Warm but wet spring looking likely in latest Niwa prediction

After an unusually wet autumn and several polar blasts this winter, spring looks likely to be warmer than average this year.

Niwa's weather prediction for the next three months showed temperatures around the whole country were up to 65 per cent more likely to rise above averages for this time of year.

Principal forecasting scientist Chris Brandolino warned a higher chance of warmer temperatures didn't mean three months of balmy weather.

"It's important to remind people it doesn't mean there won't be cold snaps or chilly days."

It was also good to remember Niwa's predictions were averages, so a warmer three-months overall could still mean a cold September for example, which could be averaged out by a warmer November, he said.

Inland Otago, the foothills of the Alps and the West Coast were the most likely to see warmer-than-average temperatures, and Gisborne, the Hawke's Bay and Wairarapa were slightly less likely than other areas.

Coastal water temperatures around New Zealand were forecast to remain above average over the next three-month period as well.

As an island nation New Zealand was hugely affected by ocean temperatures, which were predicted to be warmer than normal.

"That's a meaningful factor in what we're expecting for the next the months in terms of temperature," Brandolino said.

Air being sucked in from the north thanks to high pressure systems to the east and south and low pressure to the west was also a factor in the prediction for warmer weather.

Northern air also brought with it a bigger chance for storms.

"When we have winds from the north it's air with a lot more moisture typically.
"Combine that with the warmer ocean temperatures [which can be] fuel for storms. That can lead to a higher chance for heavier downpours."

The North Island and top of the South had a slightly increased chance of heavier rain than usual, but most of the South Island should experience normal rainfall, Niwa predicted.

While the ocean - atmosphere system in the tropical Pacific Ocean overall remained consistent with an El Niño - Southern Oscillation (ENSO) neutral state, La Niña-like signals became more prominent in the atmosphere during July and this trend continued during August 2017.

ENSO neutral conditions were likely to persist over the next three-month period, as they had in July and August, Niwa said.

Ocean and atmosphere may continue to periodically exhibit La Niña-like signatures which could account for increased rainfall, however.