The trigger, a cold front colliding with a warm and humid northerly airstream, is expected to deliver severe conditions over the next 48 hours.Heavy rain capable of producing flash floods is likely along with damaging winds, hail and lighting.
The storms are forecast to arrive first across South Australia on Thursday afternoon and evening.
"This is a rally broad weather system, with mass storm potential," senior Bureau of Meteorology forecaster Miriam Bradbury told AAP.
"Storms can be expected today across NSW, South Australia, Victoria and even the southern Northern Territory."
Severe Weather Update: Risk of severe thunderstorms for parts of central & southern Australia today. Video current: 1:30pm AEST 17 October 2024.— Bureau of Meteorology, Australia (@BOM_au) Latest forecasts and warnings: https://t.co/4W35o8iFmh or the BOM Weather app. pic.twitter.com/NwxuRFDF9wOctober 17, 2024
Casterton, in Victoria's west, copped a taste of things to come when lashed on Wednesday afternoon by what locals said was the worst storm they had encountered.
"Everyone is in shock by the sheer ferocity of it," Heidi Herbert from Herbert's Bakery said.
"I've never heard a noise like it before. It was like a freight train but worse ... I stood outside on the verandah and it was just deafening.
"We saw the clouds come in and the thunder and lightening but had no idea this was coming. It all happened so quickly, it caught everyone by surprise."
The bureau said Casterton copped a 20 to 25 minute bucketing which included strong winds, 5cm hail and flash flooding.
"There was heavy rainfall of about 22 mm in 20 minutes ... that doesn't sound like heaps of rain but when it falls in 20 minutes that's significant," Ms Bradbury said.
The State Emergency Service took more than 80 calls, 65 relating to building damage or flooding.While businesses in the historic community suffered collapsed ceilings, broken windows and flooding, Herbert's Bakery is already back open.
"I think everyone is in shock but fortunately no one is hurt," Ms Herbert said.
The hail in Ms Herbert's yard at Casterton was up to 5 cm in diameter. (Supplied by Xxx/AAP PHOTOS)
The system will continue east on Friday across inland NSW and northern Victoria, with Melbourne also likely to experience damaging winds, hail and rain.
Senior Meteorologist Kevin Parkin said severe thunderstorms would be localised, with expectations of "giant hail", intense rainfall and flash flooding in the northeast.
"You'll have some suburbs that will see a bit of weather and next door there'll be carnage with branches off trees, flash flooding, skylights damaged, solar panels damaged from large hail, crops damaged, things like that," he told reporters.
"So it can be very hit and miss."
Crowds heading to the MotoGP at Phillip Island, southeast of Melbourne, have been told to reconsider travel plans.
"Try and hunker down, make sure you're where you need to be by (Friday) morning or put your plans off through those high risk areas," Victorian SES commander David Baker said.
There's also an increased risk of thunderstorm asthma for northern parts of Victoria.
While southeast Queensland and the rest of NSW may experience storms, they were not expected to be severe.
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