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Victoria weather: Why south-east Australia is receiving a tropical deluge

Kate Doyle and Ben Deacon, Saturday December 2, 2017 - 13:44 EDT
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Over 200mm is expected for parts of Victoria and New South Wales on Saturday December 2. - ABC

South-east Australia is receiving a tropical downpour. It is part of the same extraordinary weather system that has brought heatwaves from Melbourne to Hobart and cool, wet weather in Queensland.

Three weeks ago a high pressure system settled between Australia and New Zealand.

Normally high pressure systems clear within the week, but forecasters such as the Bureau of Meteorology's Dr Blair Trewin watched in amazement as the system refused to budge.

"It is very unusual to have such a blocked situation, particularly in spring. It is something you see more often in late summer or early autumn," Dr Trewin said.

"But even then it would be unusual to have something last this long."

Meteorologists call these slow-moving systems 'blocking highs' because they block fresh weather coming through.

The blocking high directed warm air south over Victoria and Tasmania, leading to a record long spell of warm temperatures.

This month, Melbourne experienced a record nine consecutive November days over 26 degrees C, and a record 14 days over 30 degrees.

Hobart had six days in a row over 26 degrees — equalling a record for any time of the year.

The warm and calm conditions also led to exceptional ocean temperatures with water off Victoria and Tasmania up to three degrees above normal — a record for November.

While the south-east experienced warm, sunny weather, Queensland has had persistent onshore winds.

"We've had easterlies over northern New South Wales and Queensland for weeks now that's been pumping more and more moisture into the atmosphere", Dr Trewin said.

It is only now that the blocking high is finally moving east as a cold front and low pressure system, drawing tropical moisture from the north.

Dr Trewin says this low pressure system would cause a major rain event in itself by the time it reached Victoria.

What makes this weekend's forecast so extreme is that when the system reaches Victoria, it will collide with the moist, tropical air from Queensland.

"So what we are expecting to see is a subtropical or even tropical air mass come right the way down south into Victoria," he said.

"When you get that combined with the active weather system you are going to see a lot of rain."

Climate driver brings a tropical influence

Behind the unusual month of weather, a big picture climate driver called SAM, or , is at work.

Senior climatologist Felicity Gamble said the rainfall pattern bears the signature of a positive SAM.

This is normally associated with rain on the east coast in summer.

"Essentially what's happening is the ridge of high pressure that we often see over southern Australia at this time of year shifts slightly southwards and we start to get a little bit more of a tropical influence," she said.

"It extends down south over Australia into Victoria and southern areas which don't often get that tropical influence."


© ABC 2017

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