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ABC News
National

Why is it raining so much and when is it going to stop?

It has been round after round after round of rain over the past few weeks, on top of all the rain of the past few years.

So why is it just so wet out there? More importantly, when will it stop? 

Where is all the moisture coming from? 

There is always a lot going on up in the atmosphere but the big ones this year are the compounding impacts from the third La Niña in a row and the negative Indian Ocean Dipole.

Both of these drivers mean the waters surrounding northern Australia are even warmer than usual. 

These warm tropical seas evaporate easily, pumping moisture up into the atmosphere.  

"So not only is the water all the way around Australia, well above average, it's also providing multiple sources of moisture," according to Dean Narramore, senior forecaster at the Bureau of Meteorology. 

"The atmosphere is primed to produce a lot of rainfall.

"What that means is, every time we get a weather system moving through, it's tapping into tropical moisture as it moves slowly across the country."

Add to that the atmosphere's increased ability to hold moisture as the climate warms and it is a soup up there.

Why so much rain falling over the south east?  

Having weather systems swing through south-eastern Australia this time of year is not unusual but the amount of moisture available for them to throw down at us is. 

"Because there is so much moisture, it's now allowing those systems to bring widespread, heavy, and prolonged rainfall to such large areas," according to Mr Narramore.

In dry years, the impact of these systems is very different. 

"If we went back a few years ago, we actually had a similar active weather pattern like this back in 2019," he said.

"But the difference was there was zero moisture with it. So we had all these weather systems moving through, and it just brought lots of wind, heat, and gusts and obviously was one of the main drivers of our fires.  

"This is all combining to make sure that every weather system that does move through, unfortunately, brings widespread soaking rainfall to many of us across eastern and south-eastern Australia." 

When will the rain stop? 

In the short term, breaks from the rain will come in gaps between systems.

We are talking a few days, maybe a few weeks of lull if we are lucky. 

Not everywhere will be hit by every system but, generally, weather systems roll across the south of the country every week or so.

As we move into summer, southern Australia is likely to see less rain as the ridge of high pressure makes its way down. 

This happens every year and doesn't guarantee no rain, but it is the reason summer is generally dry over southern Australia.

Conversely, as the ridge moves down, it opens the door for more rain over northern Australia and the summer wet season.

So medium term, the risk is likely to shift north rather than going away. 

Longer term, the underlying drivers would need to stop pumping the atmosphere full of moisture. 

The latest climate driver update from the Bureau suggests the negative IOD is on the way out.

This is completely on form. The IOD usually collapses as the monsoon moves down at the beginning of summer. 

The La Niña on the other hand is expected to persist until early 2023.

This would be a little bit earlier than usual. La Niñas usually last until the end of summer or early autumn.

But an early end to the La Niña would not necessarily mean we are off the hook. 

Even in a neutral summer, it is the wet season for tropical and subtropical Australia. 

With catchments absolutely brimming, even if the climate drivers do break down quicker than usual in the coming months, normal amounts of rainfall could cause problems this year. 

The Murray-Darling floods will continue for months as the water slowly creeps downriver, even without any more rain. 

There is still a ways to go yet.

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