The “rain bomb” that soaked Brisbane and triggered record floods in northern NSW will develop into an east coast low and dump as much as 200mm of rain on Sydney with “life-threatening” risks on Wednesday.
The Bureau of Meteorology has more than doubled its rainfall forecast for Sydney to 150-200mm on Wednesday alone, and predicted the possibility of a severe thunderstorm and gusty winds by the evening.
“Heavy rainfall which may lead to flash flooding is forecast to develop over parts of the Hunter and Metropolitan, Illawarra, South Coast and parts of Central Tablelands and Southern Tablelands Forecast Districts from tonight,” the bureau said in a new warning. “Six-hourly rainfall totals between 80 and 120mm are likely.”
“Locally intense rainfall leading to dangerous and life-threatening flash flooding is possible with thunderstorms with six-hourly rainfall totals up to 200 mm possible,” it said.
Dean Narramore, a senior bureau meteorologist, said the east coast low was likely to be more compact compared to similar recent systems such as in June 2016 or that which triggered floods near Sydney in March 2021. The event’s intensity depends on how close the low forms and how long it lingers near the coast.
“The worst of the weather, the strongest winds, the heaviest rainfall and the worst of the beach erosion will occur near and just south of where that [system] crosses the coast, if it does, or when it comes close to the coast,” Narramore told a media conference in Sydney.
Risks include landslips and mudslides as had been experienced in northern NSW as the rain shifted south from Queensland. Hazardous surf will extend along the whole length of the state’s coast on Wednesday and Thursday, bringing the threat of coastal erosion too, the bureau said in a separate warning.
As trough shifts south, so does the rain on Tuesday, with Sydney and other areas set for a soaking. Falls of 80-120mm are possible, @BOM_NSW warns: https://t.co/TKZsfGfKV3 pic.twitter.com/4bc8zxl4wV
— Peter Hannam (@p_hannam) February 28, 2022
The source of the rain was a trough that would form into a low pressure system off the northern NSW coast before moving towards central parts of the NSW coast and deepening into an east coast low, the bureau said.
Areas hard-hit by flooding in south-east Queensland and northern NSW gained some relief on Tuesday as the trough moved offshore.
Ben Domensino, a senior meteorologist with Weatherzone, said people in a stretch of the coast from Gosford southwards, including Sydney, should prepare for significant falls.
And an east coast low could develop off shore on Tuesday, with more heavy falls then: @BOM_au pic.twitter.com/yIKQpKJ0gT
— Peter Hannam (@p_hannam) February 28, 2022
“What the models are agreeing on is that a low [pressure system] will gain strength and move towards the coast somewhere over the central or southern coast of New South Wales,” Domensino said.
“But I think for the south coast, the Illawarra, you can pretty much lock in the heavy rain and flood potential.”
The bureau predicted Wollongong will receive 150-250mm on Wednesday, and another 30-50mm on Thursday.
And over the coming eight days, some of the rainfall totals could get into the 100s of mm...in catchments that are already pretty wet, with dams that are already nearly full or spilling: @BOM_au pic.twitter.com/HBVqqBZSEj
— Peter Hannam (@p_hannam) February 28, 2022
WaterNSW on Monday said Sydney’s main reservoir, Warragamba Dam, would probably start to spill into the Hawkesbury-Nepean floodplain later this week.
The bureau’s forecasts now have Warragamba receiving as much as 100mm to 150mm of rain on Wednesday and 30mm to 50mm on Thursday.
Warragamba was at 99% capacity on Tuesday, and the catchments were either full or close to it, with several likely to spill. The city’s entire dam network was 97.6% full.
“The upper range of the predicted spill peak from Warragamba dam has increased overnight, though it remains at a rate still well below the spill experienced in March 2021,” WaterNSW said.
“[D]ownstream river level increases are likely and will also be influenced by the volume of inflows from tributaries downstream of the dam,” the agency said.
“Tallowa and Cordeaux Dams are already spilling and other Upper Nepean storages likely to spill in the days ahead include Nepean, Avon, and Cataract, along with Woronora Dam to Sydney’s south,” it said.
The event could bring damaging wind gusts peaking in excess of 90 km/h over the coastal fringe in the warning area from Wednesday, the bureau said.
With sodden ground, strong winds could bring down trees, adding to the risks and also disrupting power supplies.
According to the bureau’s regional floodwatch, Hawkesbury and lower Nepean rivers face minor to moderate flooding. The upper Nepean River is likely to have moderate to major flooding, while the upper Coxs and Colo river is set to have minor to moderate flooding.
