ClimateWatch: SEPTEMBER outlook & how SPRING is shaping up (+16 Maps)

Published Wednesday — We’re seeing an uptick in high pressure across both NZ & Australia as we head into spring – but there are still rain and snowmakers in the mix too.

Generally speaking there is plenty of high pressure departing the Indian Ocean, tracking over southern Australia and then into New Zealand over the coming month(s).

There are rainmakers too, but for the first half of September they do appear quite broken up compared to the intensity of the lows and rainmakers we saw in August.

For New Zealand this means some much needed ‘breathing room’ following August’s record breaking rainfall, slips and flooding in a number of regions. We also see no immediate looming “atmospheric rivers’ driving down very wet tropical rain into NZ.

LA NINA
There is still chatter of La Nina reforming this spring for a third time – but modelling also suggests it may not be very powerful or long lasting if it does occur. Worthy of keeping a close eye on – but as we’ve seen with previous La Nina events there is still a lot of unknown science between La Nina forming and how it impacts NZ (due in part to our small size but also the fact we’re halfway from the equator to Antarctica…and La Nina is measured at the equator).

BoM in Australia is still in a La Nina Alert – indicating La Nina is possible, but still not officially here yet.

BoM’s “Model of models” – showing the world’s most reliable public models (NZ’s public agency, NIWA, does not contribute to open data for their own commercial reasons so therefore NIWA/NZ Govt modelling cannot be included).
This modelling shows there is consensus for La Nina to officially come back again for a third time this spring — but that it may be mild and short lived, perhaps gone again by January 2023.

HOW SEPTEMBER IS SHAPING UP AIR PRESSURE-WISE:

**Week 1** (Thursday Sep 1) kicks off with a lot of high pressure. What this map doesn’t show you is the significant southerly blast moving into the South Island of NZ on Sunday to Tuesday (in the first week of Sep). Check your local RuralWeather.co.nz forecast to drill down deeper.

In **Week 2** high pressure that helped drive in a wintry southerly to NZ during Week 1 is now centred east of NZ and this now pulls down a mild northerly flow for the beginning of Week 2.

**Week 3:** By the middle of the month more high pressure dominates but there may be a break in the link as stormy low pressure in south western Australia develops, large lows in the Southern Ocean track by and high pressure ‘thins out’ to NZ’s north a lot more. There are also some signs of low pressure in the northern Tasman Sea to help bring in some rainfall to northern NZ.

RAINFALL:
New Zealand’s rainfall looks “closer to normal” this month (and for the season of Spring). We’re seeing a healthy mix of high pressure and low pressure and the spring pattern tends to drive in more westerlies, so rainfall is often higher along the western side of NZ than the eastern side. That’s certainly what we’re seeing in the NZ rainfall maps coming up.

Even the long range maps from IBM show NZ is “generally” in the normal range for rainfall. Some regions lean a little drier, others a little wetter – but at this stage no ‘atmospheric rivers’ are in the forecast to significantly alter expected rainfall totals beyond normal.

**Next 7 days Departure from Normal RAINFALL map** – in other words, compared to the first week of September in previous years, how much wetter or drier will this one be. **Red** = Drier. **White** = Usual rainfall. **Blue** = Wetter than average. *Data: US Government.*

Rainfall out to Sept 15. White boxes highlight minimal to no rainfall.

Closer up view of NZ’s expected rainfall for the first half of September shows a classic spring set up, where rainfall is enhanced on the western side of NZ but reduced on the eastern side.

Remember to drill down deeper with rainfall totals at your place by visiting RuralWeather.co.nz – it will help bring these maps to life for you.

**IBM’s SEPTEMBER RAINFALL outlook** – shows most of NZ hovering around the ‘normal’ zone, give or take 12mm compared to average.

**IBM rainfall compared to average for the season of SPRING (Sep, Oct, Nov)** – shows most of NZ’s rainfall close to average. Perhaps some signs the South Island may lean a little drier than usual, especially in the south west corner (**Fiordland**, **Southland**, parts of **Otago**). Also, some signs of La Nina type weather showing up in the north and north east of the North Island (**Northland**, **Coromandel** **Peninsula**, **Auckland**, **BoP**, **Gisborne** and **Hawke’s Bay**).

**Current Soil Moisture Levels around** NZ show most places are either normal, or a bit wetter than normal. A fairly healthy soil moisture map as we head into September. Map courtesy of the NZ Taxpayers and the *National Institute for Water and Atmospheric Research* – a well funded NZ Govt Agency.

**IBM’s bigger picture for SPRING RAINFALL compared to normal.** Note how the colours in NZ are in the middle of the rainfall key (indicating fairly normal rainfall) where as eastern Australia and Papua New Guinea have more rainfall than normal in blue shading(and extreme dry in red for eastern PNG).

TEMPERATURES:
Once again NZ leans warmer than average nationwide, by half a degree to 1 full degree. This based on what has historically been recorded at this time of year over the last generation at least. Even with the wintry blast expected in some areas for the first week of September, we still expect all regions to end the month warmer than average overall.

**IBM departure from norma**l shows NZ’s temperatures leaning about 0.5 to 1C above normal for the next 3 Months ahead. Whereas a large portion of Australia is cooler than usual, including Brisbane, Sydney, Adelaide and Melbourne. Not to mention Alice Springs!

SEA SURFACE TEMPERATURES:

**SST (Sea Surface Temperature) anomaly map** shows it is cooler than average along the equator (La Nina-like) but it’s warmer than average around much of NZ and to our north, especially the Coral Sea off the Queensland Coast and towards Vanuatu and Fiji – a breeding ground for tropical lows in the warmer months of the year. *This map courtesy of BoM in Australia, with thanks.*

**SST difference compared to normal in the NZ area**, shows the Marine Heatwaves carry on in a number of regions – or close to it. Data made freely available with thanks to the **Moana Project**. Marine heatwaves can make it more pleasant for swimming – but also increases overnight temperatures in coastal areas and may also enhance slow moving downpours in the area.

Current Sea Surface Temperatures (SST). Courtesy: Moana Project.

These ClimateWatch updates are made independently by WeatherWatch.co.nz – in association with RuralWeather.co.nz and our official business partnership with IBM and *IBM Watson (*the world’s most accurate forecaster).

We use open public data to create a free public resource. Thank you to those outlets/Governments that support Open Data.

RuralWeather.co.nz / WeatherWatch.co.nz

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