A catastrophic fire danger is forecast for Greater Sydney, Greater Hunter, Illawarra and Shoalhaven regions on Tuesday, prompting the New South Wales Premier to declare a seven-day state of emergency.

Key points: Cold fronts, which push hot air ahead of them, have been responsible for catastrophic bushfires in the past

Cold fronts, which push hot air ahead of them, have been responsible for catastrophic bushfires in the past A cold front from WA and central Australia is expected after NSW experiences south-easterly winds, which means the wind direction will change rapidly

A cold front from WA and central Australia is expected after NSW experiences south-easterly winds, which means the wind direction will change rapidly This can mean people who think the fire danger has passed may get caught out when the wind changes suddenly

Of course there are underlying conditions, but why is Tuesday looking so bad?

It's a cold front.

Cold fronts have resulted in disastrous fires again and again in Australia.

It was a cold front that ramped up the fires on Friday, and it was a cold front that brought on Black Saturday (2009), Ash Wednesday (1983), and Black Friday (1939).

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It starts with hot, dry air being dragged across from the red centre.

"You're dragging an air mass that's been sitting over Western Australia and just being, in a sense, baking," Grace Legge, senior forecaster at the Bureau of Meteorology, said.

"There's not been a lot of cloud over there, so it's just been warming up over the past week or so. And then when we get these cold fronts moving through, they start to drag this heat ahead of them."

Then the wind changes.

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"You expect the fires moving along with the wind as they're pushed along," Ms Legge said.

"Tomorrow with the winds you'll be seeing a more south-easterly direction.

"Once that change comes through, it changes the direction that the fire's going, which means that you normally end up with a large fire front, as the wind changes direction and moves the opposite way.

"You'd start to see the fires moving more north-easterly once a change has gone through."

When northerly winds are replaced by a cold front bringing strong south-westerlies, anyone on the eastern side of the fire is in danger. ( ABC Weather: Kate Doyle )

So, even though it appears the fire has moved past it does not mean you are out of danger.

"The changing wind direction is often where people can get caught out. So you've always just got to listen to the advice of emergency services," Ms Legge said.

Wind change not the only risk

Jason Sharples, Associate Professor at the University of New South Wales and bushfire dynamics expert, said that fires that formed under catastrophic fire conditions had erratic fire behaviour.

"They are going to move extremely quickly. There'll be a lot of spotting involved. Winds will be blustering and the fire will actually start to interact with the atmosphere to a certain degree, depending on how strong the winds are and how hot the fire gets," Dr Sharples said.

"That means this whole extra level of unpredictability to what your fire's going to do."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 44 seconds 44 s NSW RFS chief outlines catastrophic fire risk

Along with the wind changes outlined by Ms Legge, another potential danger associated with a cold front is the trough — or area of low pressure — that comes through just before the front, according to Dr Sharples.

"We're just learning now that these sorts of atmospheric fixtures can have really important consequences for the fire. They produce extra lift, which means that plume can be pushed higher up into the air," Dr Sharples said.

"That can bring in a whole load of other processes like pyro cumulonimbus and all the ensuing effects from that as well."

Pyro cumulonimbus are when a thunderstorm forms over an active fire, generating erratic fire behaviour and even lightning — which can start new fires.

It's nothing but red across Lighthouse Beach. ( Supplied: Laura Begbie )

But Dr Sharples said the strong winds that are forecast may make pyro cumulonimbus less likely.

"The plume is pushed over by strong winds, which makes pyro cumulonimbus less likely, but there is still a possibility that they may form if the fire makes an intense run or as the change comes through," he said.

"So the whole wind has this downward motion to it. This will keep the plume laying over, which makes it very efficient at pushing embers ahead of the fire."

Beware embers and spot fires

Dr Sharples said these sorts of conditions could cause bits of burning bark and small branches to get drawn into the plume.

"They then get pushed by the wind down ahead of the fire. And if you've got really dry conditions where the ember lands and if it's still got a bit of heat in it, that can start fires very quickly," he said.

"Typically they'll go a few hundred metres to kilometres but under really extreme conditions like Black Saturday you had spot fires starting tens of kilometres ahead of the fire."

Waiting to see the flames could be leaving it too late.

Another problem is that the dangerous fire conditions on Tuesday are over mountainous terrain.

"Once [the wind] gets into the mountains or the rugged bits, then the wind is getting diverted by the hills, it's getting pushed up and over and in between topographic features," Dr Sharples said.

"The fire can then interact with those flows, to produce some really weird fire behavioural effects. Adding the topography just adds another level of complexity and another level of unpredictability for what the fire is going to do."

It is not the day to be complacent.

When will it end?

Ms Legge said Tuesday will definitely be the peak of the fire danger. Conditions may ease for New South Wales on Wednesday, but it is far from over.

"We've got a colder air mass moving through, for at least New South Wales," Ms Legge said.

"The colder air mass will start moving through on the Wednesday and the winds will start to ease on Wednesday.

"They may remain a little bit gusty overnight though, especially the further north you go, depending on where the change is.

"So it will ease off slightly, but we will remain very high, pretty much throughout the area for the next few days at least. It's definitely not over yet."

Another cold front could be on the cards for late next week but there is no certainty it will bring rain.

"The fires will probably be ongoing and, realistically, until we get enough rainfall to really change the fuel loads and allow them to moisten up, we will probably see these conditions continue for a while," Ms Legge said.