Why is there so much rain, and when will it end?

You are right to think the constant rain in NSW is unusual. You are also likely wondering when it will end. Here’s what you need to know.

For some areas of the east coast, it just feels like it won’t stop raining.

On Thursday morning, Sydney passed its annual average rainfall (1214mm) in just over three months, which Sky News chief meteorologist Tom Saunders said is “easily” a record.

“It’s been the wettest start to a year on record,” he told news.com.au, noting it wasn’t just Sydney seeing the wet weather. “The whole central and eastern (areas of) NSW has been exceptionally wet so far this year.”

And you’re right in thinking that it is unusual.

Where is all this rain coming from?

La Niña is a climate driver in the Pacific Ocean that has the power to affect weather worldwide.

Mr Saunders explains “a long way away from Australia” in the central equatorial Pacific Ocean, the ocean temperatures are colder than normal, while off the northern coastline of Australia, the ocean temperatures are warmer than normal.

Sydney has already passed its average annual rainfall for the year. Picture: NCA NewsWire / Gaye Gerard

“For Australia, the impact is we get a strengthening of trade winds, easterly winds through the tropics, and because those easterly winds are stronger it means more moisture is blown from the Pacific towards Australia,” Mr Saunders said.

“That sets up the background environment that there’s more moisture.”

However, what we are seeing in this current La Niña is “a little unusual”.

“Often La Niñas will bring good rain to northern, central and eastern Australia, and the state that has the highest chance of seeing above average rainfall is Queensland,” he said.

“But that hasn’t happened on this occasion. It has actually been drier than normal through central and tropical Queensland the last few months, but NSW has seen, in many cases, record rain.”

Areas of NSW like Lismore have suffered through more flooding while cleaning up from the last major flood disaster. Picture: Dan Peled/Getty Images

So, why is NSW copping the rain?

Apparently these things called upper low-pressure systems are to blame.

Low-pressure systems, the ones you see on weather maps on the news or online, are at the surface of the earth.

A few kilometres above the surface, Mr Saunders explains, we’ve been having frequent ‘upper low-pressure systems’, also known as cold pools.

“They’ve been pushing up from the Southern Ocean over NSW and then as it moves in, they’ve run into all that moisture La Niña has provided, which is what has led to these frequent rain events,” he said.

The frequent rain events have been devastating for communities. Picture: Darren Leigh Roberts

What happens next? Is it going to stop?

The simple answer is: “There’s no guarantees that all this rain will stop”.

The rain in this weather event is easing, but NSW will probably see more rain next week with another upper low likely. It is too soon to say if it will be a flood event.

Heavy rains are expected to ease over the weekend, until the next event. Picture: Sky News Weather

Beyond that, La Niña is starting to weaken and Mr Saunders said it will probably break down over the next month.

Unfortunately, that does not mean the intense wet weather will be suddenly over.

While the ocean temperatures may return to normal, it will take time for the atmosphere to react, meaning the weather patterns will not automatically return to normal when La Niña ends.

“Sometimes there is a lag of a few weeks. Potentially we’re looking at well into May where it’s likely to be wetter than normal,” Mr Saunders said.

“Then going into winter and spring, it’s a case of watch and wait because there is every possibility another La Niña will form.”

That would make it three in a row, and would mean more heavy rain and flooding.

The current La Niña has persisted beyond what the Bureau of Meteorology originally forecast. Picture: NCA Newswire/ Gaye Gerard

In the Bureau of Meteorology’s latest climate driver update last week, it said La Niña had “weakened slightly” over the past fortnight and climate outlooks indicated a return to neutral levels (no La Niña) late in the southern hemisphere’s autumn, which is later than what was originally forecast.

In November, when La Niña became “established”, BOM predicted it would be “short-lived” and would end in late summer or early autumn.

In an update released mid-March, BOM said the while the current La Niña would eventually weaken and dissipate, its “retreat had stalled”.

La Niña isn’t the only climate pattern that Australia will need to keep an eye on.

The Indian Ocean also has dry and wet phases, which could bring above average rainfall in winter and spring, but it is too early to forecast whether there will be an impact this year.