NSW weather stuck in a rain streak and what's behind the weather pattern

Heavy rainfall may trigger more flash flooding.  (Supplied: Niels Kraaier)

If you’re living in south-eastern Australia, the weather has probably been feeling a bit like Groundhog Day.

Since the start of May, Sydney and other parts of the New South Wales coast have been hit with uninterrupted rain, delivering accumulated falls between 50 and 100 millimetres of rain over the course of the week, including heavy downpours over the Illawarra and Hunter regions on the weekend.

And current forecasts indicate it’s far from over, with the potential for Sydney’s main weather station to break the record number of days of rain in a row by the time the event ends, according to the Bureau of Meteorology (BOM).

“This pattern has been stagnant for six days, and is expected to remain there for at least another seven days, continuing these drizzly coastal conditions,” BOM meteorologist Edward Medlock said.

“The thing we’re looking at right now is whether this will lead to a record of number of consecutive rain days in a row [in Sydney].

“The record so far is 16 days, which was when Lismore flooded back [in] February and March 2022 … so we’re not there yet but it could happen if this high decides to stick around.”

Meanwhile over Tasmania, and inland parts of New South Wales and Victoria and South Australia, it’s been a steady pattern of clear skies, mild days and cool nights, with a copy-paste forecast for the next week ahead.

So why has Australia’s normally flowing and transient weather pattern all of a sudden come to a halt?

‘Blocking high’ jams Australia’s weather

According to climate scientists, it’s all to do with something known as a “blocking high” – a critical part of Australia’s weather pattern and one which climate scientists say is important to better understand future rainfall.

A “blocking high” refers to a strong high pressure system which remains near stationary for an extended period of time – ranging from several days to several weeks.

In particular for Australia, it’s one which forms just south of the continent where the storms normally pass, “blocking” the path of rain-baring weather systems that flow from east to west.

“They’re a bit like a rock in a stream,” CSIRO climate scientist James Risby said.

“And the flow of storms, which normally moves along the stream, has to go around that block.”

The impacts of blocking highs vary depending on the strength and location.

In the current set up, the main impact is for rain, with the high directing a constant stream of onshore winds over the east coast, which favours showery weather for the region.

But they can also drive bouts of extreme heat and dry.

Earlier this year, a blocking high located in the Tasman Sea helped drive a four-day autumn heatwave over south-eastern Australia, with record-breaking overnight minimum temperatures close to 30 degrees Celsius in some locations.

Though blocking systems are a common occurrence for Australia, Mr Medlock said what made the current event more unusual was the strength of the high pressure system, and the time it was taking to move on.

“There’s always slow highs and fast highs, but something like this [is] fairly unique,” he said.

“If they stick around for you know, a week long, it’s probably uncommon.”

Why do they matter for Australia?

Some of the most extreme blocking events in the world occur in the Northern Hemisphere, where its common for them to last for several weeks, having been associated with extreme flooding and deadly heatwaves.

Several extreme examples of this played out last year, with record-setting heatwaves and flooding events spanning North America, Europe, North Africa and Asia.

Blocking in the Southern Hemisphere tends not to be as prolonged as its northern counterpart, because there is more ocean and less land mass.

But Dr Risby said blocking highs were still a “critical” part of Australia’s weather pattern, in particular their contribution to rainfall by their association with “cut-off lows”.

Cut-off lows are low pressure systems which have broken away from the main belt of low pressure, and are associated with sustained, and often heavy, rainfall and can produce strong and gusty winds and high seas.

“Part of the storm tracks that come along can split around the blocking high, and the bit that splits around the high and travels toward the continent, we would often call a cut-off low.

“Those are really important in bringing rainfall to southern Australia, both in the south-western and south-eastern Australia.”

On the flip side, he said they can also set up dangerous heatwaves and fire conditions, by fanning hot winds from the north into south-eastern Australia.

“They also generate extreme temperatures blocking highs,” he said.

“For example, in south-east Australia the typical weather set up for a heatwave is that there would be a high in the Tasman Sea region which would be directing a strong northerly flow into south-eastern Australia.”

What will it mean in a changing climate?

Dr Risby said these links to extreme weather meant the future behaviour of blocking systems under climate change was important to understand.

This included questions about future frequency of blocking highs in regions like Australia, and their preferred positions.

“Particularly from our perspective in Australia, we’re really sensitive to how often blocking occurs in the Tasman Sea region,” he said.

“So, will we get more or less blocking in the Tasman Sea region? Will it be at a higher latitude or the same latitude?

“Small differences in where the blocks tend to form under climate change will have a big impact on our regional rainfall.”

He said what scientists were confident with … was that the heat impacts felt by the same blocking set ups in future, would be exacerbated in a warming climate.

“Blocking highs in future could lead to more intense heat waves,” he said.

“For example, in south-east Australia the typical weather set up for a heatwave is that there would be a high in the Tasman Sea region, which would be directing a strong northerly flow into south-eastern Australia.

“So if it blocks, that will occur under conditions where the continent itself is warmer, and the atmosphere and ocean is warmer because of global warming, and so that will lead to more intense heat waves than in the past.”