Rain system that drenched parts of central Australia responsible for downpour in Bundaberg

Updated October 03, 2017 17:21:41

Large parts of Australia have been getting a drenching over the past few days, from Oodnadatta in South Australia with 59mm, across to Bundaberg, Queensland with 344mm.

There were a handful of places in northern South Australia that had totals over 50mm late last week, including Todmorden, north of Oodnadatta, which recorded 85.2mm, and Tieyon, which got 67.8mm.

Across the border Yulara, near Uluru in the Northern Territory, received 46.4mm, but places further north recorded less, with Alice Springs getting just 9.7mm in the gauge.

The break in the dry has been welcome for many, but where is all the rain coming from?

Bureau of Meteorology forecaster Sarah Fitton said the rain in the red centre and the flooding rains on Queensland's mid coast had all been part of the same system.

"We had a rain band that has gradually moved eastwards," she said.

"It has moved through the northern parts of South Australia and the southern parts of the Northern Territory last week then over the weekend."

"Then it started to move over southern parts of Queensland.

"It produced some quite significant rainfall yesterday and into this morning before it started to move offshore today.

"One of the main drivers for it [is] we had an upper trough that has been gradually moving eastward."

Troughs are weather features that show up on the nightly news as dotted lines. They are areas of low pressure, where air is lifting away from the ground.

But troughs moving across Australia happen all the time and it stays dry, so why did this one produce rain?

Ms Fitton said the upper level trough had been adding to a lower level trough.

"If you just have a surface trough and it is very dry, you might not have the right conditions," she said.

"But if you have an upper trough and a surface trough combining, that makes conditions much more favourable [for rain]."

Why do troughs lead to rain?

"For one thing you often have convergent winds. So winds meeting at one point and they don't have anywhere else to go but up," Ms Fitton said.

"It's a lifting mechanism. It helps the air that is close to the surface to rise, and once you get air that starts to rise you get clouds forming and you get showers.

"It helps to give it a little bit of a push."

Surface troughs can also create an unstable atmosphere, so when the air does start to rise it tends to keep rising.

In this case, northerly winds also dragged in moisture.

"There was a lot of moisture that was coming in from more tropical air," Ms Fitton said.

This moisture, lifted by the troughs, generated rain in the red centre.

Why did Queensland get so much?

The rain seemed to double in strength as it made it to the Queensland coast.

Ms Fitton said that was because as the system approached the Queensland coast, the upper trough developed into an upper low.

Lows are areas of low pressure like troughs, but they are generally stronger.

"So when this combined with a series of surface troughs — one on the coast and two further inland — there was lots of lift for that rain and thunderstorm activity," she said.

"We also had some moist easterly air coming off the ocean that was producing lots of extra moisture."

Aditi Sharan, a forecaster at the bureau's Queensland office, has been looking at the rainfall totals from the past two days.

"The event total for the two days was 344mm over Bundaberg," she said.

"The 24-hour rainfall for Bundaberg was 319mm, so it has actually broken the October record of 218mm which was set in 1953."

It has been a similar story further south, with Noosa receiving 129mm and Doonan Creek getting 112mm.

But despite the big totals, Ms Sharan said it was not an unusual event.

"Basically spring time is when we start seeing more rainfall," she said.

What's next?

The rain has contracted offshore, so for today there will be some showers, and from tomorrow there is a medium to high chance of showers and a possible thunderstorm over the southern quarter of Queensland.

"At this stage there is no other system coming through, but that could change in the next few days," Ms Sharan said.

Topics: weather, rainfall, bundaberg-4670, oodnadatta-5734, alice-springs-0870, noosa-heads-4567

First posted October 03, 2017 17:05:43