Science of the autumn heatwave

The trend has continued into an unusually balmy May, in which temperatures have been averaging 1.5C above normal. Photo / Warren Buckland

What started warm will end on a much cooler note, says meteorologist.

It's been one of the warmest starts to the year that New Zealand has ever experienced, but Kiwis should start pulling out their winter woollies.

Since January, meteorologists have been chalking up record after record, including the hottest February yet seen.

The trend has continued into an unusually balmy May, in which temperatures have been averaging 1.5C above normal.

MetService meteorologist Georgina Griffiths said the first four months of the year were the warmest on the books for Wellington, while Auckland had also seen one of its hottest ever January to April periods.

"Most places have been running remarkably warm over an extended period and four months is not to be sneezed at, that's a third of a year."

She put the pattern down to a high number of northerlies, a virtual absence of southerlies and unusually warm seas around the country.

"When you have settled highs, with a lack of wind and lots of sunshine, you tend to have things not over-turn."

The El Nino system that peaked over summer was now all but over and played just a small part, she said.

This month temperatures broke records in some places - Dunedin and Ashburton, particularly, had seen days in the mid-20s.

"Most people might have noticed that they still haven't lit a fire, or that they've barely put a sweatshirt on."

The above-average temperatures hadn't affected the fruit industry, although the dry weather had proven a problem, a Horticulture New Zealand spokeswoman said.

And at Wanaka skifield Treble Cone, opening day would still go ahead as planned on June 23.

"It's still six and a half weeks away," Treble Cone sales and marketing general manager Nick Noble said. "It's a long time. A lot can happen in that time."

Ms Griffith said a cool change would come toward the end of this month, pulling temperatures back into a normal seasonal pattern.

"This May will be a month of two halves; the first half running 3 to 4C warmer, but we'll see more typical temperatures shortly," she said.

"The remainder of this week and next week are still going to be pretty breezy, but there's a good sou-westerly in the mix later to reduce those temperatures."

WeatherWatch.co.nz has already declared that autumn is finally here, after yesterday saw conditions more typical of the season prevail around the country, which it put down to a strong westerly change coming through behind a frontal system from the Tasman Sea.

A strong northwesterly airflow would lie over the country today, meaning gale northwesterly winds were again possible from afternoon, the website reported.

The North Island would see both sunny areas and some cloud, along with westerly winds for most places.

Despite the change in pattern, Niwa has predicted that temperatures between May and July will still "very likely" be above average in all regions of the country, but with frosts still occuring from time to time in cooler places.