A large trough and cold front is creating wild thunderstorms as it collides with a humid, unstable air mass.
Severe weather warnings are in place for much of western NSW and Victoria's central third including Melbourne and the northeast, with damaging winds over 125km/h, hail and heavy rain expected on Friday afternoon.
"We could see heavy rain, which leads to flash flooding, and we could see large or even giant hail up to five centimetres or more," Bureau of Meteorology forecaster Angus Hines said in an update.
The bureau has also issued marine wind warnings for most of northern Tasmania.
Severe Weather Update: Severe — Bureau of Meteorology, Australia (@BOM_au) #thunderstorms possible for much of eastern Australia on Friday. Video current: 11:30am AEST 18 October 2024. For the latest forecasts and warnings, go to https://t.co/4W35o8iFmh or the BOM Weather app. pic.twitter.com/kaVlwnoGcuOctober 18, 2024
Wild winds, hail and flash flooding hit parts of South Australia overnight on Thursday, with gusts of more than 137km/h at Port Pirie and a 36mm deluge in just an hour at Mount Horrocks.
Storms have been easing in and around Adelaide but just under 4000 customers were still without power on Friday afternoon.
Over 3000 homes and businesses in western Victoria were facing outages and while AusNet Services had restored power to some in the east of the state, it had extra crews ready for further disruptions.
Extensive damage to transmission towers between western NSW's Buronga and Broken Hill have left more than 1600 powerless, with some temporarily connected via backup generators.
Victoria's State Emergency Service duty officer Shane McBride urged motorists to be vigilant and rethink unnecessary travel.
"Anyone today who's thinking of travelling around, it's really about planning a trip (and) thinking about whether you need to be on the road at all," Mr McBride said.
He warned people not to drive, ride or walk into flood waters.
"It only takes 15cm to float," he said.
"If you have to be on the road, be very mindful. Be patient. There will be traffic delays."
Casterton, in Victoria's west, copped a 20-minute bucketing on Wednesday featuring high winds, 5cm hail and flash flooding
"Everyone is in shock by the sheer ferocity of it," local Heidi Herbert from Herbert's Bakery said.
"I've never heard a noise like it before. It was like a freight train but worse."
The SES had received more than 260 assistance requests between Thursday night and Friday afternoon, with more than 100 reports of building damage, 86 flooding incidents and 35 trees down statewide.
Golf ball-sized hail fell at Casteron. (Supplied by Xxx/AAP PHOTOS)
Addressing reporters in Ballarat, premier Jacinta Allan would not be drawn on whether there would be a government support package to help Casterton residents clean up.
"We need to let the assessments be undertaken," she said.
"They're being undertaken right now, noting there is still a lot of ... difficult and potentially dangerous weather that is coming through the state over the course of the day."
Flood watch warnings could be issued in the coming days but the front itself is expected to drag the severe weather offshore by Saturday afternoon, with showers easing despite gusty, southerly winds.
Crowds heading to the MotoGP at Phillip Island, southeast of Melbourne, have been told to reconsider the trip.
There's also an increased risk of thunderstorm asthma for northern parts of Victoria.
The wild weather is expected to ease in eastern and southeastern Australia by Sunday, for a mostly dry and relatively sunny close to the weekend.
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