Australia's east coast is set to receive a drenching this weekend as moisture streaming in from the tropics links up with a weather system from the south.
"We are expecting showers along most of the east coast," Bureau of Meteorology senior forecaster Christie Johnson said.
Heavier falls over the weekend may be focused on the New South Wales coast south of Sydney, according to the BOM's modelling.
But many regions from Darwin to Hobart can expect rain between now and early next week.
Ms Johnson stressed that flood-affected regions in the Northern Rivers and South East Queensland were not considered to be at serious risk – for now – of extreme rainfall this weekend.
"We don't want people to be concerned — we just want them to be alert, stay up to date with forecasts, and watch for any warnings that do get issued, because our hydrologist teams are watching it really closely," she said.
A very wet dry season
The unseasonable rain will begin in Darwin, which is forecast to receive up to 35 millimetres over the next few days.
It comes at a time of year when the average monthly rainfall is less than 2mm.
"It's the dry season up there," ARC Centre of Excellence for Climate Extremes climatologist Kimberley Reid said.
The tropical moisture will then stream across Western Queensland and cause rain across much of the state.
"We could see rainfall totals of 10 to 40 or even 50mm through that area on Friday, which obviously is pretty unusual given the rainfall that that we normally see through that part of the world at this time of year," Ms Johnson said.
When systems collide
The Top End rain will hit the tropical north coast of Queensland first on Thursday and Friday, bringing potentially heavy showers.
From then on, the storm will link with a weather system to the south, triggering highly unpredictable rain.
"There could be rainfall pretty much anywhere in the east of Queensland," Ms Johnson said.
Brisbane can expect up to 15mm of rain per day over the weekend, possibly increasing early next week.
As the two systems link up over the weekend the rain is predicted to centre on NSW, where there could be localised heavy falls.
"The current system is evolving in such a way that it looks like there could be a focus for the weekend over that southern part of the coast of NSW — the Illawarra and the northern part of the South Coast," Ms Johnson said.
Sydney can expect daily rainfall of up to 50mm between Friday and early next week.
'Wave-breaking event'
The dry season rain has caught the eye of climatologists who say the weather pattern resembles something usually seen in February rather than June.
"This is all one big event that's connected," Ms Reid said.
"We're at the start of what we call a wave-breaking event."
She said the jet stream had split in half, sending one jet flowing north into the tropics and one across the south.
"You've got weather that you'd expect over the mid-latitudes appearing over the Northern Territory," she said.
"So the waves of the atmosphere are breaking like an ocean wave.
Ms Reid said the uncertainties in the forecasts reflect how different computer models had predicted the development of the weather systems.
"Where exactly the rain will fall depends a lot on the position of the low pressure system over NSW," she said.
"So if it moves 100 kilometres further north than we might predict, then that's when you get massive errors in the forecast.
"As we get closer to the events happening, ideally the models should start to converge and should start to say the same thing.
