And right at the center of danger are Sydney and the Illawarra, which could be hit by a nasty East Coast Low as early as Saturday.

With up to 300mm of rain in the meter, that would easily add up to more than a month of rain in one day.

And it’s not just New South Wales, Queensland is also clearly going to be soggy for the next few days.

A higher level trough is causing the weather drama, pushing cool air up from South Australia.

That trough will interact with tropical moisture further north to produce these windy and wet intense weather systems.

“A trough is developing just off the coast of NSW,” saidSky News Weather channel senior meteorologist Tom Saunders.

“We expect that trough to deepen into a full-fledged East Coast Low this weekend.

“These systems are notoriously difficult to predict exactly when and where they will strike, but at this stage it appears that it is that central part of the NSW coast with rain that is also affecting the northern parts of the country,” he said. .

Floods ‘essentially guaranteed’

East Coast Lows have many of the characteristics of cyclones but are not classified as such as they are in subtropical waters. But they can still cause massive amounts of rain, storms and coastal erosion.

“Sudden and river flooding are essentially guaranteed,” said Mr Saunders of the fallout from this weekend’s weather.

“Damaging winds and power outages are possible, especially towards the end of the weekend through early next week.”

So how much rain can fall? Well, it depends on the meteorological model, but 200-300mm is a possibility with a chance of up to 400mm in remote areas and around Sydney, Wollongong and across the Blue Mountains. And that’s really not great.

“That covers the Hawkesbury Nepean catchment area and that catchment area is still quite wet and that would certainly be enough rain for major flooding,” he said.

If that happens, parts of Sydney’s western and northern fringes could be flooded.

The Bureau of Meteorology has issued a severe weather warning for heavy rainfall from Saturday around Ulladulla to Sydney.

NSW SES Commissioner Carlene York said that because dams and rivers were already full, there was “significant” concern about what’s to come for communities across the state.

“This significant downpour could have major impacts on communities from Port Stephens to Batemans Bay, so we are preparing and putting out community notices letting them know they are in some of these high-risk areas and ready.”

The Bureau predicts a drop of 60-90mm on Saturday in Sydney, 100-150mm on Sunday and up to another 80mm on Monday. Maximus should be around 17C.

Only on Sunday it can be even heavier in Wollongong and the Blue Mountains with 150-200 mm possible.

Newcastle are looking at 35-50mm on Sunday and 50-80mm on Monday. Further north, there could be some showers over the weekend for places like Port Macquarie and Grafton, but totals will be much lower. Rain may also fall inland and south of NSW, but again this will be much less heavy than the Sydney basin.

Canberra is forecast to receive 10-25mm of rain between Saturday and Monday.

Queensland to get soaked

Widespread rain continues in Queensland. On Saturday it will decrease but on Sunday it should be bubbling up again with the heaviest falls early in the week.

Over the northern parts of the coast, rain totals of 100mm are on the map.

Townsville will see some showers as the weekend progresses, but Monday will bring it down, with 40-60mm forecast, and more for Tuesday.

In Brisbane, Saturday looks to be the wettest day, with up to 25mm of rain and less on Sunday. But there could be showers again on Tuesday. The other capitals will experience a very different weekend, with Melbourne, Adelaide and Hobart all remaining dry and partly cloudy with nearly uniform mid-teen highs.

Darwin will see some blue skies with maximums of 29C. But it is Perth that will be gloriously sunny. But it will be relatively mild with temperatures just approaching 20C.

— with Ben Graham.