ClimateWatch: September & spring looking mild for NZ (+16 Maps & Video)

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ClimateWatch: September & spring looking mild for NZ (+16 Maps & Video)

1/09/2021 9:15pm

> From the WeatherWatch archives

Here’s the NZ outlook for the month of September – and the season of spring, brought to you in association with IBM (our business partners) and RuralWeather.co.nz.

RAIN:
A normal September set up with westerlies picking up more and more (although easterlies will impact northern NZ a little). The West Coast has around 300mm coming in the first half of the month and the bulk of that heavy rain is where it should be – the southern half of the coastline. Overall, the month may actually lean drier than average for the West Coast due to highs returning from the west. Elsewhere and eastern areas of both islands look quite dry for the next couple of weeks.

For the rest of September there is variety in rainfall – but mostly around the South Island, with the North Island looking more settled with few wet day events.

The rest of spring looks similar, although perhaps leaning drier than normal for some regions still – due to the big uptick in high pressure moving in from the west.

Drier than average for the top half and eastern side of the South Island and most of the West Coast (except Fiordland). Elsewhere, average to even slightly wetter than average.

Most of NZ leans drier than average to average rainfall for the season of Spring. The green shaded areas may be a sign of some brief wet weather – but it’s not enough to give great confidence of anything significant at this stage. Please note, super long forecasts like this have about a 60% accuracy rate. They are best used as a general guide – not a specific forecast.

CURRENT SOIL MOISTURE (Sept 1):

TEMPERATURES:
Leaning warmer than average for September by 0.5C in both main islands of New Zealand.

This is the same outlook for October and November too.

Data provided by IBM.

AIR PRESSURE:
Where are the highs and lows tracking? Here’s how the next 3 weeks kick off…

The big high east of Dunedin (1038hPa) dominates NZ and destroys that weak low near Auckland and Northland (that brought heavy rain, thunder and gales on Aug 30/31 in Auckland). A huge storm south of Australia keeps their southern coastline active and keeps the sea surface temperatures down. A remarkable air pressure gradient (of 100 hectoPascals) between the High at 1038hPa and the low south of Aussie at 939hPa.

More air pressure giants with yet another 100hPa air pressure gradient, between an enormough several thousand kilometre wide high pressure zone exiting Aussie and a deep low in the Southern Ocean (1029hPa vs 92phPa!). For NZ this means dry in the north, wet in Fiordland and windy westerlies dominating.

Looking a little more like La Nina with the easterlies at the top of the country – but most of NZ remains dry under very large and dominant highs streaming out of Aussie.

LA NINA OR NO LA NINA?
We remain in a Neutral zone – and while still some hints at La Nina, it doesn’t look too major. The Indian Ocean Dipole (basically the Indian Ocean’s version of La Nina) leans towards negative – which encourages more rainmakers into northern and north western Aussie. Some of these rainmakers can reach NZ but they are few and far between. We may notice more tropical storms forming in Nov/Dec if sea surface temperatures (SST) remain just a little more warmer than usual.