Updated: May 25
Well just as the Bureau call the event over (based off their model) the Indian Ocean is waking up and in line with International Guidance which I am strongly supporting, always developing the IOD through August, building it in September and then peaking the event in October and easing from November and waning in December.
That seems fair, I always like to review all model data and measure that against real time data, observation and short and medium term modelling. We are seeing the models pinging moisture building over Indonesia and being drawn south and southeast into WA and then onwards through the nation.
SHORT TERM DATA
18Z GFS Precipitable Water Values - Next 2 weeks
These are not the anomalies I usually share, but available moisture that can be utilised by the atmosphere to produce rainfall. The warmer the colour tones the more moisture laden the atmosphere. You can see a very different look in the Basin to where it was 3-4 weeks ago, with much more available moisture rushing through the Indian Ocean and then being propelled through the mid latitude westerly winds. This will increase rainfall chances.
Note the moisture being drawn south - September 29th 2021
The moisture is more likely to be drawn south and southeast through the mid latitude westerly winds, ahead of cold fronts, broadcasting that moisture throughout Australia, lifted by cold fronts and troughs to increase rainfall chances nationwide, especially southern and central parts, from later this month. Also note the ingestion of moisture east of Africa through the roaring westerly winds over the Southern Ocean. (PW values of 60-70mm does not mean you will get 70mm of rainfall, but says the airmass is saturated and very productive in terms of rainfall)
Rainfall Anomalies for the coming 2 weeks
The last time we saw values of this nature was back in June and July where we saw large impacts of moisture streaming south from west and south of Indonesia. Note the surge coming south of Java later in the month. The light blue shading is maxing out the anomalies of 150-200mm above normal. There are pockets of drier weather throughout the tropics in line with the monsoonal break over parts of India.
Rainfall for the next 2 weeks
As a result, all that moisture surging into the nation supports more productive rainfall over mainland Australia, where the spread of rainfall comes for the last week of September is obviously the million dollar question, but the take away is that the moisture is building and supporting more rainfall for late September and into October.
LONGER TERM DATA
Euro October SST Anomalies
High chance of above average SSTs over the IOD region, supporting the peak of the phase in October and thus leading to more rainfall and cloudiness being projected across the nation. The broader waters over the north are warmer than normal which will help to broadcast moisture across northern and eastern Australia in association with the IOD.
Euro November SST Anomalies
The event begins to wane as the seasonal shift takes place, the northern wet approaches. That is normal. What is abnormal is the temperature anomalies strengthening off QLD which could be the Euro coming on board with the idea of La Nina developing.
Euro December SST Anomalies
The IOD is resolving and neutralising in line with the seasonal shift, but note the SST anomalies offshore QLD and through the Arafura Sea, supporting the La Nina theory, but also supporting the moisture spreading south and east through Australia in Summer time. Could add more fuel to the storm season and cyclone season.
Euro January SST Anomalies
The IOD is now returned to seasonal in 2022. The elevated SSTs over the Coral and Arafura Seas are interesting and could provide some excellent rainfall for those thirsty parts of QLD. If La Nina develops, the SSTs over the east coast could be warmer further south east of NSW.
IMPACT ON RAINFALL FOR THE REMAINDER OF THE YEAR.
Seasonal Rainfall October-December 2021
The swath of rainfall above average linked to the IOD event (note the very high chance of above average rainfall over Indonesia) and this being drawn south and southeast through October and November and lesser extent in December. This modelling does not support the La Nina impact on Australia, which means that the forecast rainfall could be well above average in future runs. Finally, we are talking about the wettest time of year for southeast areas of the nation and wet months over northern and eastern parts of Australia, so to be above average during the wettest time of year is significant.
Seasonal Rainfall November-January 2021
You can see as the IOD relaxes and resolves that the impact on rainfall is reduced for Summer 2021/22, but still expected to be above average in many locations, and this model predicting a higher chance of above average rainfall in the peak of the wet, which means wild card systems could be drawn south and east through the nation from the monsoon, and predicting the impact of that ahead of time is like throwing darts in the dark at at moving target while blindfolded. The point of showing you the data, is that the IOD is likely to play a part in elevating rainfall chances, and then we hand over to the La Nina potential in summer time.
SOIL MOISTURE - As of September 15th 2021.
There is a lot of work needed for QLD and parts of SA to repair the soil moisture issues after a drier spell in pockets, especially through central and western QLD. Around the Goyder line it is as expected for this time of year. The east is wet and this will lead to an increased risk of flooding through NSW and VIC as expected, this increasing through the back half of Spring. The IOD a good chance of leading to this flood risk increasing in the coming 6-8 weeks if the above data verifies.
More details coming up tomorrow on Climate including an update on my Spring and Summer Outlook.