Thunderstorms, small tornadoes and heavy rain are possible in western parts of the North Island this morning as a low moves across the country.

The same system has been responsible for the unseasonably warm weather in Auckland.

Over the past five days Auckland has had an average high of 21C - higher than November's average and just below December on 21.6C.

Whangārei has been the pick of the bunch, topping out at 23.9C on Monday - the fifth warmest May day since records began in 1967, and 22.6C yesterday.

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Niwa principal scientist Chris Brandolino said the main driver behind those warm temperatures was the position of the low in the Tasman Sea.

It was driving warm northerly winds over the country, pushing temperatures up across the North Island, but also bringing rain and thunderstorms to some areas.

However, as that low moves further east today it will drive cooler air north, dropping temperatures as a southwest flow moves in.

MetService meteorologist Matthew Ford said the low was still sitting just to the west of New Zealand this morning.

It would move slowly southeast today, and cross the middle of the country about midday bringing cold air and showers to most areas.

Western parts of the North Island and the top of the South Island are in for a wet day with potential thunderstorms.

Some of these thunderstorms could become severe about Waikato, Waitomo, Taumarunui and Taranaki, bringing localised downpours of up to 40mm/h and the possibility of small tornadoes.

Heavy rain watches are in place for coastal Waitomo, Taranaki, Nelson and northern parts of Marlborough through to this afternoon, with isolated thunderstorms and localised downpours possible.

There is also a risk of significant rainfall about Wellington and the Kāpiti Coast, including western parts of the Tararua Range mainly in the afternoon today.

Auckland, Northland and the Bay of Plenty will escape the worst of the weather but could see some heavy falls and potential thunderstorms from this afternoon.

A southwest flow moves over the South Island today, dropping temperatures into the low teens and bringing showers to most areas. Snow is forecast down to 800m for a time this evening.

Tomorrow a weak trough crosses the North Island, while a front moves on to the lower South Island.

"Once this low rolls by, over the next five days the country moves into an unsettled southwest flow, bringing periods of rain, and it could get cold at times," Ford said.

The front moves slowly up the South Island on Friday with potentially heavy rain about Fiordland, Westland, Buller and the Southern Alps, and severe gales in parts of Canterbury and Marlborough, through to the weekend.

It makes its way on to the North Island on Saturday and Sunday, with a moderate chance of heavy rain about the Tararua Range, Mt Taranaki and the hills and ranges from inland Taranaki across to Tongariro National Park. Many other parts of the North Island are also in for a wet weekend.

Another front is forecast to move over the South Island on Sunday.

Today's weather
• Whangārei

Showers, possibly heavy and thundery from afternoon. Northerlies turning westerly afternoon. 22C high, 12C overnight.

• Auckland

Showers, possibly heavy and thundery from afternoon. Northerlies turning westerly afternoon. 20C high, 13C overnight.

• Hamilton

Showers, possibly heavy and thundery from afternoon. Northerlies turning westerly afternoon. 20C high, 10C overnight.

• Tauranga

Showers, possibly heavy and thundery from afternoon, clearing evening as northerlies change westerly. 20C high, 12C overnight.

• New Plymouth

Periods of rain, chance heavy and thundery, easing evening. Northerlies turning westerly afternoon. 18C high, 10C overnight.

• Napier

Sunny spells. Chance evening shower. Northwesterlies. 22C high, 10C overnight.

• Wellington

Periods of rain, with some heavy falls and thunderstorms, especially this afternoon when northerlies turn southerly. 17C high, 9C overnight.

• Nelson

Periods of rain, possibly heavy or thundery, clearing afternoon. Northerlies. 17C high, 8C overnight.

• Christchurch

Occasional rain clearing late afternoon, then a few evening showers. Southwesterly developing morning. 13C high, 5C overnight.

• Dunedin

Cloud increasing. Brief afternoon rain, then a few showers. Southwest. 12C high, 4C overnight.