Tasman low helps usher in tornado

Storm clouds hang heavy yesterday over Auckland but Rangitoto is still in sunshine. Picture / Lynn Clayton

The tornado that struck Waitakere yesterday was a result of a combination of daytime heating, warm, moist and unstable air brought over by a Tasman low and a trough moving down Auckland, the MetService said.

"All those factors combined to produce some impressive convection," said MetService duty forecaster Leigh Couper.

She said it was hard to predict if there would be another tornado but the potential existed.

The MetService said up to 45mm of rain had fallen at its Auckland airport gauge in the six hours to 8pm. The highest reading was 11mm in 10 minutes. It was believed that up to 40mm or 50mm might have fallen in an hour in areas that were heavily flooded such as west Auckland.

On the North Shore, 30mm of rain was recorded falling between 4pm and 5pm at Birkenhead Point.

The MetService warns of rising river levels and the likelihood of surface flooding in Northland, Auckland and the Coromandel Peninsula.

In Northland and Auckland, the heaviest falls were expected yesterday evening and this morning, where up to 30mm of rain could fall in an hour.

The Coromandel Peninsula gets its heaviest falls from later this morning, with up to 30mm falling an hour.

Ms Couper said a slow-moving front was also loitering over the upper South Island so most of the country would be quite damp and dreary until later in the week.

The wild weather comes as September marked a month of climatic extremes, with record low rainfall and high mean temperatures. While areas such as Fiordland and Westland received high rainfall, including 294mm in four days at Milford Sound, it was extremely dry in the east from Wairarapa to Otago.

Some parts of Canterbury received only a tenth or less of their average rainfall. There was no measurable rainfall at Kaikoura and about 10mm or less in some parts of Marlborough and Otago.

Rainfall was also at only 50 per cent or less of average in parts of Northland, Auckland, Thames, Nelson and much of the west of the North Island from Taranaki to Wellington.

- Additional reporting NZPA