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BOM forecast wet start to summer for NSW, but not drought-breaking

By Ashleigh Raper, Thursday November 15, 2018 - 20:18 EDT
ABC image
Grey skies could be the December norm over the Harbour City. - ABC

There is some good news for New South Wales farmers struggling with drought — the Bureau of Meteorology (BOM) is predicting a wet start to summer.



In its latest climate outlook, the BOM said it was expecting December to be wetter than average.

"It's a pleasant surprise for areas, especially for areas that have been quite dry," said Agata Imielska, a senior climatologist at the BOM.

The BOM is predicting a 65 per cent chance of above average rainfall next month but it may not be received by all.

That means areas will get at least 50 millimetres of rainfall, while some may be up to 400 millimetres.

Eastern, and south-eastern parts of the state, including Sydney, are more likely to see rain than western areas.

The amount of rain is also not expected to break the drought.

"It would be nicer to see the signal across the entirety of the state and particularly west of the Great Dividing Range," Ms Imielska said.

"Still better than seeing a dry outlook obviously," Ms Imielska said.



Multiple climate drivers at play

There is currently a 70 per cent chance that an El Nino will form this summer, which is triple the normal likelihood.

The El Nino phase of the El Nino Southern Oscillation is usually associated with dry conditions in Australia.

So how can there be both above average rain and an El Nino?



Dr Andrew Watkins, manager of long-range forecast at the Bureau of Meteorology, said there was more than one climate driver at play.

"We can see that we have our weather systems generally further south than normal for this time of year and that's called a positive SAM, a positive Southern Annular Mode," he said.



At this time of year that means high-pressure systems are expected to rest over the south Tasman Sea.

"Think of it as an anti-clockwise rotation in the Tasman Sea and that's basically pushing air off the sea onto the land along the New South Wales coast," Dr Watkins said.

That air that comes off the warm ocean brings moisture and when triggered by a trough or front can generate rainfall.



Dry weather systems still dominate

The positive Southern Annular Mode may provide relief for parts of NSW in December but overall dry climate drivers are expected to dominate over the next few months.

Dr Watkins said the expected El Nino would block cloud development, allowing the ground to heat up more, particularly inland.

When combined with dry soils widely present this year, Dr Watkins said this leads to a "double whammy" heating the air up more than normal.

It is not just El Nino. We are currently experiencing a positive Indian Ocean Dipole (IOD), which is like an El Nino but over the Indian Ocean.

During a positive IOD there are cool waters to the north-west of Australia.



"The weather systems, they go where the warm water is, "Dr Watkins said.

"They tend to move away from the cooler-than-normal water near us and they head towards the warmer-than-normal water off Africa.

"That's great for the droughts and things they've had over there in Somalia and other places in the last couple of years… not so great for us."

Even though the positive IOD is likely to break down over the next few weeks, the next few months are expected to be dry for Western Australia and there is little relief in sight for those in drought in western Queensland.

Prepare for the heat



People in NSW should brace for hot days and nights, as the outlook points to warmer than average temperatures throughout summer.

The BOM also predicted there would be an increased chance of extreme weather, such as heatwaves, because the state had been so dry.

It said the increased risk of bushfires that had been predicted would not change, despite the rain expected in December.

"Sometimes that rainfall can actually stimulate some growth which can then dry out and actually pose a potential bushfire risk," Ms Imielska said.


- ABC

© ABC 2018

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