August 2023 – A chilly end to winter
New Zealand’s weather pattern during the final month of winter saw frequent impacts from cold southwesterly weather systems that brought bouts of wind, rain and snow across much of the country, very characteristic of developing El Niño conditions. Auckland experienced the coldest month in over a decade, while the North Island ski fields saw frequent top-ups which led to the best conditions in recent years.
Not to be out-done, the South Island also saw below average temperatures with Nelson recording the coldest month since 2004. Snow also fell to low-levels multiple times during the month and impacted the high-level roads on numerous occasions.
The final fortnight of winter saw predominant high pressure in the Tasman Sea bringing generally fine weather across the board, with hard morning frosts across the South Island and parts of the North Island, intermixed with the odd westerly front. However, this period of relative calm is unlikely to persist into spring as weather systems line up from the north and west to take aim at New Zealand.
Climate Drivers
The United States weather agency NOAA declared the start of El Niño conditions with the potential for a significant event by summer, while their Australian counterpart BoM is likely to confirm onset over the coming weeks. Despite this discrepancy, New Zealand is likely to see continuing El Niño-related impacts over the coming months, which include a more active and volatile westerly flow. This may be bolstered by a Positive Indian Ocean Dipole event, which is now looking likely to develop this spring which would enhance the westerly flow of weather across the country.
September 2023 Outlook
Spring begins with a subtropical low driving southwards across the upper North Island, with heavy easterly rain likely for those regions. Meanwhile, high pressure reigns supreme further south with below average rainfall likely over the South Island.
Temperatures should trend well above average in the north, while southern regions see cold and frosty mornings followed by pleasant afternoons under largely blue skies.
The second week of September sees high pressure depart eastwards, which allows a build-up of westerly fronts to sweep across the country. These features will generally be quick-moving, with most regions seeing near-average rainfall and temperatures. However, the southern part of the South Island may trend slightly drier (and warmer) as weak ridging attempts to build in-between the fronts.
A weak low pressure anomaly returns across the South Island and part of the North Island during the third week of the month, indicating a slightly more active southwesterly weather pattern.
An uptick in quick-moving fronts bringing bursts of heavy rain, wind and potentially late-season snow about higher-levels is possible, along with temperatures trending a bit below average with a possible cold snap.
This anomaly moves offshore during the final week of the month, with New Zealand likely to see a mix of weak westerly fronts and drier weather. Cool mornings are likely to persist as the month of September comes to an end.
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