture throughout the nation as we go through Spring.
But lets focus on September and see what the drivers are doing.
Southern Annular Mode - SAM - Neutral trending more Positive
The SAM has been quite volatile of late, there has not been great confidence in the members coming into agreement until just in the past 2 days, and that is why we are seeing the models respond to a period of neutral/positive. That leads to a weak influence on the rainfall over the eastern seaboard and settled weather with high pressure generally dominating the south and we are seeing that for the coming fortnight. Looking further ahead, the neutral phase is likely to continue until the end of the month - but again the skill in forecasting this is low - ALL YEAR ROUND. But on my current look at the full data sets underpinning this chart, the neutral phase is more likely. WILD CARD - The SAM can turn sharply positive during this time, and I suspect that is more likely as an outlier than things turning sharply negative. If this happens, then rainfall may creep back into the forecasts for eastern Australia.
Indian Ocean Dipole - Remains Negative (but has cooled in recent weeks)
This has been probably the biggest let down in the past month, and indeed leading into Spring, as I have been saying since early August, I am not satisfied with the influence this is having on the weather across the nation, with limited deep moisture passing through the nation from the Indian Ocean. Yes there has been moisture streaming into systems over WA, but it has been mainly elevated and lower latitude and for a proper IOD, it needs to be coming directly out of Java and further west from the tropics, and we are not seeing that. So while it is negative, it is a great time to once again point out (as I have for many years) that one climate driver is not a blanket rule for widespread rainfall for all areas of the nation. I can give you a guide but the confidence behind the forecast influence is equally as important. Given where we are in the year, the IOD should be having more of an influence no matter what is happening across the nation synoptically and I am not seeing that in the coming 2 weeks at this time and this will mean a trend towards above average temperatures and drier than normal weather until mid month. Things can still change and as we are seeing over the east of the nation, some locations.
I have outlined in a circle, the shift in the past month from where the BoM model was forecasting the IOD to where it is this week. Again I am following the Euro Model as I have stated since March on social media and on this site when I launched it. I am still leaning towards the Euro, but even then, we should be seeing more influence and not cooling of this part of the basin so this will underpin the forecast for the coming month on Tuesday next week.
Euro peaks during October and has not budged in it's guidance for the past 4 months.
The BoM conclude the event during October, where the other climate agencies keep it going through November but in a weakening phase. The CanSIPS outlook changes daily so it has reverted back towards more of the Euro outlook, but has lower skill score.
All models as they should, conclude the event, and seeing them move into positive territory is no cause for alarm during the monsoonal phase as the temperatures fluctuate based upon tropical activity. CanSIPS is very reactive like the BoM model so they are usually very dramatic from run to run.
ENSO - Neutral
But as outlined last week, the global agencies, and the one that I rate as the best agency, still is forecasting a ~70% chance of a weak la nina developing during Summer 2021/22. I am not leaning towards that occurring, however I just want to flag it as this forecast could change rapidly in the coming 4-6 weeks and that means we have to adjust forecasts very quickly to alert the impacts of a la nina developing, so pay attention to the forecasts in the coming weeks. Again I am stressing a neutral phase will persist at this time and that means no major impacts over the nation under such a forecast. However we now have 3 models globally that are suggesting a weak La Nina, that is up from 1 in early August. Keep watch.
Euro model spread for the NINO 3.4 SST Anomaly.
Note most within the plume are below 0 for the summer so we may see some further adjustments in the forecast model spread coming up from the Euro next week.
I am leaning again more with the Euro solution, but I will also add, NOAA is a highly reputable agency, and they usually are trigger happy when they call La Nina/El Nino phases, but they rarely miss. They have placed the Pacific on La Nina Watch and I think that is the right call given the guidance. The next 4-6 weeks of data will be critical.
HOW DOES THIS IMPACT THE SEASON AHEAD?
Again I do think it will be wetter as we go through Spring, which you would say about most Springs anyway. The warmer the weather, the more moisture it can hold and if you are living over the east, you are experiencing the warmth, but many are starting to see the storms with sharp bursts of heavy rainfall - this all spreading into large areas of rain.
Note that this may occur once again through the second half of September given the temperatures are expected to rise to be above average from this time next week, now we just have to see how the moisture pools over the nation while we wait for the next lifting mechanism to knock the high out the way.
Rainfall for the 3 months ahead - Euro
Temperatures for the 3 months ahead - Euro
My forecasts for September as they stand from last week - this will be updated Tuesday.
Rainfall Outlook for September
% chance of exceeding median rainfall for the month of September 2021
Temperature Outlook for September
% chance of exceeding median temperatures for the month of September 2021
I again caution you to not interpret climate data without consulting professionals, a lot of people that I have seen on social media suggesting the wet spring is a dud, 3 DAYS INTO A SEASON, is dangerous, just like saying it will be flood city! So those of you in SA in particular, it is frustrating watching the rainfall again over the eastern states, but that does not make a season! I hope you have read the notes with the pictures because that certainly helps too. Weather forecasting is not perfect as it constantly changes as you can see above.
From the BoM to me, through to any other global agency, it is never perfect!