New Zealand’s bout of severe weather is being driven by a tropical low moving south, bringing large volumes of warm, moisture laden air over the country at the height of summer.
Four red-level heavy rain warnings have been issued for areas of the North Island: Northland, the Coromandel Peninsula, the Bay of Plenty including Rotorua, and Gisborne from Tolaga Bay northwards.
Nine districts have declared states of emergency, including the six making up the Bay of Plenty region.
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Earth Sciences NZ meteorologist Chester Lampkin told 1News the conditions might feel out of place for this time of year but this type of weather could occur during the summer months.
"Summer is still here. We are right in the middle of summer and, on occasion at this time of year, we do get tropical lows — the remnants of tropical cyclones. They can move south across the country," Lampkin said.
The main driver of the weather was a tropical low pressure system drawing moisture down from the Coral Sea and the South Pacific.
Lampkin said the moisture was being pulled south over New Zealand, primarily affecting the North Island, though parts of the South Island may also feel the effects as the system moves further south.
"As that low drives southward, it's going to bring in all that moisture from the Coral Sea, from the South Pacific and, as it hits the elevated terrain of New Zealand, it just squeezes out that moisture like a like a sponge or like a towel, and you just get heavy rain events."
Heavy rainfall has already affected Northland, Coromandel Peninsula, Gisborne and the Bay of Plenty, where MetService had issued red rainfall warnings.
The weather also reflected a La Niña pattern, which had been forecast for several months, Lampkin said, adding tropical intrusions of "warm, moist air" were more likely at this time.
"When that air comes in from the north and interacts with the land here in New Zealand, you get heavy rain, and that’s what we’re seeing."
He said the combination of a tropical low and La Niña represented a classic summer set up, rather than an early end to the season.
"This doesn’t mean summer is over. One part of summer is getting these northerly and north easterly flows that can bring heavy rain."
Although the focus is on the upper North Island for now, he said impacts in the South Island cannot be ruled out as the system moves south.
He also said it was too early to say exactly where records may be broken, but that rainfall totals could be substantial in areas under red warnings.
"Some places could see between 100 and 200mm of rain and, in the ranges, that could exceed 300mm," he said.
"That would certainly be toward the record-setting end."
This summer, La Niña increased the likelihood of wetter than normal conditions in the north, while parts of the south and west of the South Island could remain relatively dry.
Lampkin also commented on the influence of climate change.
"When you have a warmer background state, you're likely to have a warmer climate, therefore you're likely to have more extreme weather, in a sense."
People should pay close attention to MetService warnings, including rainfall and wind alerts, and take practical steps to stay safe, he said.
"Know your safety plans, and check on your neighbours, family and friends. This is when communities really come together during extreme weather."
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