Temperatures across the country are set to take a dive today as a cold southerly sweeps over New Zealand.

Sizzling summer days in the mid-30s and restless nights thanks to a humid 20C will seem like a distant memory as temperatures plummet to the teens and overnight lows hit single digits.

Those in the lower South Island might want to wear an extra layer of clothing and throw the duvet back on the bed as the forecast high in Queenstown today is only 13C and drops to a shivering 3C tonight.

Dunedin is currently sitting on just over 11C, a big drop from the 35C recorded a month ago, and isn't expected to get over 20C until Tuesday. Christchurch isn't much better, with a high of 15C forecast and an overnight low of 8C.

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The North Island isn't looking at any better treatment. Much of the lower North including Wellington, Palmerston North and New Plymouth, will be in the mid to high teens through the day, and Palmerston North is set to get as low as 5C tomorrow night.

The top of the north, including Whangārei, Auckland, Hamilton and Tauranga, will sit in the low 20s during the day, and overnight lows are expected to get down to
7C in Hamilton and 11C in Auckland - a massive change from those sticky, humid nights a few weeks back.

Strong winds and "plenty of rain" is also set to hit eastern parts of both islands today, with severe weather warnings in place for Hawke's Bay and eastern Marlborough.

The area of Hawke's Bay north of Tutira is expected to get 75mm to 100mm of rain and thunderstorms are possible.

Eastern parts of Marlborough should expect a further 20mm to 30 mm of rain to accumulate on top of what has already fallen.

Heavy rain watches are also in place in Gisborne, Hawke's Bay, Tararua District and Waipara.

The watches are valid in Gisborne until 9am tomorrow, Hawke's Bay until 3am tomorrow, the Tararua District until 9pm today, Wairarapa until 3pm today.

An earlier heavy rain watch in the Canterbury foothills has been lifted, and rain has now eased to showers.

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Today's weather forecast is for deteriorating conditions and as such the Wings team has made the difficult decision to...

Posted by Wings Over Wairarapa on Saturday, 23 February 2019

Day two of air show Wings over Wairarapa today has been cancelled because of the bad weather.

"Today's weather forecast is for deteriorating conditions and as such the Wings team has made the difficult decision to cancel the show," the website said.

"This is because of our concerns for public safety and for the safety of our pilots flying in minimal conditions."

The heaviest falls were earlier expected about the Kaikoura Coast and Ranges, where a warning was in force. It has now been lifted.

MetService meteorologist Stephen Glassey said 65mm of rain had fallen in the area since 4pm yesterday - a lot of it since midnight. Lighter showers are expected to continue into tomorrow.

Overnight, Porters Pass and Methven received the most rain: 70mm.

WeatherWatch reported today's southerly winds will be made stronger and colder by a large high in the Tasman Sea.

Glassey said strong southerly winds were being felt in Cook Strait as well as eastern parts of central New Zealand.

Auckland is forecast for occasional showers, possibly heavy, throughout today and a high of 21C.

Tauranga will also have intermittent showers and a high of 23C.

Wellington can expect rain for the duration of today and a high of 16C.

Christchurch is forecast for showers in the morning, to ease by 4pm, with cold southerly winds, and a high of 15C.

Tropical cyclone Oma has passed

In good news, Cyclone Oma is "definitely not" heading to New Zealand and the tropical air which has fuelled it, creating a drizzly nuisance across the North Island this weekend, is set to dissipate today.

WeatherWatch says Oma is trapped in the Coral Sea for the coming week, remaining "well away from New Zealand", as it heads back north potentially towards New Caledonia.

MetService meteorologist Rob Kerr reiterated this, noting the humid air that caused a wet weekend across Northland and Auckland passed off to the east yesterday evening.

Kerr noted that Oma had diminished in intensity to the point where all of the characteristics which classify it as a tropical cyclone had passed - except for gale-force winds.

"The good news is Cyclone Oma is definitely not heading to New Zealand, but we're going to see all the temperatures dive right across New Zealand tonight and be much cooler than it has been over the last few weeks," Kerr said yesterday evening.

Kerr said by late Monday, early Tuesday this week, another high-pressure ridge would push off the wet southerly winds, creating fine conditions.

WeatherWatch said this high-pressure system returning to New Zealand mid-week would also protect us from new tropical storms potentially generating east of Fiji.

"While Fiji itself isn't currently in any firing line or risk for a storm the next tropical cyclone may well form east of Fiji around the middle of next week," WeatherWatch said.

"As for New Zealand, high pressure returns to NZ next week which means any tropical storms should stay well north in the tropics.

"Our forecast, generally speaking, is for drier-than-average weather to continue next week once the southerly has gone by the end of Monday as the following high-pressure system looks very solid."

MetService's Kerr, however, was more circumspect about the prospect of a new Cyclone forming in the pacific this week, saying: "I wouldn't want to say with any confidence this far out."

Kerr did, however, allow for the possibility that Cyclone Oma would re-intensify as it moved north, away from New Zealand, this week.