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BOM's latest outlook signals soggy spring weather for eastern Australia

The skies are expected to keep letting lose this spring. (Brenda Connelly)

The Bureau of Meteorology's official outlook suggests we are heading towards a soggy spring for eastern Australia but relatively dry in the west.

There is a strong, over 80 per cent chance, of above-average rainfall for the majority of the east of the country over the three months of spring.

Not what communities still recovering from the flooding of the last few seasons will want to hear, as catchments up and down eastern Australia remain sodden. 

"At the moment, unfortunately, large parts of eastern Australia, of course, [are] still very wet and the rivers are high and our dams are still quite full," said Andrew Watkins, head of long-range forecasts at the Bureau. 

"So, unfortunately, that means that any significant rainfall falling in those areas in eastern Australia do raise the risk of flooding." 

In contrast, parts of Western Australia are expected to be relatively dry, with regions south of Perth having only a 25 per cent chance of exceeding median rainfall.

Western Tasmania, already quite dry, is also expected to be unusually dry. 

Much of the country can expect above average rainfall over the next three months.  (Bureau of Meteorology)

"In terms of temperatures, we are looking at warmer days than normal across parts of Northern Australia, and also down in some parts of the south-west and Tasmania as well," Dr Watkins said.

"But for many parts of central and south central eastern Australia, we are actually looking at cooler days than normal over a pretty broad area, which corresponds to expectation of having more cloud and wetter soils." 

But the country's extreme north and south are still expecting above-average daytime temperatures this spring.

Cooler daytime temperatures are expected through the centre of the country but it is still expected to be above average elsewhere.  (Bureau of Meteorology)

Overnight temperatures are likely to be above average for much of the country.

The wet conditions have long been foreshadowed. With widespread speculation over the potential return of the La Niña ever since the last one declined at the beginning of winter.

We are currently sitting at La Niña alert, with the BOM indicating there is a 70 per cent chance of a La Niña forming by the end of the year.

La Niña, or La Niña-like conditions do generally encourage wet conditions for the east and the north of the country, as we have seen to devastating effect over the last few years.

But it is not the only driver that can encourage rain.

Whether or not the La Niña does end up forming, its cousin the negative Indian Ocean Dipole is riding strong this spring.

"That negative Indian Ocean Dipole is really the key player in our climate at the moment. That's really what's pushing the odds towards wetter than average, particularly over parts of eastern Australia," according to Dr Watkins.

IOD looms large this spring

While La Niña, and her partner El Niño, operate over the Pacific Ocean the Indian Ocean Dipole (IOD) is a similar set-up over the Indian Ocean.

A negative IOD has been in full swing since around June and declared in the beginning of August.

The negative phase of the IOD occurs when there are warm waters off the north-west Australian coast.

That warm water then evaporates, charging the atmosphere full of moisture.

Moisture that can then be tapped into by cold fronts to form north-west cloud bands that funnel that tropical moisture right the way across the country into the south-east.

While every year is different, the areas shaded in green usually receive wetter than normal weather during a negative Indian Ocean Dipole — particularly the south-east of Australia. (ABC: Shakira Wilson)

"With the negative IOD you tend to get more rainfall in lots of Australia and eastern and south-eastern Australia. But it depends on where you are," according to Nicky Wright, paleoclimatologist at the University of Sydney.

The south-west's rainfall in particular is not increased by the influence of the negative IOD. 

"For this specific negative IOD, it's hard to say whether it's influencing our weather so far because we need to have attribution studies to actually confidently say that." 

The negative IOD is expected to remain in the Indian Ocean until the monsoon moves south and disrupts the system in early summer. (Bureau of Meteorology)

But the moisture-encouraging conditions are expected to hang around for a while longer yet. 

"Projections currently indicate it's going to get a little bit stronger, September and October, and then by December it should have hopefully, gone back to neutral," according to Dr Wright.

Compounding effects of multiple drivers

A positive SAM in winter or the beginning of spring generally decreases rain from frontal systems across the south. (Bureau of Meteorology)

In more bad news for those hoping for a break in the rain this season, if both drivers occur at the same time their impacts can compound, bringing even wetter conditions.

The outlook also says warm sea surface temperatures to our north are also contributing to the wet outlook in the north and east.

The only potentially drying factor on the forecast right now is the strongly positive Southern Annular Mode (SAM), which means frontal systems are more likely to stay further south.

But don't count on it to reduce rainfall in the east. 

A positive SAM means that currently drier places like southern WA and SA are likely to stay dry while pushing more moisture onto the east coast.

So whether or not the La Niña does eventuate, there are still factors pushing us towards a wet spring.

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