VICTORIA is bracing for a dangerous storm that will batter the state, dumping three months' rain in three days.
An army of 5000 State Emergency Service volunteers is standing by, as experts warn the downpour could match the rain produced by the cyclones that ravaged north Queensland.
"It poses a threat to life,'' weather bureau senior meteorologist Scott Williams said.
"There will be a massive amount of lightning, there will be roads cut, floodwaters."
"I think this event will turn farms into lakes."
More than 300mm of rain will fall across all parts of the state, starting in western Victoria and arriving in Melbourne by midday.
Rain hit the state's southwest on last night as a trough of low pressure churned towards Melbourne.
A swathe of western Victoria was affected, from Horsham to Cape Otway on the coast.
The Bureau of Meteorology issued a renewed warning late last night for heavy rain, scattered thunderstorms and flash flooding.
TIMELINE OF EVENTS:
- From 6am, rain is expected in western parts of Victoria
- From 8am to 11am, rain is expected in Geelong and western parts of Melbourne
- By lunchtime it will pass through Melbourne
- By 5pm, it will be in eastern parts of the state
- 150mm to 200mm for Ballarat and Bendigo
- Melbourne will probably see, at the very least, 50mm to 100mm
- The surrounding hills and catchments, will see 100mm to 150mm
The freak weather event - as a fierce low pressure-system slams into a stifling layer of hot air - is predicted to outdo anything seen in Melbourne since 2005, and in regional Victoria since 2010.
Severe weather, flood-watch and thunderstorm warnings have been issued for the entire state, vast areas of which could be at risk of being completely cut off.
Sporting events and festivals were among a growing list of events that have been cancelled or postponed, amid fears of flash flooding and road closures.
A slow-moving trough will stall as it moves over the state, deepening into a low-pressure system drawing in very moist tropical air from the northeast, producing the high rainfall.
Melbourne is likely to receive more than 150mm of rain over a 72-hour period, and the state's northeast and alpine districts another 300mm.
Some city and regional councils had begun preparing sandbags in case of storm surges in low-lying areas.
Farmers in grain-growing regions were racing to prepare as the system moved in.
"This could be disastrous,'' Victorian Farmer's Federation grain group president Ross Johns said.
"The fear is that this rainfall will significantly reduce yield and damage the quality.
"So it has been 'all systems go' as farmers try to harvest as much as they can before the weather hits,'' he said.
The federation's president, David Jochinke, said the humidity following the rain would be quite challenging for farmers and pastures could be leached of nutrients with the deluge of water.
Senior forecaster Richard Carlyon said the slow-moving nature of the weather system made the situation serious.
He said the heavy rain band would reach Victoria's western border overnight and extend eastwards today.
The heaviest rainfalls and thunderous conditions will develop in Melbourne this afternoon and into Saturday.
WHAT WE KNOW SO FAR:
- All parts of Victoria has been issued with a severe weather warning.
- There will be thunderstorms in every district across the weekend.
- At least 300mm of rain will fall across the state between Friday and Sunday.
- Storms will begin in western Victoria on Friday morning.
- Storms will reach Melbourne Friday midday.
- The worst of Melbourne's rainfall will take place on Friday evening and Saturday morning.
- WORST HIT: North East Victoria
- WORST IMPACT (by people): Melbourne
The Mallee will cop a drenching first before the intense low moves to the northeast on Saturday. The highest rainfall is predicted for alpine regions, before the system moves east of Bass Strait.
Mr Carlyon said Melbourne was in the firing line, but regional centres to the north, including Bendigo, Echuca, Shepparton and Albury, would also be drenched. The northeast ranges could get more than 250mm, which could bring a rapid rise in river levels.
Premier Daniel Andrews urged Victorians to be ready.
"This will be a very challenging period for our state and we will be able to deal with those challenges because of the bravery, the training of our emergency services," Mr Andrews said.
"And I hope for common sense and good judgment from all Victorians,'' he said.
"Please listen to those warnings. Heed them. That is the best thing that you can do to keep yourself and your family safe."
Emergency Services Commissioner Craig Lapsley said "vulnerable" Victorians in low lying areas should consider relocating for the weekend.
"Now is the time to ask for family support, neighbourhood support," he said.
"We also need to reach out and be good community citizens. We need to reach to the people who might need our help."
People were also warned not to park cars in underground carparks and to ensure gutters were cleared.
SES operations manager Tim Wiebusch urged people not to drive through floor water.
"We are particularly keen to make sure people don't drive through flash flood waters," he said..
"It only takes 15cm of water for a small vehicle to float.
"We don't want to unnecessarily tie up our valuable emergency service resources over the next few days with vehicles stranded
"Now is the time for Victorians to start being alert to our weather conditions over the next three days."
Mr Williams, who works at the Bureau's Extreme Weather Desk, said the weather event would be muggy, but cool southerly winds would roll in on Sunday.
"It will be very humid and unstable across the state," he said.
Bureau of Meteorology senior forecaster Richard Carlyon said he expected to see conditions "never seen before in parts of the state".
Mr Wiebusch said holiday-makers should take precautions to avoid being caught in floods.
"Make sure you're not setting up your camp alongside rivers across Victoria … because we are likely to see moderate to major flooding particularly in the northern parts of the state and the Gippsland area," he said.
"Heed the emergency services warnings, stay tuned to your emergency service broadcasters and stay tuned to the emergency.vic.gov.au website for the latest emergency information."
7 News Meteorologist Jane Bunn said flash and river flooding would strike during the "significant weather event" and it had to go somewhere.
"We have had that amazing month of November," she said.
"We had that high ... which jumped to the southern the Tasman Sea where it sat for a while which allowed the north-easterly that settled over here."
Ms Bunn said the scale and scope of the flooding would be akin to the floods of 2010 which hit central Victoria, particularly Bendigo, extremely hard.
Ms Bunn said Melbourne was right in the middle of the low-pressure system moving across the state too.
Sky News meteorologist Tom Saunders said there was a volatile mix of warm and cold air colliding.
"Before the rain arrives, southeast Australia is ending one of the hottest Novembers on record with another spell of extreme heat."
Mr Saunders said the unprecedented heat had been due to a blocking high over New Zealand which could be the result of weak La Nina conditions forming over the Pacific.
"Warm air colliding with cold air is a volatile mix and will lead to the formation of a deep and complex low pressure system over southeast Australia on Friday," Mr Saunders said.
"The low will remain in the vicinity of southeast Australia until early next week and lead to well over a month's worth of rain in just a few days for some regions."
"I don't think I've ever seen such a deep (low pressure system) form over Victoria while we've got all this moisture around … we are in uncharted territory," Mr Williams said.
"We're likely to see the development of a pretty major low pressure system over Victoria on Friday and into Saturday morning coming from the west which is going to lift all this tropical air and as a result produce widespread thunderstorms (and) very heavy rain.
"The most likely period for flash flooding and widespread heavy rains is Saturday morning, including the early hours."
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