Many areas have already seen that warm bias and wetter bias to kick off September, as modelled by climate agencies for months in advance. However some areas are still tapping their toes and waiting for more rainfall to develop to lift up values to at least near seasonal values, let alone above average rainfall.

So what are the chances?


Rainfall Outlook September 2021

% chance of exceeding median rainfall for the remainder of September (note some areas in the green shading over eastern and central Australia have exceeded the monthly rainfall for the month - some chance these areas could record another months worth of rainfall during the coming 3 weeks.) This forecast remains unchanged from my last update in August as the weather looks to be on track to support the wetter bias further east at this time.

Rainfall to date - BoM. Clearly showing that bias over the eastern inland so far, it is now a question of whether that rainfall can spread further west through SA and parts of WA and QLD during the coming weeks to verify my forecast above, and will review this at the beginning of October.

Rainfall Outlook October 2021

% chance of exceeding median rainfall for October

Rainfall is still anticipated to increase in line with the latest guide that the IOD is expected to peak during mid to late October into November. This will see rainfall heaviest over Central and Eastern areas of the nation. The elevated SSTs over northern waters of the nation from the Indian Ocean through the Timor and Arafura Seas through to the Coral Sea and northern Tasman/Pacific off the east coast will play a huge part in injecting higher levels of moisture through the nation as will the Indian Ocean IF the guidance from the Climate models verifiy. The international models are unchanged in their guide of peaking the event through the coming 2 months.

Temperature Outlook September 2021

% chance of exceeding median temperatures for the remainder of September.

The remainder of the month has shifted away from the cooler bias, at this time, with the SAM trending positive or remaining neutral for the remainder of the month. This is leading to a trend of easterly winds dominating from next week, the westerly wind belt suppressed leading to colder air remaining further south. So seasonal conditions temperature wise along the east coast overall, after what has been a warmer start to the month. The heat will continue to be well above average over the northern half of the nation as has been the trend for recent months. The slightly negative IOD phase which is expected to strengthen, is a driving force behind this.

Temperature Outlook October 2021

% chance of exceeding median temperatures for October

A first look at October suggests that with the increasing rainfall signal that the temperature anomalies will moderate over the eastern and southeast inland at this early stage with the bias of above average temperatures continuing with the early build up conditions over the northwest of the nation. That does not mean we will not see heat (as we saw last week) during this period, but the overall trend is for seasonal weather in the east overall with a warmer month over the northwest and west.


SAM - Remaining Neutral for the foreseeable future, though the volatility of spring continues for the remainder of September.

The neutral to slightly positive SAM has a minimal influence on rainfall and temperatures throughout the nation, but the spike in the positive phase of the SAM through mid month could see an increase in wet weather over the east coast next week and into the fourth week of September, rendering the rest of the nation on the drier side with high pressure moving in. However in Spring, we have to factor in that the broad scale signals from climate drivers/modelling are lower confidence with competing airmasses and moisture coming together occurring at any time. But on current guidance, the rainfall bias with cooler would be on the east coast in the coming 2 weeks and then spreading out beyond there, but as I have said in the medium term forecasts, watch this space. This is why the forecast for September remains largely unchanged rainfall wise.


The Indian Ocean remains such an interesting beast and difficult to predict ahead of time, clearly not going to plan for the second year in a row. However as outlined last week, the global modelling does suggest it will trend deeper into a negative phase through September, peaking October and waning through November and concluding through December which is typical. But how much of an influence it will have on our weather remains to be seen, and currently, it has been quite poor in recent weeks, with modest injection of moisture from the northwest but the major rainfall events have been born out of the Coral Sea and northern Australia moisture.

ENSO - Neutral

Neutral though there are stronger signals for a La Nina to form in the Summer.

The modelling is still split globally through the second half of Spring with some agencies increasing the risk of a La Nina forming through the latter part of Spring and persisting into Summer. Though other agencies are not as aggressive. That said, this climate set is usually very reliable, and even through my forecast remains neutral for the coming Spring and Summer, the next set of data due out next week will be make or break for this event to kick off. So watch this space. Currently the impact of the ENSO on Australia's weather is minimal for the coming September and October.


Euro Rainfall Next 7 Weeks - From September 7th through to October 18th 2021.

You can clearly see the rainfall picking up for the back half of the month and inland areas through October in line with the IOD strengthening and the weather warming over the nation, the tropics waking up and sending moisture south. Remember, we have not seen inland troughs just yet and they should start to appear in the coming 6-8 weeks driving the storm season into gear over much of QLD, NSW and VIC. That could lead to elevated rainfall chances there. For SA and the NT, troughs moving out of WA will lift moisture available from the north and northwest IF the Indian Ocean is going to play ball and influence our weather.

The Rainfall Anomalies for this period.

Correlating CFS - Rainfall for the coming 7 weeks - Ending October 21st 2021

Rainfall Anomalies for the remainder of September.

CMC Rainfall for the remainder of September 2021

Can see a lot of rainfall coming if this model is correct over the next 4 weeks. I did pick the system through the weekend which dropped the rainfall over Central Australia, so discounting that rainfall, the east is still yet to see that rainfall analysed as is much of SA and follow up falls for areas of the southeast. It also has the rainfall coming up during the week for SWLD of WA and that runs into TAS with those excessive falls there. Increasing the tropical moisture as well over the NT and QLD.

Corresponding Rainfall Anomalies for the remainder of September.


Spring 2021

Still has a wet bias over much of the nation, though pockets of the south MAY be drier, but this is conditional upon where systems eventually develop and travel to. As we saw over the weekend, it takes ONE system to bring a months worth of rainfall at the moment, and this can be replicated in areas that are shaded in white and yellow, not just in the green and blue zones. However it would be more likely to see rainfall above average in the areas shaded in green and above for the Spring Season, with a wetter October and November. Not great for those wanting dry weather for harvest.

Temperatures - This model for Spring continues to be supported by other agencies, with the trend of warmer weather over the northern half of the country continuing with the elevated SSTs over northern waters. The cooler than average spring has moderated as weather agencies factor in a shorter duration IOD and on observed weather I am in agreeance with this. There could be pockets of below average temperatures still over inland NSW and the ACT through Spring but the coverage area has shrunk.

More details coming up on the La Nina risk later this week ahead of the next round of data next week and the importance of the Coral Sea and waters off northern Australia during the Spring and Summer and how these areas influence rainfall and temperatures through the warmer season.

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