Victoria is facing a "major weather event" in the coming days as thunderstorms threaten to dump torrential rain and cause flooding in almost all of the state.

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Forecaster Scott Williams, from the Bureau of Meteorology (BoM) said thunderstorms developing over western Victoria yesterday evening would move to other parts overnight and into today.

"Those thunderstorms will gradually all weld into a massive, great rain band, and that band will spread down across the state on Friday night and Saturday morning," he said.

"This is a vast, intense, high-impact event for this state."

Mr Williams said the rain would fall heavy and fast, with the potential of more than 50 millimetres in just an hour.

The biggest concern was for the north-east of the state, with "huge" amounts of rainfall to peak there on Saturday.

"It's not just 300mm at the top of the mountains, it's 300mm possible anywhere from Wangaratta to Hotham Heights," Mr Williams warned.

"It'll be over more quickly in the west, the Mallee and those areas, the main impacts [there] will be the thunderstorms, so the rain total out there will probably be generally 50mm to 100mm.

"There may be some local 200mm falls.

"As you go further east in the northern country towards Bendigo and down towards Ballarat, it's much more likely that Friday night they'll see 150mm to 200mm — heavy thunderstorm activity."

He said the rain would also push down into Melbourne overnight on Friday and into Saturday.

"Melbourne will see at the very least 50mm to 100mm. The surrounding hills, the catchments of the Yarra and Maribyrnong river probably more like 100mm to 150mm on Friday night," he said.

Winds will pick up on Saturday night, and Gippsland will be the last to be hit by the rain. Some eastern parts may see 100mm rain on Sunday, but it was too early to give an accurate forecast.

"In Melbourne on Sunday morning we could get another burst of 50mm, 60mm-plus of rain, that could coincide potentially with floodwaters from the the night before coming down the Yarra and Maribyrnong," Mr Williams said.

"If we're going to get gale-force south-westerly winds, a high tide, 150mm of rain in Melbourne over two days — that's three times the December average — we're going to have massive flooding."

Asked to rate the storms out of 10, Mr Williams said: "I'll take the punt and say it's a 10 for Victoria."

"So we're going to have major, major, major flooding problems right around this state, probably many roads cut … I suspect [infrastructure] will be impacted as badly as most of us have ever seen."

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Premier Daniel Andrews said Victorians needed to prepare for a "very challenging period", urging people to follow official warnings.

State and local emergency control centres have been activated as emergency services work on a four-day plan to cope with possible flash flooding.

"This is a very serious matter, and one that will pose a real challenge to communities right across the state and will be a direct challenge to public safety," Mr Andrews said.

"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."

Emergency Management Commissioner Craig Lapsley said authorities would get final details on the weekend forecast today, but asked people to think about where they would be and the conditions around them when the rain hit.

Thunderstorms are common in Victoria in November, but not on this scale.

Farmers at Natimuk, near Horsham, are working to harvest as much as they can before the rains. ( ABC News: Jessica Black )

Farmers on edge

Growers fear the deluge has the potential to wipe out an entire season of crops.

Victoria is the last place in the country to harvest grain, prompting a race against the clock to get as much of the yield in before the bad weather arrives.

The forecast rain could destroy the crops, pushing them over into the mud and making them impossible to harvest.

Stone fruit growers are facing the prospect of brown rot. Peaches and nectarines also suck up the moisture through the soil, which causes the fruit to split.

Farmer Mark Uebergang has only been able to harvest a quarter of his crop. ( ABC News: Jessica Black )

Cherry growers were expecting a bumper harvest heading into the Christmas period, which was good news for consumers, but a massive drenching could lead to splitting.

Chiltern orchardist Bill Hotson plans to take the drastic step of getting a helicopter from nearby Albury, in southern NSW, to blow air on his cherries after the rain to dry them off.

"The ones that are most affected are the ones that are nearing ripeness — the sugars come up and cause the skin to crack," he said.

Farmers in the state's wheat belt have abandoned their crops to torrential rain, after a frantic effort to get them safely into storage before the storms rolled in.

Grain receival sites from far west New South Wales to the Wimmera were forced to close early Thursday to protect the grain before the rain set in.

Mark Uebergang farms 500 acres at Natimuk, near Horsham. By late Thursday afternoon, more than three quarters of his crops were still in the ground.

"It is what it is, and it's just the nature of farming," he said.

"No one likes to see money slipping out. These beans that are in front of us. These could be under a metre of water, but I know that's the country I've got and I know it could happen some time."

'We just have to get into action'

Matt Zagami from Avagrow Farms, at Wairewa in east Gippsland, is trying hard to harvest his snow peas before the rain hits.

"[I'm a] little bit anxious. We've got a lot of vegetables in the ground and if we get too much rain at this time it can cause a lot of damage," he said.

Three or four days of high humidity will cause just as much risk to the crop as the rain itself, increasing the chance of blackspot and other diseases.

At Dicky Bill Farm in Maffra, spinach crops are most at risk of damage.

Harvest and production co-ordinator Beth Cotterill said a 15-minute hail storm wiped out 150 tonnes of spinach at the farm two weeks ago and there was a lot of nervousness about the days ahead.

"We just have to get into action to do what we can," she said.

Workers at Dicky Bill Farm in Gippsland are trying to pick as much spinach as they can. ( ABC News: Isabella Pittaway )

Ashley Mills, from the CFA in north-east Victoria, said the rain could push the fire danger period back by three weeks in some parts of the state — but he warned against complacency.

"If we've got high 30-degree temperatures and low humidity, we'll still see the potential for fires to develop quickly, even within three weeks after an event like that," he said.

The heavy rainfall could also pose a problem if it triggers growth, he said.

"So later on in the season we may well see some fuel loads that may present as a bit of an issue."