Come Wednesday, the grey clouds and rain that have become all-too-familiar visitors in recent times are due to roll in again, along with a good chance of another east coast low developing, meteorologists say.

The NSW State Emergency Service also warned residents, particularly in the mid-north coast and Northern Rivers regions, to prepare for a low pressure trough and possible east coast low developing off north-eastern NSW. Such a system could bring "intense rainfall and strong winds".

"Now is the time for residents to prepare themselves and their homes to help reduce such impacts as we have seen across the Hunter and Sydney regions last week," Adam Dent, NSW SES Commissioner, said in a statement on Monday afternoon.

Weather modelling shows between a 60 and 70 per cent chance of an intense low-pressure system forming off the east coast of Australia between Thursday and Monday.

Barry Hanstrum, regional director NSW at the Bureau of Meteorology, said the bureau's forecasting models are producing "a range of scenarios" that are likely to firm up in coming days.
 
"What we do know is that most of the forecasting guidance shows a low pressure system developing off the northern New South Wales coast, but we are not able to determine the exact position, timing and intensity of the system this far out," Mr Hanstrum said in a statement.
 
"Current guidance suggests that rain will increase this week, and peak between Thursday and Saturday," said Mr Hanstrum, adding that the heaviest falls may be in north-eastern NSW and south-east Queensland. "Further rainfall is likely for catchments that have already experienced flooding during last week's severe weather event.

In Sydney, as much as 80 millimetres of rain could fall between Wednesday and Saturday, the Bureau of Meteorology said in an updated forecast on Monday afternoon.

Friday and Saturday may be the wettest of those days, with as much as 25 millimetres predicted to fall on both days.

Towns and cities further up the coast will most likely fare worse, as the forecast rainfall map for Friday shows below.

As much as 205 millimetres of rain may fall on Byron Bay for the Wednesday to Saturday period, the bureau said.

Flood-hit Dungog may get another drenching, with 80 millimetres of rain over those days, including as much as 30 millimetres on Saturday. Newcastle may collect as much as 115mm over the four days.

With many areas of the NSW Central Coast sodden after last week's rain, further falls could lead to flash flooding, meteorologists warn.

The SES responded to 19,950 requests for assistance in just over a week, Mr Dent said.

Rob Sharpe, a senior meteorologist at Weatherzone, said there was a "fairly good chance" of an east coast low developing, but it would not be as ferocious as last week's.

"It's not going to be nearly as strong, it's not going to stay near the coast nearly as long, and the winds might become strong at times on Friday or Saturday, but they aren't going to last very long and they're not going to be peaking above the 100km/h wind gusts at this stage," he said.

Mr Sharpe said Sydney would get showers fairly consistently from Wednesday until at least Saturday, and possibly Sunday.

"Today and tomorrow are going to be pretty nice weather in Sydney," Mr Sharpe said.

"We're not going to get any of those big thunderstorms or east coast lows today or tomorrow, which will be nice for Sydneysiders to get a bit of clean-up done after the big storms we have had."

(Source: meteorological models posted by Ken Kato)

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