Rain on horizon as negative IOD looms

More rain on horizon as negative IOD looms | Weather In Focus

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Almost all of NSW will see some rain this week as a north west cloudband sweeps across the state.

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ALMOST all of NSW will see some rain this week as a north west cloudband sweeps across the state.

A north west cloudband is simply a vast band of cloud that stretches across Australia from the north west to south east.

They form when warm and humid air from the tropical Indian Ocean drifts towards the southeast and passes over Australia.

As this warm, humid air moves further south, it rises above cooler air in the mid-latitudes and produces rain-bearing clouds.

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The oceans off the north west shelf of Australia are currently warmer than usual for this time of year.

This warmth is enhancing the amount of moisture in the atmosphere and providing the fuel for this week's north west cloudband.

Part of every state and territory will see some rain from this week's cloudband, including most of NSW between Wednesday and Friday.

Drier weather will then return to NSW from the weekend as a high pressure ridge develops over the state.

But this won't be the last north west cloudband we see over NSW this season and there could be plenty more rain in the next few months.

There are growing signs that much of Australia will experience unusually cold and wet weather this winter and spring, including NSW, with a negative Indian Ocean Dipole (IOD) emerging in recent weeks.

The IOD is an index that measures the difference in sea surface temperatures on either side of the Indian Ocean.

When this sea surface temperature gradient is large enough across the Indian Ocean, it drives rising and descending moisture and air on both sides of the Indian Ocean basin.

There are three phases of the IOD: negative, neutral and positive.

Each phase dramatically impacts Australia's weather.

In general, negative IOD events are cool and wet for Australia, while positive IOD events are drier and warmer.

While the IOD is currently neutral (average conditions for Australia), it has recently dipped into a negative phase and some computer models indicate that a negative IOD will be officially declared before the end of winter and persist into early spring.

If this happens, it will increase the likelihood of above-average rain and below-average daytime temperatures in NSW during the rest of winter and spring.

So, north west cloudbands like the one we are seeing this week could be a prominent feature during the next several months.

Meanwhile, if you're sick of winter already, you might be pleased to know that Monday was the winter solstice.

But despite its name, it's not actually the beginning of the end for those dark mornings.

Sadly, there's still a few days left of later sunrise times.

They'll get slightly later until the end of the month (by about a minute - sunrise is at 7am now in Sydney, it'll be 7:01am on July 2), then we start heading back towards earlier sunrises.

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