It's the summer that never really ended.

On the back of what was the second warmest February in New Zealand history, the first month of autumn has been proving much balmier than usual.

The heat that yesterday pushed temperatures in many cities to the mid to upper 20s has been characteristic of a month in which Auckland's mean has been 20.C - 1.8C above average.

It's been a similar story in centres such as Whangarei, Tauranga, Hamilton, Taupo, New Plymouth and Christchurch, where temperatures have soared as high as 32.9C and reached the upper 20s yesterday.

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"The reason for the warmth is pretty simple," said Chris Brandolino, a forecaster at the National Institute for Water and Atmosphere (Niwa).

"We've had a persistent flow of air from a warm region - and that's meant more northwesterlies and northeasterlies."

When he and his colleagues sat down at the end of February to analyse what the data was saying about the coming season, the indications weren't weak, but strongly convincing.
The outlook they issued at the start of March has held true: warm air flows from the tropical Pacific have delivered above average temperatures for the entire country.

"We've had the good fortune of high pressure to the east and to the north of the country, and that's something that's probably going to persist for most of autumn," Mr Brandolino said.

"So you may sneak in a few more days at the beach, but there will be cold snaps as winter gets closer to us - especially for southern areas."

MetService meteorologist Georgina Griffiths said a blocking high that's been in place to the north of the country since January, coupled with warmer seas heated by a lingering El Nino climate system in the tropical Pacific, had played big parts.

"And we've seen virtually no southerlies."

But as soon as the blocking high moved eastwards, she expected the "tap to turn on", delivering more bouts of heavy rainfall over the remainder of the month.

Until June, Niwa predicts a mixed bag of rainfall over the next few months, with near normal levels for the north of the North Island and east of the South Island, but normal or below normal levels for the west and east of the North Island.

Meanwhile, forecasters say parts of the country could be hit by flooding and severe gales as a tropical low packed with showers and thunderstorms moves across the country today and tomorrow.

The weather will bring rain to many parts of the country prior to Easter. Photo / iStock
The weather will bring rain to many parts of the country prior to Easter. Photo / iStock

The heaviest downpours were expected to hit this evening, bringing as much as 250mm of rainfall to some parts of the central West Coast and Tasman, and up to 200mm in places between the Bay of Plenty and Northland between tonight and early tomorrow.

Niwa reported bouts of rain could contain winds reaching 100km/h and lead to road flooding and beach erosion.

In Auckland, there was concern a storm surge combining high tide, heavy rain and pounding waves could put some coastal roadways - particularly Routes 6 (Judges Bay) and 7 (St Heliers) - at risk of flooding between 6am and 10am tomorrow.

The low is expected to move away on Friday for the start of Easter weekend, with winds turning southerly, though a few showers are expected to remain around the Nelson and Buller regions.

MetService has issued a weather warning for Northland and parts of the South Island with a potential for localised flooding, slips and hazardous driving conditions.