Weather: Colder than usual September expected for NZ

The normally tranquil swimming holes above McLaren Falls in the Kaimai Ranges were turned into a torrent of whitewater yesterday. Photo / Alan Gibson

September is going to be colder than usual for all of New Zealand after a very wet and cold winter.

MetService Meteorologist Georgina Griffiths said since the middle of winter New Zealand has generally been running colder than normal, due to a higher frequency of southwesterlies and cooler than usual seas around our coastline.

"This is standard El Nino for us," Ms Griffiths said.

"After an extremely wet winter for the west and south of the South Island, and between Taranaki and the Kapiti Coast, many people are keen to know how the start of spring might unfold," she said.

It was the wettest winter on record for Hokitika Airport, with 1092mm of rain observed.

Record-breaking rainfall was also experienced at Palmerston North this winter, with 413mm recorded, making it the wettest winter since records began in 1943.

It was the fourth wettest winter since 1890 in Wanganui, where 396mm of rain was recorded.

"After a stormy start to September, we should then see a progressive shift towards more highs over the country," Ms Griffiths said.

"The first week of the month should produce significant rain for all regions of the country, including useful rain into the relatively dry eastern areas. But, for the remainder of the month, normal to below normal rainfall is predicted for all areas."

Thunderstorms on the way

Thunderstorms bringing heavy downpours, hail and a small chance of a tornado in Auckland could be arriving today.

The first band of isolated thunderstorms is moving into western Waikato and Auckland region from the Tasman Sea.

MetService meteorologist Tom Adams said the thunderstorms would bring heavy downpours to some areas, but won't be felt in other areas.

"It's a very bubbly convection system," he said.

WeatherWatch analyst Philip Duncan said the thunderstorms would be noticed around Auckland and Hamilton between 9.30am and 11am.

MetService said the first band of thunderstorms will pass over, leaving cool air and some strong wind gusts. Hail could also fall in parts of Auckland.

"There is a small chance of a tornado, but it's not a Kansas tornado," Mr Adams said.

"But we do have it in our warnings, there are small effects with the thunderstorms that are hard to distinguish from," he said.

The rain would be short bursts of heavy rain, Mr Adams said.

"It won't be as bad as yesterday."

Once this morning's band of thunderstorms moves away, several more will follow, with growing intensity, Mr Adams said.

From evening the rain falls become more frequent and heavy for the top half of the North Island.

Temperatures are warm in Auckland currently, but will drop a degree or two when south-westerly winds arrive.

In Wellington it's fairly warm this morning on a day where showers are expected to hang around.

For the South Island, there's scattered rain that will become more frequent as a southerly wind arrives.