Another big rain event is heading eastwards, with the potential to soak saturated and already flooded parts of New South Wales and cause Sydney’s main reservoir to resume spilling.
A low-pressure trough will develop into a low, bringing widespread falls and thunderstorms, with the heaviest rain to arrive from Sunday into Monday.
For the flood-hit Lachlan catchment in central NSW, the system will drop another 30-50mm of rain, Ben Domensino, a senior forecaster at Weatherzone, said.
“It’s likely to cause some renewed flooding, particularly in the central west of NSW,” he said.
Another weekend rain event is looming for NSW/ACT. Model consensus on the details is limited but it's likely to be problematic for some already sodden parts of the state. pic.twitter.com/u8FP3NZHaE
— Andrew Miskelly (@andrewmiskelly) November 17, 2021
Sydney will also cop a drenching, with the Bureau of Meteorology forecasting 30-50mm for Sunday, “with heavy falls possible in the west” of the city.
The track the low takes will make a big difference to local rainfall tallies.
One weather model has the system tracking further north, reaching the coast near Newcastle. “Further north, there’s more moisture for it to tap into,” Domensino said, adding that path could result in Sydney collecting as much as 100mm on Sunday into Monday.
A second model has a more southerly course for the low, with regions south of Wollongong likely to receive the 100mm or so totals, he said.
NSW’s regional dams are 92% full, with some at or above capacity. Burrendong in central NSW is at 129% capacity, making use of its flood zone.
Sydney’s reservoirs are at 96.3% capacity, with Warragamba – which holds about 80% of the city’s water – at about 100%.
“Warragamba is not spilling but is in drawdown operations to reduce the storage from the full supply level,” a WaterNSW spokesman said.
“The storage is kept at one metre below full supply level, however that target is unlikely to be reached before the weekend,” he said.
Warragamba is currently receiving about 7.5 gigalitres of inflow a day and about 9GL a day is being released to create “airspace” to accommodate some of the expected falls.
The bureau’s forecast for Warragamba is currently at 25-50mm of rain on Sunday.
It's looking rather wet for parts of #NSW this weekend especially from Saturday night and through Sunday. It's looking like there could be further #warnings issued and if you're anywhere from the western slopes out to the east it's worth keeping an eye on the forecast. pic.twitter.com/IcTZe8bgnF
— Bureau of Meteorology, New South Wales (@BOM_NSW) November 18, 2021
More rain will follow within days as another low-pressure trough stretching from Queensland to Victoria moves eastwards.
That system, though, is unlikely to develop into a low, Domensino said. Rainfall will be widespread and, with catchments already saturated, more flooding could follow.
With conditions in the Pacific close to La Niña levels, these events are “the taste of what we will see this summer”, he said.
During La Niña events, easterly blowing equatorial winds strengthen, shifting rainfall westwards towards Australia. Such events increase the odds for a more active than usual cyclone season for the region.
The Bureau of Meteorology may declare a La Niña event as soon as next Tuesday. Six of the seven models used by the bureau forecast a La Niña will last until January.
