2.3 degrees hotter than usual, but chill on the way

May's weather was warm enough for basking in the park ... but it also brought a deadly tornado to Albany and a deluge that turned a Ngaruawahia street into a lake. Photo / Natalie Slade

New Zealand sweltered through its hottest May since record-keeping began ... but the weather was hardly perfect, as some areas were drenched in more than double their usual rainfall.

Figures issued today by climate agency Niwa showed that May was 2.3C warmer than usual.

The average monthly temperature was 13.1C, a heat normally expected for April.

The previous hottest May, recorded in 2007, had a mean temperature of 12.4C.

Niwa's principal climate scientist, James Renwick, said the numbers were extraordinary and unusual.

"A monthly jump of two degrees is extremely unusual - it's a surprisingly big step up," he said.

"We've had a very strong La Nina event in the tropics since about August last year, which brings weather from the north over New Zealand and warmer air down from the sub-tropics."

Mr Renwick said the warm tropical air flow also caused the destructive storms and flooding which hit the Bay of Plenty at the end of April.

"The warmer the air, the more water it can carry, which is why the areas which were the warmest, like the eastern Bay of Plenty, also had more than double their average rainfall.

"So it's been very wet in a lot of places and very warm, but with not a lot of sunshine."

At the start of May, eastern areas of the North Island were battered by torrential rain and gale force winds which caused widespread flooding and a state of emergency in Hawkes Bay.

Later in the month, Nelson's rivers were pushed to bursting point when the region had 3.5 times its normal rainfall. But its temperature was 3.5C warmer than normal.

At Whakatane, the airport raingauge showed the region had 2.5 times its normal rainfall.

Mr Renwick said the La Nina event, which was responsible for the record-high temperature, also caused the tornado which tore through Auckland's North Shore at the start of the month, killing a man at Albany.

"To get a vigorous tornado, you've got to get a vigorous thunderstorm," said Mr Renwick.

"And for that to happen you've got to have a lot of moisture in the air and energy."

He said global warming had increased New Zealand's average temperature by about 1C in the past hundred years, so other heat records were becoming more and more likely.

"It makes it easier to get a warm month because the background temperature keeps increasing."

But the warmth was unlikely to last until the long Queen's Birthday weekend, as a low pressure system would move south later in the week.

MetService forecaster Steve Rawdon said the Upper North Island would have thick cloud and showers from Friday, and the rest of the country would be mostly dry.

"It looks fairly likely that it will take place, but we've still got another couple of days of reasonable conditions at least with plenty of sunshine."

Most main centres would have temperature in the mid to late teens today, and the first week of winter was expected to be mild especially in northern New Zealand.

Mr Rawdon said eastern areas would have lots of sunshine.

"We'll lose the sting of the sou-westerly as the main high pressure centre moves a little closer.

"And what that does is bring a little bit of cloud into the eastern areas of both islands, but it will clear the weather a little in the west."

But the clear skies would also bring cold frosty nights with temperatures below zero in sheltered inland areas.

Weather Watch chief analyst Philip Duncan said the warm muggy weather predicted for the weekend would not last and a cold snap would start to move over the country next week.

He said next weekend's chilly temperatures would signal the start of winter.