Weather: Wet and windy week forecast as westerly hits New Zealand

It's going to be a wet and windy week as a westerly makes its way across New Zealand.

MetService forecasts this week won't be as warm and muggy as the last and many western areas will bear the brunt of the cold weather.

"Western areas, the West Coast in particular, are set to have rainy weather, with the odd thunderstorm added in from time to time," says MetService meteorologist Tui McInnes.

"However, thanks to our mountain ranges on both islands blocking moisture from heading across, the eastern regions are set to stay mostly dry but maybe a bit cloudy."

Although western areas are set to have a mostly wet week, Gisborne and Hawke's Bay will be mostly dry, MetService says. The upper parts of the North Island may get some winds in the middle of the week and some wet weather too.

"With this dynamic weather, there are risks of severe weather and consequently we have a few watches and warnings in place for heavy rain and severe thunderstorms," McInnes says.

There is a severe thunderstorm watch in place for western areas of the South Island, mostly near and west of the Southern Alps from Nelson to Fiordland.

"These thunderstorms have the potential to bring localised heavy rain of 10 to 25 mm per hour, hail, strong wind gusts, and possibly small tornadoes," MetService's warning says.

"Additionally, there is a moderate risk that some of these thunderstorms could become severe, especially about northern Fiordland and through Westland, bringing localised downpours of 25 to 30 mm per hour, damaging wind gusts stronger than 110 km/h, large hail 20 mm in diameter or larger, and possibly even damaging tornadoes."

MetService warns driving conditions will be hazardous since there may be surface flooding and poor visibility in heavy rain.

They add that if any tornadoes occur, they will only affect very localised areas.