Heavily populated areas of New South Wales are set for a soaking today, with wild weather and torrential downpours predicted across the state.
Sydneysiders are being urged to reconsider their morning commutes, advised by police to stay off the roads with expectations to expect months-worth of rain in just a few hours.
Airlines are telling travelers from Sydney, Brisbane and the Gold Coast to prepare for significant delays as adverse conditions make flying dangerous and difficult.
Up to 100mm could fall across Sydney, the Hunter and Illawarra, the Bureau of Meteorology says – well above the monthly average. Some places could top 200mm as an intense low pressure area in cities.
Sydney rivers including the Hawkesbury, Nepean, Cooks and Georges are all on flood watch, while the State Emergency Service is preparing to mobilise thousands of volunteers.
It's unclear where the rainfall will be the most intense, but the weather bureau has warned of damaging winds and possible flash flooding, expected to reach the NSW coast during a morning peak lasting for six to 12 hours.
"We're asking all road users to probably reconsider the need to be on the road through what a heavy rain event," NSW Police Chief Inspector Phillip Brooks told reporters on Tuesday.
"Drivers, riders, cyclists and pedestrians need to make sure they are safe on our roads."
NSW State Emergency Service Assistant Commissioner Scott Hanckel said parents should think about alternatives for the school drop-off and pick-up and advised businesses to expect workers to arrive late.
"It's a great day to work from home – if that's appropriate," he told AAP. He warned drivers not to flood into flood waters, with that being the overwhelming major cause of flood deaths.
Sheep graziers have warned that cold temperatures, rain and strong south to south-easterly winds are expected during Wednesday, increasing the risk of losses to lambs and sheep exposed to these conditions.
Areas likely to be affected include the Illawarra, Southern Tablelands and Australian Capital Territory forecast districts and parts of the South Coast, Central Tablelands, South West Slopes and Snowy Mountains forecast districts.
Simon Lewis, New South Wales Severe Weather Manager for the Bureau of Meteorology, said Wednesday's downpour could lead to roads awash with water, rapidly rising creeks and hazardous surf until Friday morning. However, "one good thing" about the system was that it would pass quickly.
"It will cross the coast somewhere between Sydney and Wollongong tomorrow and then move fairly rapidly offshore," Mr Lewis predicted. "We're not expecting a very long duration of heavy rain, but we are expecting to see very intense falls sometime tomorrow morning and persist through through tomorrow afternoon."
The culprit is a huge low pressure system, expected to rapidly intensify as it crosses the coast between Sydney and Wollongong early this morning.
'WILD SUPERCELL THUNDERSTORMS'
While meteorologists have dramatically upped the rain forecast for New South Wales, "wild supercell thunderstorms" in Queensland could hit the south east of the state on Wednesday as central areas continue to wither under a heatwave stoking bushfires.
"A dangerous rain and wind event is on the way for NSW with Sydney and Wollongong in the firing line with heavy rain, followed by powerful winds as a low pressure system comes in," saidSky News Weatherchannel meteorologist Rob Sharpe.
"It's going to be a vigorous system – short and sharp."
It had been forecast that Sydney could see up to 50mm of rainfall over a two-day period beginning on Wednesday. Now the Bureau of Meteorology is saying that as much as 100mm could bucket down the city in just a few hours as a concentrated low pressure area in the head.
To put that into perspective, November's average rainfall in Sydney is 66mm. So the city could see more than a month's rain in a single day – or even just a few hours
A DRAINCHING ACROSS THE STATE
It's not just Sydney that is about to drenching. Wollongong could see as much as 150 mm of rain, Nowra 100mm, Katoomba 90mm and Wagga and Canberra 30mm. Some isolated coastal and exposed areas could be walloped with as much as 200mm.
"Current forecasts indicate intense rain could occur in major population centers from Wednesday morning. Damaging winds and hazardous surf conditions are also anticipated, "said the BOM in an alert.
A severe weather warning is in place for the NSW coast from the north of Newcastle to the south of Nowra and inland to the Southern Highlands and Blue Mountains. Rain, though less of it, can be expected across much of the state.
Mr Sharpe told news.com.au the severe weather warning for damaging winds was also worth noting.
"The most important feature of the winds is that they peak after most of the rainfall has arrived. This means that the ground will be soggy and a little bit looser making it easier for trees to come down, "he said.
"This event is likely to lead to flooding, trees and powerlines down and so we'll also have power outages with the worst of the weather."
Other capitals will generally dry with the odd spot of rain in Melbourne. But parts of Queensland could also get a battering.