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How the weather is shaping up for September

File photo/Daniel Hines/SunLive.

Classic spring patterns are being forecast for the month of September.

WeatherWatch.co.nz says it is what they like to call an "Air Pressure Sandwich".

"This is when we have a belt of highs to NZ's north, a belt of lows to our south, and the "filling" is the windy westerlies in the middle (and yes, New Zealand is right in the middle of this air pressure sandwich!).

"The westerly lean of winds will be more noticeable from the lower North Island Island southwards, encouraging milder and drier weather at times in the east of the South Island and Wairarapa."

WeatherWatch says the highs to the north will keep northern NZ drier than usual while the westerly flow will help keep the eastern North Island a little drier - but the eastern North Island may also get clipped by a few showery southerlies lifting totals there a little more than in Northland and Auckland, for example.

"Rainfall will continue as normal - or even above normal - in the places where few of us live: Fiordland and South Westland."

The rainfall totals in Fiordland will be quite steep in the coming weeks and there will be some spillover further east in Southland, Otago and Canterbury.

But the very eastern sides of Southland, Otago and Canterbury do look to lean drier than average along with the upper South Island.

LATE SEPTEMBER / SPRING / LA NINA?

"The further we go out, the harder it is to pinpoint specific rainfall. It's worth noting that IBM's longer range rainfall models suggest "normal" rainfall does return to Northland and Auckland during spring," says WeatherWatch.

"While it's hard to see how this will happen at this early stage, there is 70 per cent confidence of La Nina forming according to Australia's Bureau of Meteorology (BoM) who WeatherWatch trusts.

"If La Nina occurs and it is also a moderate to strong La Nina, it could see more lows and rainmakers developing to NZ's north.

"It's not yet showing up in the weather charts, but we'll keep you posted on further updates."

High pressure. Blue boxes = Low Pressure. This shows a classic spring set up with westerly quarter winds dominating.

No great change as we head into the second week of September, although more highs in the Southern Ocean.

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