The severe weather watch issued for the Bay of Plenty this morning has been upgraded to a warning, while the watch remains in force for the Coromandel region.

An intense trough is expected to approach the country and cross northern and central New Zealand today.

A band of heavy rain is expectd to cross the Coromandel and Bay of Plenty today.

The MetService says a strong and moist northeast flow ahead of the trough is bringing rain with areas of heavy rain to many areas of the country, especially in the north of both islands and in the west of the South Island.

In the 12 hours from 3pm to 3am, but from 9pm east of Whakatane, 80 to 120mm of rain is expected to fall in the Bay of plenty.

Peak intensities of 15 to 25mm/h are forecast, but there is a possibility of 30mm/h in thunderstorms.

The Bay of Plenty has seen 4.2mm of rain in the last 24 hours, according to the Te Puke weather station.

'Also as the main frontal rainband crosses northern New Zealand, northeasterly gales could briefly rise to severe gale in some places,” says a statement from the MetService.

'The watch for severe gales in Northland and Auckland has now been upgraded to a warning, and a watch for possible severe gales will now be maintained for Coromandel Peninsula and the Waikato.”

A thunderstorm watch has also been issued for the Coromandel Peninsula and the Bay of Plenty as the active front moves across the country.

'This is an alert to the likelihood of localised downpours in the 25 to 40mm/hr range occurring in the vicinity of the front along with a risk of a small tornado, mainly in east coastal areas,” says the MetService.

'Rainfall of this intensity can cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips. Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.”

A severe thunderstorm watch means conditions are favourable for severe convection in and close to the watch area.