This is quickly becoming a year for the record books.
While the 2020 and 2021 instalments of La Niña broke the south-east drought, the 2022 finale is building to a crescendo of rain, storms, floods and cyclones.
All major drivers of Australia's weather patterns are stuck in a reinforced loop that is delivering an atmospheric river of moisture to our landmass.
This spring it's the Murray Darling Basin underwater, however, as we transition into summer and beyond, the extent of flooding is likely to expand to other catchments of eastern and northern Australia.
Spring report card
Historically La Niña coinciding with a Negative Indian Ocean Dipole (the Indian Ocean version of La Niña) has produced exceptionally wet springs.
The last time the double rainmakers were both well-established was 2010 and that was Australia's wettest spring on record (data to 1900).
While September 2022 was the fifth wettest on record, it was still 13mm drier than 2010.
However, this October has so far delivered incredibly high rainfall.
The first 10 days of the month produced:
- 205.4mm in Sydney, nearly three times the average and just 80mm off the all-time October record (data to 1858)
- 106.4mm in Bourke, four times the average and already the wettest October in 67 years
- 134.2mm in Darwin, nearly double the October average and second-wettest start to a wet season on record (data to 1869)
Considering the rain forecast during the coming week, this spring is well on the way to being in the top 10 — or even top five — wettest on record.
Flooding this week
The pattern of weekly rainbands and flooding will continue this week, a negative Indian Ocean Dipole (IOD) modus operandi.
A cloudband formed across southern Australia on Monday and is currently spreading heavy rain through south-east states, triggering a flood watch or warning for more than 60 rivers from central NSW to Tasmania.
Some of the worst flooding this week is likely across northern Victoria where up to 100mm could fall on Thursday alone.
The flood prediction for southern and central inland NSW is complicated by the already swollen rivers following soaking rains during the past few weeks.
Flooding for the remainder of 2022
Rainbands and flooding are likely to remain through 2022 for south-east states, until cold fronts retreat into the Southern Ocean closer to Christmas.
Although, tropical lows can occasionally bring flooding rains to south-east Australia right through summer.
For residents downstream along the Murray River, it's a nervous wait as upstream peaks from both NSW and Victoria gradually merge to form one mammoth wave of water heading west towards South Australia.
The flow rate at the South Australia-Victoria border is currently the highest in six years at around 73 gigalitres per day and climbing rapidly.
Considering the upstream floods, the flow rate could exceed the 2016 peak of 95 gigalitres per day by summer.
Will floods return to the east coast?
As soon as the word got around La Niña was back for a third consecutive year it made many residents on the east coast justifiably anxious, but the east coast is currently mostly flood-free.
There's a simple reason for that — it's spring.
And in spring most of the rain which falls over southern and central Australia is connected to cold fronts, which come from the west and therefore drop rain on the western side of the ranges before reaching the east coast.
The most severe flooding along the east coast occurs from late summer to winter when the right environment exists for troughs and low-pressure systems to dump copious amounts of rain east of the ranges.
Data from the Hawkesbury River at Windsor shows 46 floods of at least 10 metres since 1799, only five have occurred between September and January.
Unfortunately, this indicates that flooding rains are likely to return.
Southern Queensland's most severe floods generally arrive between January and April and the NSW coast from about February.
We can only hope the severity is below earlier this year.
For tropical Australia, the arrival of the monsoon is likely to bring flooding from around December.
The most severe flooding often occurs near tropical cyclones or tropical lows and the Bureau of Meteorology has predicted a 73 per cent chance of an above average cyclone season.
However, numbers have declined over recent decades and the last season exceeding the average of 11 cyclones was back in 2005-06.
While 11 cyclones on the surface sounds extreme, this covers the entire region from the Solomon Islands to west of the Cocos Islands.
The average number of cyclones which make landfall on the Australian mainland each year is only four, and many hit the coast in sparsely populated stretches around the Gulf of Carpentaria, Kimberley, and Pilbara.
When will the big wet end?
Both the negative IOD and La Niña have shown small indications of a slight weakening during the past week.
For the Pacific Ocean, this is most likely just a short-term fluctuation as most La Niñas don't break down until late summer or autumn.
For the Indian Ocean, this could be the start of a more permanent return to parity as dipole events always decay by around December when the monsoon arrives, although the negative phase is likely to still increase Australia's rainfall for at least another month or two.
The other major driver of our weather, the Southern Annular Mode (SAM), is continuing its positive trend this year, a pattern that boosts rainfall along the eastern seaboard.
A positive SAM is a contraction of westerly winds towards Antarctica and westerly winds are dry winds for most of eastern Australia.
That's three boxes ticked for further rain and flooding, especially considering our spilling dams and saturated catchments.
So, when will rainfall return to some form of normality?
The short answer — we will most likely have to wait until autumn 2023.
