The Bureau of Meteorology (BOM) is now predicting between 60 to 150mm of rain could fall on Sydney early next week, with even more on the south coast. That would be the heaviest rain for months.

On Tuesday alone, if 70mm falls – which is the upper end of the BOM’s forecast – it will be the single biggest day’s downpour on the city since early February and more precipitation than during the entire month of either June or May.

The culprit is a probable east coast low which is due to bubble up on Monday. That should bring torrential rain after a weekend which itself could be marked by showers.

The full “rain event” is set to begin today.

“This event is likely to bring severe weather in the form of heavy rainfall, damaging winds and damaging surf,” said Sky News Weather meteorologist Rob Sharpe.

A cold front is charging through South Australia, Victoria and southern parts of New South Wales today and tomorrow and that will bring showers to a wide area.

Most of that moisture will be in inland areas, Dubbo could see between 10 and 40mm, but there’s also a possible shower or two for the coast.

As that system moves off into the Tasman on Monday it’s likely to undergo a change, said Mr Sharpe

“When the upper cold air interacts with warm seas surface temperatures along the New South Wales coast line, that’s our trigger for an east coast low,” he said.

LOCATION OF LOW STILL UNCLEAR

East coast lows have many of the characteristics of a tropical cyclone except that they are not formed in tropical waters. They bring torrential rain that can lead to flooding and gale force winds.

However, they’re also cunning blighters to pin down. Next week’s system could be an east coast low – if it lingers around the coast – or if it drifts out to sea it could become a Tasman low which has a lesser effect.

“The other feature of east coast lows is that they typically have multiple smaller centres within the system that bring periods of heavier rainfall and powerful winds that are tricky for global forecast models to predict accurately and in detail, particularly well in advance,” Mr Sharpe told news.com.au.

At this point, though, it does look like it will turn towards the coast and in particular the NSW south coast between Sydney and the Victorian border. But its effects could be felt further north and well into Victoria.

Wollongong is looking at up to 80mm of rain on Tuesday, following 40mm on Monday. Batemans Bay could see 60mm on both days. The low could reach as far inland as Canberra with up to 35mm of moisture at the beginning of the week.

“On Monday, Sydney and towns north are likely to see a sunny spell as it will probably be north of the low in dry westerly winds,” said Mr Sharpe.

“However, on Monday night and into Tuesday morning smaller lows could drift northwards as the main weather system moves away from the coast meaning that bursts of heavy rain and strong winds should move further along the coast to include Sydney and Newcastle in the wet and blustery weather.”

Looking across the next few days, Sydney can expect some pockets of showers on Saturday and Sunday and then between 25-45mm of rain on Monday and 25-70mm on Tuesday with sporadic showers for the rest of next week.

Heading into Queensland, Brisbane could be very damp. A few spots on Saturday but then a possible storm on Sunday and up to 20mm of rain and as much as 45mm on Monday before clearing up.

The other very soggy city could be Perth but this time due to a different system. A sunny weekend turns to a stormy start to the week with possibly up to 35mm falling on Tuesday.

Adelaide will see showers, some heavy, and a possible storm both today and tomorrow before it settles down.

Some rain in Melbourne across the weekend, totalling up to 10mm, with storms also a possibility. Drier in a chilly Hobart but a downfall or two could pass through on Saturday.

Mostly sunny in Darwin with highs of 33C.

benedict.brook@news.com.au | @BenedictBrook