1News weather expert Daniel Corbett looks ahead at the start of the meteorological summer.
The start of meteorological summer always gets us excited when we begin to think about fine days and summer holidays.
Then the reality sinks in – because of New Zealand’s location, midway between the equator and the poles, we can get a real range of weather even in the summer.
This summer the key for your holidays will be to dig out the sunscreen but keep that raincoat handy too because it could be one of those that turns into a smorgasbord selection from the buffet table at the local restaurant.
A bit of everything will be in the mix because there will be no strong signal or driver in the atmosphere.
That said there is some significant warmth and moisture brewing to our north. The atmosphere will try and do a 180 degree turn from last summer pushing us into a weak La Niña direction – but only just.
It will be a bit like just dipping one toe in the hot bath and the other on the ice-cold bathroom floor.
Since the very weak to almost neutral La Niña will not control all of the summer weather levers, other factors will get a say in what transpires.
Just a recap: Last summer we had a weak El Niño in place which favours more westerly winds blowing across the country and more rainfall in the west.
This is all due to a difference in the sea temperature anomalies across the Pacific Ocean which in turn affect how and where the clouds grow in weather systems. That didn’t stop significant warmth from making good inroads across the country last summer.
This summer we have a very weak La Niña/neutral set up. This will favour more oomph or lift in the atmosphere over the western Pacific and tropics.
It also can push our summertime high further south across the country which then brings the more humid northerlies into the north of the country.
The other factors we watch out for is blocking, which slows down approaching weather systems and causes them to linger in one place and dump potentially significant amounts of rain.
Two years ago, it was a strong La Niña and blocking during the summer of the Auckland Floods and Cyclone Gabrielle. This extreme La Niña set up is not likely this summer, but you will want to keep the raincoats handy.
For many of us, expect some decent fine spells at times from the passing highs. But all the warm seas to our west and north, along with more oomph in the atmosphere, could bring heavy rainfall events at times, especially if they linger in place.
So, enjoy the warmth but – as is quite often the case – have a Plan B if the rain does fall on that BBQ you’ve arranged.
And, whatever the weather delivers us, enjoy the summer.
NIWA’s forecast for the season
NIWA has also published its outlook for December 2024 to February 2025.
KEY POINTS:
- A La Niña Watch remains in effect.
- Recent signals in both the ocean and the atmosphere point to the tropical Pacific transitioning to more distinct La Niña-like conditions over the course of December. There is however still uncertainty as to whether conventional La Nina thresholds will be exceeded: international guidance indicates about equal chances for La Niña (50%) and neutral (49%) categories during December 2024–February 2025.
- Regardless of whether La Niña is formally declared over the forecast period, weather patterns broadly consistent with La Niña conditions are expected to become more established around Aotearoa New Zealand as the summer season unfolds.
- In early-to-mid December, a period of stronger-than-usual westerly winds is expected to bring rainfall, possibly heavy, to most western regions of the country. Thereafter, more typical La Niña-like conditions are expected to develop resulting in increased northeasterly quarter winds.
- As the season progresses, a lean towards more frequent rain events is expected from about the new year, particularly for northern and eastern parts of the North Island. Monitor NIWA for updates.
- Air temperatures are very likely to be above average across the country. Both anticyclonic conditions and increased northeasterly flows, which will be the themes for the season, may lead to spells of unusually warm temperatures, including warm overnights.
- Coastal sea surface temperatures (SSTs) remained generally above average around the country, ranging from about 0.5°C to 1.0°C above average during the past 30 days, but marine heatwave conditions have receded in both extent and amplitude recently. Global climate models’ forecasts are for SSTs to remain above average around New Zealand during the coming summer, and marine heatwaves will remain possible. Monitor the SST update for updates.
- December-February soil moisture levels and river flows are most likely to be near or below normal in the east of the North Island and the north and west of the South Island, and near normal in the east of the South Island. For the north of the North Island, below normal soil moisture levels are most likely, and near or below normal river flows are equally likely.
- Soil moisture levels at depth continue to be below normal in the east of the North Island.
