The Southern Annular Mode (SAM), an index that influences wind, rain and snow in Australia, has just shifted into a negative phase. So, what does that mean for weather in NSW in the coming week?
The SAM is an index that's used to monitor the position of a belt of westerly winds that flow from west to east between Australia and Antarctica.
These westerlies carry cold fronts and low pressure systems that produce much of the rain that falls over southern Australia during winter.
When the SAM is in a negative phase, these westerly winds, and the cold fronts and low pressure systems they carry, are located further north than usual for that time of year.
When the SAM is positive, as it has been for the past fortnight, the westerly winds, cold fronts and low pressure systems are located further south than usual.
The SAM can have a notable impact on Australia's weather. Negative phases of the SAM during late-autumn and winter often increase rainfall over southwestern and southeastern Australia.
They also reduce rain in parts of eastern Australia, including much of western NSW, and cause stronger westerly winds across the southern half of the country.
By contrast, positive phases of the SAM in late autumn and winter can suppress rainfall in Australia's southwest and southeast, while increasing rain over eastern Australia. Positive SAM phases can also make East Coast Lows more likely near Australia's eastern seaboard.
Also take note if you are one of Australia's alpine addicts, because the SAM can influence snow.
More snow is expected to fall in the Australian alps when the SAM is negative and less snow is likely when it's positive.
So, what's the SAM doing now?
After being in a positive phase for much of the last fortnight, the SAM has just shifted into a negative phase.
This transition coincided with a strong cold front crossing southern Australia during the last few days.
This system caused hail and a dusting of snow in WA on Monday, followed by widespread rain and damaging winds in parts of southeastern Australia on Tuesday.
The SAM is likely to stay negative through the final days autumn and may return to neutral levels in the opening week of winter.
This means that we could see some dynamic weather in the Australia region as we approach the start of our cold season.
Forecast models are already suggesting that a deep low pressure system could develop over the Tasman Sea in the final days of May, producing large surf and blustery southerly winds in eastern NSW.
However anywhere west of the ranges in NSW should stay dry until at least Tuesday next week.
As we have seen during the past couple of weeks, the SAM can shift quite quickly, so it's worth keeping an eye on as we head into the opening weeks of winter.
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