New Zealand’s weather has a reputation for being unpredictable and keeping both locals and forecasters on their toes.
However, behind the famous “four seasons in one day” catchphrase is a perfect storm of geography, winds, and oceans.
“New Zealand is an isolated, island nation in the middle of the roaring 40s, where fast moving weather systems are not uncommon, contributing to our highly changeable weather,” said MetService.
The roaring 40s refer to a belt of strong westerly winds that circle the Southern Hemisphere.
These prevailing winds, paired with air masses from both the subtropics and the polar south, mean New Zealand is constantly caught between weather extremes.
1News’ meteorologist Daniel Corbett said this positioning makes New Zealand the “perfect breeding ground” for unpredictable weather.
“We’re getting cold, we’re getting warm, all mixing together.”
Corbett said weather systems can often “get stuck” and don’t move across the country.
“Sometimes these areas of cold higher up, these cold pools as we call them, move through and then they get surrounded by warm air. And it’s like a car sitting in a lump of mud, spinning its wheels. It doesn’t move, so you get the rain for several days.”
North to south variations
New Zealand’s long and narrow shape, stretching more than 1600 kilometres from Cape Reinga to Bluff, means varied seasons and vastly different climate conditions between the north and south.
A MetService spokesperson said this led to “highly changeable day-to-day weather” that was often influenced by multiple weather systems.
“It’s rare for every region in New Zealand to be experiencing similar weather conditions.”
Mountainous terrain and surrounding ocean also play a role.
When westerlies push rain onto the South Island’s West Coast, the Southern Alps shield the east from bad weather.
“However, it’s those same Alps that also create the Foehn wind effect with Canterbury hit by strong and warm northwesterlies.”
The Tasman Sea provides a “breeding ground” for low pressure systems that often produce heavy rain, strong winds and thunderstorms, MetService said.
Systems from New Zealand’s northwest can bring warm or mild air, while systems from the Southern Ocean bring cold air from the sub-Antarctic regions and are responsible for cold snaps.
To keep up with these systems, MetService operates a high-resolution weather model down to 200 metres, and receives satellite imagery every 10 minutes from a Japanese geostationary satellite near the equator.
Despite that, radar coverage can be patchy due to mountainous terrain and the curvature of the Earth.
“There are still gaps in coverage in the lowest parts of the atmosphere,” the national meteorological service’s spokesperson said.
“Sudden changes in weather, particularly thunderstorms, are challenging to forecast anywhere in the world, but it is even more challenging in mountainous countries such as New Zealand where the weather varies rapidly from one place to another.”
What next?
The future of forecasting in New Zealand could benefit from AI advancements, MetService said.
“Computers are getting faster, including the super computers that run weather models, which means higher resolution outputs are possible, providing a more detailed picture of the incoming weather.
“This finer detail is especially helpful when looking at smaller features such as thunderstorms or local wind conditions.”
For now though, MetService advised Kiwis to stay vigilant to “highly changeable” weather.
“Weather can often deteriorate quickly. Keeping updated with the latest MetService forecast and severe weather information, puts you in the best position to stay safe and keep those around you safe.”
Watch Daniel Corbett’s explanation of our unpredictable weather on TVNZ+
