Month’s worth of rain could fall in one day in east coast black nor’easter.

Forecasters are warning Friday could be a very dangerous weather day as a black nor’easter descends with particular fears for Sydney.

More than the average rain for the entirety of April could fall on Sydney on Friday as a so-called “black nor’easter” weather system bears down on the country’s east.

A 71-year-old man died on Thursday as the same system swept through south east Queensland.

Now forecasters are warning that so much rain could fall over the three day rain event, which reaches its zenith on Friday, that several Sydney rivers are in danger of bursting their banks in the city’s west and north.

“We are well and truly into the 200mm plus range (of rain) in Sydney and the Illawarra – and in parts of the south coast 300mm,” said the Bureau of Meteorology’s (BOM) Angus Hines.

“It is very, very wet: flash flooding and riverine flooding are both possible”.

The BOM’s severe weather warning are stretches 500km in NSW. Picture: BOM.

Black nor’easter

The cause of the treacherous weather is the meeting of cooler moisture from the south with tropical moisture from the north.

The clashing of the two can lead to what’s known as a “black nor’easter”.

This is when strong north easterly winds combine with all that moisture creating dark brooding clouds full of rain set to burst.

It’s more than just the clouds colour that’s menacing: they also bring drenching rains, slow moving storms, gale force winds and hazardous surf.

Rain has been steadily pouring across Sydney overnight but it is expected to ramp up during Friday.

The BOM has issued a severe weather warning for a huge area of the New South Wales coast stretching some 550km from just south of Newcastle to Merimbula, on the south coast.

The warning area includes the central coast, Sydney, Wollongong and Nowra and pushes as far inland as the Blue Mountains, Bathurst, the Southern Highlands and Canberra.

“Dangerous and life threatening flash flooding is possible,” the organisation said with potentially up to 300mm falling on the Illawarra escarpment.

The conditions could last well into Saturday afternoon.

Gosford on the central coast is looking at 90-110mm on Friday followed by up to 35mm on Saturday.

By Saturday, 300mm of rain could have failed in some areas near Sydney and Wollongong. Picture: BOM.

Month’s rain in a day – or just hours

Sydney could see between 50-100mm on Friday and up to a further 50mm on Saturday.

According to the BOM, the median rainfall for the CBD in April is 92.5mm meaning an entire month’s worth of rain could fall in Sydney just on Friday.

It could be even heavier in the city’s west with 35-140mm for Penrith on Friday. In Katoomba, atop the Blue Mountains, Friday’s rain could peak at 150mm.

On Friday, Wollongong is predicted to receive 35-130mm followed by 20-80mm the following day. The city’s average April monthly rainfall is 75mm so it could almost double that in a single day.

Major flooding could occur on the Nepean and Hawkesbury rivers in Sydney. Picture: BOM.

Real fears for Sydney flooding

The sheer amount of water has to go somewhere and there are real concerns for river levels. The good news is major flood warning are only (yet) in place for three NSW rivers – the bad news is all three could heavily affect Sydney.

The Nepean, Hawkesbury and Colo rivers could all overflow potentially engulfing buildings and roads in Sydney’s west such as the suburbs of Richmond and Windsor. In the past, flooding in these locations has claimed lives.

Heavy cloud over Brisbane as the weather system passed through. Picture: Lyndon Mechielsen/Courier Mail

Death in Brisbane

This bout of weather has already claimed the life of one person. Peter Wells, 71, died on Thursday when his car was swept away by flood waters in Greenbank, in Brisbane’s south.

The white ute of Mr Wells, who was a grandfather and respected and well known community member, was found on a river embankment.

Brisbane is still seeing heavy rain with 15-45mm expected on Friday and potentially top ups of 25mm on Saturday and Sunday. Heavier falls still are possible on the Gold Coast.

There are a number of flood warnings in place for rivers across SEQ.

The rain will become markedly less heavy the more inland you go. You would hardly call Canberra dry this weekend, but its predicted maximum of 40mm of rain on Friday, and the same again on Saturday, is many times lower than costal communities.

The wet weather will eventually push into Victoria but the worst of it is likely to be isolated to the state’s far east.

Mallacoota is expected to see between 10 and 45mm on Saturday with Wodonga on perhaps 15mm.

Melbourne will be relatively untroubled by the black nor’easter with a few showers on Sunday.

Western areas of Tasmania could see some heavier showers toward the end of the weekend including some moisture for Hobart.