Victoria weather warning for heavy rainfall and possible flash flooding as La Niña arrives

A rapidly intensifying system over central Australia is forecast to dump up to 120 millimetres of rain and could cause flash flooding in parts of Victoria.

The system could also produce wind gusts of up to 100 kilometres per hour off the coast.

The Bureau of Meteorology (BOM) is warning the rain could lead to flash flooding in western and central parts of the state late today, and in the north-east ranges early on Thursday.

Forecasters have been monitoring the low-pressure system, which is linked to tropical moisture levels not seen for years.

The low is expected to rapidly deepen this evening over the northern part of the state before moving south on Thursday.

BOM senior forecaster, Kevin Parkyn, said the weather pattern heralded the start of La Niña.

During a La Niña phase, Australia's northern waters are warm, typically resulting in increased rain for eastern and northern Australia.

"The atmospheric conditions are just ripe for that system to intensify tonight over Victoria," Mr Parkyn said.

"We call that process cyclone genesis."

There could be flooding on the north-east ranges, with falls of between 80 and 120mm and heavy bursts of up to 60mm at times, he said.

Mr Parkyn said that might not sound a lot but it could cause other problems.

"The concern is we'll see these sort bursts over a few hours, particularly in the north and about the dividing range were we could see falls of 50 to 60 millimetres," he said.

"That's a real concern overnight."

A flood watch is current for north-eastern Victoria and a severe weather warning for heavy rainfall is current for the Mallee, Northern Country, North Central, Wimmera and parts of the Central, South West and North East forecast districts.

There will be widespread falls of 10-30mm across the state.

Melbourne is forecast to get up to 20mm of rain, potentially surpassing its average annual rainfall almost three months before the end of the year.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume.

Watch

Duration: 1 minute 22 seconds1m 22s

Don't make a 'deadly' decision: SES

On Thursday, the focus will shift to damaging winds over the south-west, central coasts and possibly Alpine areas.

"It looks like it will be most intense on Thursday morning just off the south-west coast and that's probably where we're most concerned of these 90-100 kilometre per hour gusts," Mr Parkyn said.

Premier Daniel Andrews appealed to Victorians to be aware of the conditions.

"That just makes the job of our emergency services heroes … just that little bit easier," he said.

Tim Wiebusch, the chief officer of operations at the State Emergency Service, urged people not to drive through floodwaters, calling it a "deadly decision".

"Research shows most fatalities … occur in floodwaters. It only takes 15 centimetres of water for a small car to float," he said.

"That's the size of the average pen."

Three people, including a four-year-old boy, were killed by falling trees in wild weather in August.

He said people should stay indoors where possible.

"If you don't need to be outside during the storm, don't be outside," he said.