Sydney has been warned to prepare for localised flash flooding and traffic issues this week as heavy rain is forecast to hit the NSW coast.
Sydney is likely to pick up its monthly rainfall average of 126mm during the next four days, with most of the rain falling tomorrow and Wednesday, Weatherzone meteorologist team leader Alex Zadnik said.
When 100mm falls in less than two days, low-lying roads tend to go under water and cause traffic issues around the city.
Rainfall rates of 15mm an hour or greater are possible for the city and suburbs, Mr Zadnik said.
These rates are sufficient to cause flash flooding.
The rain may continue into Thursday as well, although this will depend on the precise position of a low pressure trough near the coast, he said.
The trough system should move away from Sydney on Friday, causing the rain to ease back a notch, although showers will linger.
Much of the NSW coast is in for a wet week, potentially the wettest week in a year, bringing the threat of flooding, Weatherzone meteorologist Brett Dutschke said.
It may turn out to be the wettest week for coastal areas since July or even March last year, when many roads, homes and properties were inundated.
"The system is showing some similarities to the one in March 2011, which dumped flooding rain from the Hunter to the south coast over three or four days," Mr Dutschke said.
During that period more than 400mm of rain fell in parts of the Illawarra and south coast and more 200mm in parts of Sydney, the central coast and Hunter.
Wollongong had its heaviest rain in 20 years, gaining about 250mm in just 24 hours.
"This week's trough system should bring falls in excess of 100mm to an area from Tweed Heads to Merimbula by the weekend. The wettest area looks like being between Port Macquarie and Nowra. Some areas are a chance to accumulate more than 300mm by the weekend, more than double the monthly average," Mr Dutschke said.
Showers and storms are already affecting the north-east of the state and are becoming more frequent as the low pressure trough forms over the region.
Newcastle picked up 23mm of rain overnight, its heaviest fall in two months.
"Over the coming few days, this trough will deepen just offshore and slide slowly south, taking bursts of heavy rain with it. It will be fuelled by warmer-than-average seas."
From Friday some drying out will occur, initially in the north and later in the south, but it will not be long before rain returns.
"There will only be about one day's break from the rain before another trough moves in from the west and brings another burst.
"Thankfully, that rain will only be relatively brief and light. Most of it should fall over inland NSW, mainly the north, where it has been fairly dry in the last couple of months.
"Looking ahead deeper into autumn and winter, the chance of flooding rain is reduced. This is largely due to cooling waters and a neutral phase taking over from La Nina, which has only recently ended," Mr Dutschke said.
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