A severe weather warning has been issued for all of Victoria as the bureau prepares for what could be the state's most significant rain event in more than 20 years. The city will be lashed with more than 100 millimetres of rain – almost double its December average rainfall – over Friday and Saturday alone with thunderstorms and hail likely to accompany the downpour. Melbourne's catchments could receive 150 millimetres, causing serious flooding. The heaviest rainfall for Melbourne is set to come on Friday night with more than 10 millimetres an hour expected. "Half the inhabitants of Melbourne have probably never seen something like this," Mr Williams said.

"This is a vast, intense, high impact event for this state." State Emergency Service deputy chief officer Tim Wiebush said Victorians should prepare for the potential for flash flooding and riverine flooding between Friday and Sunday. "SES volunteers are on high alert with sandbagging collection points starting to be set up at various points across the state," he said. Victorians were warned not to go camping this weekend and were reminded that 15 centimetres of water on the road was all it took for a car to lose traction and float.

Major flooding expected After a scorching end to spring, Melbourne is set to get possibly an entire summer's worth of rain in the season's first three days. On a scale of one to 10 "it's going to be pretty close to a 10", Mr Williams said. "If you wake up [on Friday] and think it isn't going to happen you'll just have to wait a while. They didn't think the Titanic would sink either but it did." Mr Williams said the biggest risk for Melbourne would be extensive flooding, which can occur with more than 100 millimetres of rain. He said the city could receive that amount on both Friday and Saturday nights.

"We're going to have major flooding problems around the state, probably many roads cut. I suspect it will be impacted as badly as most of us have ever seen," he said. A 'double whammy' A low pressure system is also set to develop over eastern Bass Strait, which is expected to bring more rain on Sunday morning in southern and central Victoria with Melbourne likely to cop another 50-60 millimetres. Mr Williams said Sunday's deluge could coincide with floodwaters from Saturday coming down the Yarra River and Maribyrnong River. He said Port Phillip Bay would also be at a high tide, increasing the risk of floods.

The effect of both weather systems was a "double whammy" on Melbourne, he said. "The ground will be soaked after the first night, there will be a lot of flash flooding, the second night, the real worry turns to rivers," he said. Heavy rain is forecast to arrive in Melbourne by Friday. Credit:AAP The bureau predicts that all the major metropolitan river systems are at risk of major flooding, including the Barwon, Maribyrnong, Yarra and Bunyip rivers. Hardest hit areas

The north-east of the state, particularly places like Wangaratta, are forecast to be among the hardest hit with some areas expected to have more than 300 millimetres of rain over the weekend. The Yarra River, Maribyrnong River, Ovens River and Murray River are all included in the list of catchments that could experience flooding. From Swan Hill across to Albury Wodonga and in the alpine regions including Mount Hotham and Falls Creek a drenching of more than 250 millimetres is expected, which could cause flash flooding and rising river levels. Ballarat, Geelong and the Latrobe Valley are also expected to bear the brunt of the rain. Premier's warning Premier Daniel Andrews used Question Time in Parliament on Thursday to warn Victorians to exercise caution during the downpour, and to let emergency services workers and volunteers do their jobs.

"This is a very serious matter and one that will pose a very severe challenge to communities right across the state and will be a direct challenge to public safety," Mr Andrews said. He urged all Victorians to follow the advice of VicRoads, Public Transport Victoria and Victoria Police and not to drive into flood waters. "Please, listen to those warnings, heed them and that is the best thing you can do in order to keep yourself and your family safe and what's more, to not put our emergency services in harm's way having to come to your aid," Mr Andrews said. Andrew Gissing, a researcher with the Bushfire and Natural Hazards research centre, said it was difficult for people to judge the depth and current of floodwaters. "The biggest problem we see these days is people deliberately enter floodwater either on foot or in vehicles. The people in four-wheel-drives are the biggest risk we see at the moment," Mr Gissing said.

"Very few people know what sort of flood conditions your vehicle can drive through. If you misjudge the depth of flood water by just a few centimetres you can end up in a lot of trouble." Flash flooding on Elizabeth Street in February 1972. Credit:Neville Bowler Record-breaking rain Senior forecaster Richard Carlyon said the deluge was likely to be the most intense rain event since 1993 when the city received 89 millimetres of rain in one day.

"Across the state I'd say many records will be broken," he said. "We may remember it for a long time, it's hard to gauge how bad it will be. "It's like it had a calendar, it's uncanny in a way, starting on the first day of summer." The weather will be so extreme and potentially dangerous the bureau released its warning earlier than usual. "We don't normally give this much lead time, but it's 48 to 72 hours because we are concerned about safety," Mr Carlyon said.

"The type of rainfall we're talking about is unprecedented." The intense rain comes after the mercury reached nearly 35 degrees on Thursday just before 2:30pm, marking the 14th pre-summer day above 30 degrees, breaking a 2009 record. Flooding in Elwood after a heavy rainstorm on December 29, 2016. Credit:Wayne Taylor A disaster for farmers Farmers will be among those to suffer the most from the weekend's wild weather with many frantically trying to speed up their harvests ahead of the deluge.

Linc McClelland, a farmer from north-western Victoria's Mallee district, told radio station 3AW he was harvesting his crops of wheat, barley, canola, chickpeas and lentils until 4.30am on Thursday to try to beat the rain. "We've finished everything other than the wheat," he said. "Another two or three days and we'd be right, but the rain will probably beat us. We won't finish. "You don't often get long-range forecasts like this, so you try to do what you can. You just have to prioritise what's there." Victorian Farmers Federation president David Jochinke said it was "absolutely devastating to get this weather this time of year".

Flight delays, polo cancelled Flight delays are expected at Melbourne Airport on Friday but Airport spokesman Grant Smith, said it was not known how widespread they would be until the storm hit. He said the Airport would act on the advice from the bureau and SES.