Warning issued as Cyclone Debbie remnant collides with cold front

The Bureau of Meteorology on Wednesday issued a severe weather warning for heavy rainfall and damaging winds for the coastal fringe of NSW from Sydney north to the Queensland border.

"A low pressure system over central-eastern Queensland and a high pressure system over the Tasman Sea are dragging a humid tropical air mass over parts of NSW," the bureau said. "Heavy rainfall over northeastern NSW is likely during Thursday and Friday as a cold front approaches from the south-west and interacts with this humid airmass."

On current forecasts, Sydney can expect a warm, sunny Wednesday with a top of 29 degrees. The unsettled conditions, though, will usher in "an extended run" of cooler conditions from Thursday,as daytime temperatures drop back to 25 degrees and then just 22 degrees for several days afterwards, said Brett Dutschke, a senior meteorologist with Weatherzone. 

For Sydney, widespread rainfall totals are likely be about 20-40 millimetres "with an outside chance of more than 50 millimetres for some areas", Mr Dutschke said.

The heaviest falls in NSW are likely to be in the state's north-east corner from about Coffs Harbour north.

"24-hour totals in excess of 100 millimetres are expected over the Northern Rivers district during Thursday, and it is likely that some locations will exceed more than 200 millimetres," the bureau said.

Sydney's rainfall is likely to push the March total above 300 millimetres, or more than twice the norm for the month. It is already the city's wettest March since 1984.

The warning of coming rough weather for Sydney is more from the wind than rain, the bureau said: "Damaging winds averaging 65km/h with gusts in excess of 90km/h are possible along the coastal fringe north of about Sydney from Thursday afternoon, extending northwards during Thursday night and Friday". 

Dangerous surf conditions are also likely, particularly on Friday, Mr Dutschke said.

Saturday is likely to be generally dry but the arrival of another cold front on Sunday should see showers redevelop "with the odd heavy falls on the coast", he said.

The slow-moving nature of Cyclone Debbie and its remnant low have brought sustained intense winds and heavy rainfall to parts of Queensland.

Unofficial totals of more than a metre of rain have been recorded in the hinterland ranges near Mackay, Mr Dutschke said. The 263 km/h winds at Hamilton Island were the strongest recorded in Queensland and second-highest in Australia.