Australia bushfires: Rain forecast, but is it enough?

It's been a long time coming, but a dousing of rain is finally coming to many of Australia's bushfire-ravaged zones - the question is, will it be enough to make a difference?

Starting on Wednesday, heavy rain is forecast for the east of the country and is expected to last into the weekend. Accumulated rainfall of between 30mm and 80mm was possible across the east of Australia, according to the Bureau of Meteorology (BOM).

While the forecast is bringing hope for many, meteorologists say only time will tell if it will be enough to put out the fires. 

Tom Saunders, chief meteorologist at Sky News' Weather Channel, said just how effective the rain is depends on many factors.

"It's a very difficult question to answer, because it does depend on the size of the fire, how intense the rainfall is, and also the types of vegetation that fire is going through," Saunders said.

Heavier rain, between 20mm and 50mm, would be needed to impact the fires, but it would need to fall steadily over a couple of days, Saunders said. 

"If it's from a thunderstorm, though, a lot of the rain will just run off if it falls quickly," he said.

"It won't soak into the vegetation. So 50mm of rain, if it falls in one or two hours, will not extinguish a bushfire.

"More than 50mm is generally required to slow down an exceptionally large fire, and to extinguish most bushfires you'd need well over 50mm."

According to BOM meteorologist Sarah Scully, a deepening low-pressure trough is expected to develop over central and western New South Wales, extending down into Victoria, over the coming days.

"So with the instability that's created by this trough, plus the increased humidity, all those ingredients go together to create a really unstable environment with increased showers and thunderstorms," Scully said. 

"But it's a bit of a double-edged sword, as heavy rainfall and gusty thunderstorms bring the potential for flash flooding, particularly in the burnt-out areas of New South Wales and Victoria, which are now vulnerable to landslips and trees coming down." 

On Monday, firefighters in the NSW could hardly contain their joy to the news that rain was on the way.

"If this Bureau of Meteorology rainfall forecast comes to fruition then this will be all of our Christmas, birthday, engagement, anniversary, wedding and graduation presents rolled into one," NSW Rural Fire Service wrote on its Facebook page. "Fingers crossed."

On Tuesday, the fire service warned even with the forecast rain there were still widespread areas of high and very high fire danger across NSW.