Spring is certainly here and we’re experiencing what I think is a classic September weather pattern, dominated by storms over the Southern Ocean and strong to gale-force westerly winds surging off and on.  

The storms south of New Zealand remain enormous – both in size and depth of air pressure. Most lows south of the country have been around the 940 to 950 hPa range now for several weeks. Usually we get a few like that, then that’s it. 

You’ve probably seen and heard the words “polar vortex” used a lot this year. The polar vortex is like elastic holding the cold in place around Antarctica – and this year it’s lost that tightness and has more of a wavy shape to it. 

Each time the wave goes up from Antarctica it shifts the boundary of cold air and low pressure into the NZ area – and this week that produced a significant winter outbreak over the country with hail, snow and bitterly cold wind chill for newborn livestock.

There is no sign of these storms coming to an end any time soon. The lower air pressure to the south of NZ and Australia isn’t just there only – it exists all the way around coastal Antarctica, with low air pressure storms south of South America and South Africa. 

With so much low pressure south of us it means more isobars on the weather maps in our part of the world – and that’s why we’re experiencing the windy westerlies much more this year. They tilt northwest, we get warmer and windier days. They tilt southwest and the Southern Alps get another dusting of snow and Southland and Otago shoulder the colder gloom again for most of the country.

Over the coming week the weather pattern takes a bit of a breather – at least to start with. High pressure looks to briefly cross the country bringing a spell of calm after so many weeks of wind. But it’s short lived, and by later this week the severe gale nor’westers return with the chance of more rain and cold fronts. 

While not locked in at the time of writing this, there was evidence in the long-range maps that by next weekend a low will form over NZ as another colder, wintry, southerly comes back in. 

There is some good news for those who want a break from the unsettled weather. Long-range data does hint that as October arrives so too does high pressure from Australia. I’m reluctant to lock it in just yet, but GFS modelling out of America suggests New Zealand may well get a calmer period of weather to kick off October thanks to a powerful anticyclone that may be closer to 1040hPa. We shall see. Always good to have a silver lining amongst all this chaos. 

Upcoming Highlights from September 23:

• Brief high pressure

• Windy nor’westers and rain

• Possible weekend low

• Possible powerful high-pressure zone moving slowly across NZ next week

One of the messiest weather patterns so far this year is now tracking around New Zealand and it means a changeable forecast – for some, a highly repetitive one. Messier doesn’t mean stormier – but it does mean we’re not in a settled, calm period. 

Over winter we had a season of “big” weather systems from at least two large low-pressure systems that engulfed the entire nation for several days at a time, to the most powerful high-pressure zone ever recorded in New Zealand. Big weather systems can take weeks to move through.

Fast-forward to September and while this new season looks to also be a season of “big”, this month in particular also looks to be the month of “changeable”. That’s because weather systems (anticyclones and depressions) are moving through much faster now – classic spring! 

There are two things standing out about our weather pattern this month: One, just how very powerful the storms over the Southern Ocean are this year. In recent weeks most storms have had air pressure between 920 and 950hPa, which does happen at this time of year but it’s the frequency and large number of storms reaching this depth that is standing out to WeatherWatch this year. And two, the warmer-than-average airflows in the mix. 

The warmer-than-average weather won’t be consistent – the volatile weather south of NZ and the generally faster-moving, more chaotic pattern we have means NZ will still get regular cold injections – but because things are moving fast from west to east, these cold air injections may not linger long, and may struggle to go north – limiting northern frosts.

Storms south of NZ will send several cold fronts into NZ so the lower South Island in particular may feel winter this year is taking longer to ease, while those in northern NZ may be talking more about how fast the grass is now growing as an early spring starts to establish itself.

High pressure may cross NZ giving us a pause from this all – and a return to cold nights with a heightened frost risk. At the time of writing this, high pressure was forecast to cross NZ on Monday September 3 – but modelling was conflicted about if it would linger very long or directly affect both main islands. Outside of that, many highs may skirt to our north for a time, allowing for more cold fronts to come in from the west and south.

Due to the dominating westerly flow, rainfall over the next 15 days looks to be mostly on the West Coast with over 300mm likely around mid to south Westland, and around 100mm for parts of the western North Island, like Taranaki and Waitomo. Gisborne and Northern Hawke’s Bay may have the lowest totals.

Upcoming Highlights:

• Monday/Tuesday kicks off colder with a southwesterly flow across NZ

• Wednesday/Thursday has milder west to northwest winds returning

• Another cold front moves up NZ on Friday/Saturday

• Most wet weather in the west to southwest of both main islands

In Focus Podcast: Full Show | 13 September

Heading into in the second week of September, New Zealand’s early spring weather continues. Monday, September 9, kicks off with yet more windy westerlies, which will be strong and gusty and may even reach severe gale in some places, south of the lower North Island.

Heavy rain on the West Coast and rain or showers for many other western and southern areas. Tuesday is similar, although it becomes colder in the South Island with snow levels lowering for a time around the mountains and ranges. Wednesday may be another windy day for some of you in the south of the country. 

After more than two weeks of strong winds coming from the west these conditions will finally ease by late next week. But the weather pattern beyond this is starting to look messy. 

As we’ve been saying for a number of months now, we’re in a “neutral” season and while there is a lot of chatter going on about a possible La Niña, that doesn’t mean that our weather here in New Zealand is going to be affected yet. (And it’s possible La Niña may still not even officially be declared this year.) 

The Southern Ocean has been especially stormy this year, creating one major storm after the other. Storms in late winter/early spring are normal, but this year it is the frequent nature of so many deep air pressure lows that’s made things different. 

Not only that, but many of these storms and low-pressure zones have been brushing the lower part of NZ – they haven’t all been well south down towards Antarctica. In fact next week some of the severe gales around NZ are due to more deep low pressure just south of Stewart Island.

I’ve said a few times that winter this year was defined by the word “big”. We had big high-pressure zones and big low-pressure zones and many of them lasted for two weeks in the NZ area at a time. 

Now that we’re going through September we are still seeing big systems, perhaps not lingering as long as they did in the depths of winter, but they are still large and we are seeing large lows around Australia and around New Zealand coming up too – whether they hit us is a different question, but they are likely to be around.

Earlier this week, WeatherWatch issued our September ClimateWatch update where we track things like La Niña and temperature trends and rainfall trends for the month ahead. The month of September is looking generally warmer than average but with an increased risk of a snowstorm or frost event this year, and while that’s normal most springs, the chance of one being a strong one is a little elevated due to the storms south of New Zealand.

Spring may be a bit warmer than average but could still bring some stormy and severe weather at times, which hasn’t always been the case in recent years.

The season of stress is upon us! Spring weather brings all sorts of challenges to many of you (and to us as weather forecasters, too). Of course, one of the biggest issues in early September is dealing with snowstorms for newborn livestock and frosts for those that don’t want damage to plants, vines and buds. Many of us are in a heightened sense of alert for the next couple more months. 

The weather pattern at the moment is highly chaotic. Major storms down over the Southern Ocean have helped dominate our weather pattern for the past week and look to continue for September’s first week.

These storms have had central air pressure down around 920 hectopascals, which is incredibly low. But high-pressure zones over Australia, the north Tasman Sea and near New Zealand put the lid on just how far north those storms can go – and that’s what creates those gigantic windy westerlies that stretch all the way from south of Australia across the Tasman and then over New Zealand.

We said goodbye to El Niño back in April and we’ve had a “neutral” winter season, evident with the huge high-pressure zones that we had and the many large low-pressure zones too. 

I’ve said a number of times that this has been “the winter of big air pressure”. Now that spring is here (at least on the meteorological calendar), will we still get these “big” weather systems? It certainly appears for the start of September that, yes, they will continue with big storms south of us and large highs out over Australia putting NZ on the edge of it all. 

At the time of writing this column the climate driver update from Australia’s Bureau of Meteorology suggests that we’re still in a neutral season but that we are getting closer to a possible La Niña. 

It’s worth noting when you look at the various models from all around the world it still shows that the mean average keeps conditions in “neutral” for the rest of 2024, although it does certainly flirt with La Niña during spring (especially October and November). 

But those same long-range models suggest that we’re heading back into neutral as we head towards summer. 

To make sense of all of that confusion, it basically means not a lot is going to change with our weather pattern. There might be a bit more life to the north of New Zealand and we’ve certainly seen a few low-pressure zones over the past two weeks (and they are worth keeping an eye on because the wet season in the tropics doesn’t normally start for another month or two, so this may well be a sign of a very weak La Niña forming). 

Either way, New Zealand remains two small mountainous islands partially stuck in the Roaring Forties … in other words, don’t get too hooked on what might happen months from now, but my general feeling is more chaos before there is a stronger pattern emerging.

In Focus Podcast | Rewiring rural New Zealand’s approach to power

Rocketing power prices and uncertainty about generating more are keeping many people up at night, but Mike Casey reckons farmers have everything they need to power up right now. The chief executive of Rewiring Aotearoa has transformed his cherry orchard into a solar powered operation and he reckons every farmer should do the same. 

To Mike, it’s just a sound business decision as his power bills have plummeted, the capital outlay will be repaid in five years and he’s sorted if a storm knocks out the regional power supply.

This week, New Zealand weather kicked off with a southerly off Antarctica. Three days later, the airflow coming into NZ was from out of Australia. There aren’t many parts of the world that get the weather off two continents in such a short time – especially two very different ones (one super cold, the other fairly warm). 

Most Kiwis might see this week as more winter weather, but I see it as a sign of spring: the cold blast, the windy weather, then the uptick in temperatures that pushed NZ above average by mid to late this past week. 

But to me, the biggest telltale sign that spring’s weather pattern is coming in are the windy westerlies. 

We’re now heading into the final week of August. September kicks off next Sunday and so too does the start of spring on the meteorological calendar (with spring not starting until September 23rd on the astronomical calendar). Also, our coldest weather is in June, July and August – leaving September about a degree milder and giving us a bit more spring in our step. 

September is also the month that gains a lot of sunlight. About 20 minutes of extra daylight every week, meaning we gain one hour more daylight in September. Then, of course, we get daylight saving towards the end and that gives us another hour of daylight in the evening. 

Although daylight saving has absolutely no effect on the weather (obviously!) it is a sign that in just a month’s time we’ll have turned another big corner on winter.

In my view spring this year comes with a higher-than-usual risk of a snowstorm or frost event. This is due to the extra stormy weather over the Southern Ocean and south of NZ lately. 

In fact, at time of writing, there is a storm that computer modelling was picking central air pressure to be between 916 and 926hPa for next week. That’s more powerful than most Atlantic hurricanes. Thankfully the storm itself will be near the ice shelf, but the winds will stretch up to Aussie and NZ. 

Storms like this that far south tend to produce gale westerlies from Tasmania to New Zealand. Looking at long-range maps that sure seems to be the pattern off and on for the next two weeks. Westerlies streaming into NZ, sometimes with airflows out of Australia, sometimes the subtropics north of NZ, and other times a cold south to south-wester. 

The good news is that this set-up should drive in plenty of rain to the West Coast and spill over into our low hydro dams. Several hundred millimetres or more of rain will fall there over the next week or two. 

The bad news? Windier, drier westerlies for Canterbury, and a heightened risk of a sudden severe snow or frost event in September. The milder and windier spring westerlies may arrive early this year – but remember: the cold air south of us has only just peaked and hasn’t gone away yet.

Next week kicks off with an Antarctic blast for some, but for the rest of August a new pattern is forming – a spring weather pattern. This is different to the astronomical seasons, which are evenly spread around the year as the earth rotates around the sun. 

Instead, a spring weather pattern is when we as weather forecasters start to both see and feel a shift from the “dark and dormant” winter to the “windy westerlies and injections of warmth” that spring brings.

In previous years we have declared the spring weather pattern to be here by late July. This year we’re running a few weeks later, which, to southerners especially, won’t come as much surprise. 

One of the biggest pushbacks we get to any early spring weather declaration is that any snow or frost is “proof we’re still in winter”.  But that’s not proof of winter. Winter is defined as a season with death and decay and the coldest weather …  spring is defined as “moving, rising, life beginning, blossoms, buds, newborn animals”.

In the northern hemisphere some of the biggest snowstorms in places like Canada or Russia or northern Europe can occur in early spring. This is due to the coldest air of the year now meeting longer hours of daylight and more warmth. After all, we’re now officially out of the solar winter – the three months of the year with the least amount of available sunlight. No wonder you’re noticing the longer, brighter, early evenings now.

As Reserve Bank Governor Adrian Orr said this week, “it’s darkest before dawn … and it’s dawn now”. That sums up the current weather pattern. We’re still in winter technically, but we’re seeing hints of spring’s weather arriving. 

So what is a “spring weather pattern”? I define it as being dominated by more westerlies, more injections of Australian and/or subtropical airflows, and still stormy at times. In winter we tend to have more southerlies, more injections of sub-Antarctic air.

It does feel a bit weird to be writing about spring weather when, at the time of writing this, a significant Antarctica blast is possible on Monday. But to anyone who says “so much for spring” when you get a frost or heavy snow, that is PART of spring! 

If spring was warm, dry and sunny it would be summer. If it was snowy, cold, and had little plant life or growth, then we’re in winter. But I think many of you will see over the rest of August that we do have a more westerly-driven, milder at times, weather pattern – but spring brings that added risk of moisture + cold, which can lead to snowstorms and sudden severe weather events. 

We generally usually see New Zealand’s weather pattern calm down in two months from now, by late October. 

So we’re in a moderate to high risk for frost and snow events until then – but the weather is likely to have more of a spring in its step moving forward.

Soil moisture levels around New Zealand are, generally speaking, smack bang where they should be for this time of year. In mid-August most regions should be wet underfoot – but not necessarily hearing squelches when you walk on the ground, or having large areas of ponding or puddles. 

There are some parts of NZ that are a bit muddy, but there should be far larger areas that aren’t that wet.

According to the Crown’s soil moisture maps, instead of being at field capacity (puddles on the grass, large muddy areas), a majority of the nation is at about 50% soil moisture storage. Hawke’s Bay, Coastal Otago, South Westland, the very eastern coastal side of Marlborough and parts of Northland all fare a little wetter.

But it’s Canterbury that is driest compared to usual for mid-August. 

Usually in the depths of the darkest days of winter farmers around NZ, especially in the south, don’t want much rain. This year we’ve had farmers from Canterbury (and other regions) ask if more rain is coming. The concern isn’t so much directly for today – it’s about what is going to happen once spring arrives.

Usually in spring NZ gets the traditional westerlies blowing through – and this creates a bigger rain shadow for those in the east, thanks to our mountains and ranges. In other words, spring isn’t often the best season to drive rain into eastern areas. Parts of Canterbury are now well below where they should be from a soil moisture point of view.

It’s also worth noting the lack of rain falling on the West Coast, especially the northern half or pushing deeper into the mountains; this will also limit rainfall spillover into lakes and other waterways.  

There have been some good snow events this winter, to help in spring when that melts, but it’s not as big as some might think despite the snowy snaps we’ve occasionally seen.

At the time of writing this the forecast for every region in NZ was to lean drier than average for next week up to about Friday, and that included some rain falling in the forecast. 

Even if we look at the rainfall forecast for next week, there is not a huge amount of rain coming specifically for Canterbury. The best chances may be next weekend (August 17 and 18) for rain on the West Coast and some spillover to the east.  

The long-range forecast taking us closer to the end of August shows some very big high-pressure zones moving through (especially this coming week) but there is still variety in the weather pattern and we do see low-pressure zones and cold fronts still coming into the country. 

It’s not all dry. This long-range guidance suggests most rain will lean to the western side of NZ, and that we still have some very large areas of high pressure to come through just yet. We remain in a neutral weather pattern – likely for the rest of this year.

New Zealand’s in pretty good condition as we round the corner to spring weather conditions. Whether you go with September 1 or September 23rd as your official start to spring, it’s clear August is still winter technically – but NZ’s location on Earth means we can experience spring weather conditions this month (just ask someone who suffers from allergies thanks to pine pollen, which can often start in August). 

The last week of July encapsulated what we said in our May update of ClimateWatch (our monthly outlooks). We expected a series of large highs and large lows to come through this winter, bringing real variety in weather, and in the final week of July we had a low pressure zone in the Tasman Sea which had a sub-Antarctic feed coming into the South Island and a subtropical feed coming into the North Island. No wonder New Zealanders can’t agree on if this winter has been much colder than usual or much warmer!

The larger an air pressure system is, the more “reach” it has – by that I mean it can scoop up air from Antarctica and dredge it into NZ, or drag down tropical air over us. Big lows are often seen as “stormier” but in reality they can be “lazy” and just hang around for a long time. 

And “big” is the theme of this winter – because July not only had a high pressure zone so powerful it broke air pressure records in NZ that were 135 years old, but we’ve also had a number of very large low pressure zones that have brought heavy snow into both Australia and NZ, and warmer than usual rainfall for northern NZ.

So, back to August. There’s no shortage of big high pressure zones and this month has kicked off with a high moving in – the very same anticyclone that, a week ago, dredged up the snow and frosty South Island change and made Sunday and Monday (August 5) colder in the North island. 

It is actually splitting in half. The second half of this high (the main part of it) crosses NZ this week bringing changeable west to southwest winds over the lower half of NZ – but still plenty of settled weather too. 

At the time of writing this column, the seven-day outlook for NZ was drier than average for most regions, with the exception of Northland and Fiordland – a sign that the incoming high is going to be elbowing away most rainmakers as we go into this new month. 

The GFS model out of the United States picks this high pressure zone to linger over NZ until the middle of August, meaning colder nights but milder and much drier days. While we can’t lock in a forecast for two weeks away, the modelling I’m seeing right now shows this anticyclone finally departing NZ mid-month with a sub-tropical airflow coming in, then a low from the Tasman Sea. 

We’ll see if that pans out or not, but either way, the winter of “big and slow” high and low air pressure zones continues.

The eastern South Island needs rain, and at the time of writing this column there were some chances of wet weather for Canterbury and surrounds as we go through next week. We’ve had a number of people tell us spring feels like it’s arriving early – despite a snowy cold blast potentially moving in right after I write this. 

Usually spring in New Zealand is defined by an uptick in westerly winds, and more days where people say “I feel like spring is here!” –  usually those sunnier, milder, days where you notice the odd flower in bloom or new buds forming, or pasture taking off again. 

Spring is about life returning. Winter is death, decay and being dormant. This is why I firmly believe NZ barely has a winter. For those who have travelled abroad you may well agree, because in the depths of winter in Canada, or Norway, or Scotland you’re not talking about flowers out, or pine pollen, and you’re not wearing shorts and a singlet for work. 

NZ barely has a winter. It’s much more like an elongated autumn and spring combined. In fact, early spring is often full of some of the snowiest weather in northern hemisphere nations. That peak of cold air arriving with longer sunshine hours is part of the reason spring can be such a volatile season. 

So why am I talking so much about spring when we’re in the depths of winter? Because if you need rain, spring can be a tricky season, and August for northern NZ heralds an early spring weather pattern – more changeable weather mixed in with some really mild days. 

Spring can throw us weather more extreme than winter sometimes, just like it can in the northern hemisphere. But it does it with longer sunshine hours and a much higher chance of a warm bounce back a few days later.

For lower South Island readers you may not be in agreement – although a farmer from south Westland wrote to me last week to say daffodils are out everywhere. Another farmer, in Hawke’s Bay, told me the big spring daffodils are out now, not the small winter ones. And I have orchids in Auckland that normally flower in October looking ready to flower in August. 

Around NZ we have a mixture of people saying an early spring is arriving, while others say this is the coldest winter they’ve had in 20 years. 

August can be a strange time in NZ. We’re technically in the peak of winter, yet some see hints of spring. But it’s not surprising that the last week of July and the start of August may well kick off with peak winter weather. We’re not out of the woods from snow risks until October. 

Despite the cold blast coming, however, I’m optimistic some of you are getting a spring in your step now. Here’s hoping the chaos of it all brings rain to those who need it before spring warmth and wind dry soil moisture out further.

Just what is a “normal winter” in New Zealand? It’s something I’ve been thinking about a lot since a government agency recently started to talk of stratospheric warming over Antarctica. 

While that is a very real thing and is so significant scientists around the planet are talking about it, what does it REALLY mean for New Zealand’s weather? The last time it happened, a few years ago, no one noticed it weatherwise in NZ. 

This time around we may do. When there is warming occurring high up in the stratosphere it can make the storms that circle Antarctica more “loose”. Rather than fitting tightly around Antarctica, this year it’s like someone stretched the elastic and it means the Southern Ocean weather has a greater chance of sending out lows, fronts, southerlies and even large highs. 

But whether they hit NZ, Tasmania or the land-free Pacific Ocean to our east is the million-dollar question.

When someone says this August may be stormier in NZ – in the depths of winter – my first thought is “What do we expect?” What is “normal” weather for two large mountainous islands all alone at sea, smack-bang halfway between the equator and Antarctica with more than half of our nation in the Roaring Forties belt of windy westerlies circling latitude 40? 

As Jamie MacKay from Newstalk ZB’s The Country always says, “Forewarned is forearmed” – but because NZ is so small, we can sometimes buck the international trends. We recently had an El Niño event that, despite the hype, turned out to be no where near as dry as two of the three previous La Niña events that were headlined as being “wet” but instead brought major widespread drought.

My point is, chaos IS New Zealand’s normal. The Southern Ocean, which brushes the South Island, is the stormiest body of sea BY FAR on the planet. The gales that roar south of NZ can blow right around the entire globe and never touch land once. That’s impossible to do in the northern hemisphere. 

All that wild energy – with polar blasts of air that are from places -50degC heading our way, at the same time as a tropical cyclone from near Fiji might be too, or a calm and settled high pressure zone exiting Australia that acts like Superman holding these frigid and tropical air masses from colliding over us, giving NZ those frosty blue dome days so many of us love in winter …

NZ is the “Goldilocks” belt between the freezing airmass and the tropical airmass with anticyclones that sometimes protect us from both – or encourage more from both. 

This warming up high in the skies over Antarctica may very well unleash us more storms, so it’s good knowing there’s an increased chance of severe winter weather and simply having that in the back of your mind to be on the look out for them. 

But also, hearing winter may be stormy feels like someone also saying “the sea may be a bit wet”. Perspective is always key.