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An “atmospheric river” is starting to wash over parts of both islands with more than 31 hours of rain - and a whopping 450mm - expected in one region over coming days. 

A heavy rain watch has been issued for Auckland and Northland - news that forecasters say will be “welcomed” by farmers in dire need of moisture. 

Western regions of the South Island now lie under heavy rain warnings with more than a day of rain to start falling from 10pm tonight. 

MetService meteorologist Lewis Ferris said the “deep low” had begun crossing the country, dragging over humid air and heavy rain from the tropics. 

A heavy rain watch has been issued for Auckland for 18 hours from 10pm Thursday. 

Northland is also under a watch from midday Thursday for 24 hours as is Waikato from 3am Friday for 14 hours. 

At the top of the South Island, as much as 450mm is expected in Tasman northwest of Motueka for 31 hours beginning 3am tomorrow. 

Forecasters are warning there will be a moderate chance of the current orange warning to be upgraded to the highest red level. 

Ferris said the rain would be “welcomed” by farmers as they deal with dire drought conditions. 

He said although the rain would not be enough to be a droughtbreaker, it would be a “good help”. 

“So good news for the farmers and the people in need of the moisture, bad news for the people that want to see a continuation of the summer.” 

Ferris said the Tasman region is expected to see the largest deluge with 350 to 450mm or possibly more in the ranges and 150 to 250mm elsewhere. 

A heavy rain warning has also been issued for Mt Taranaki from midday Thursday, the Buller District from 3am Thursday and the ranges of Marlborough from 11am. 

These warnings are also expected to last approximately 30 hours with MetService urging residents to remain vigilant for surface flooding and slips. 

Thunderstorms are possible in these regions along with 150-200m of rain in Buller and Westland and 200-300mm in Marlborough. 

A drought was declared in the Northland, Waikato, Horizons, Marlborough-Tasman and Taranaki regions, with farmers buying extra feed and sending stock to greener pastures to give their paddocks a respite. 

Meanwhile, Watercare’s drought management plan was activated last month after ongoing dry weather pushed the levels of the Auckland region’s dams down. 

Meanwhile, warm, humid nights are also tipped for later this week as a moist northerly airflow covers the country. 

MetService said overnight temperatures in the mid-to-high-teens could be expected for many – a drastic change from recent single-digit temperatures that had brought frost to some areas. 

“Auckland can expect night-time lows of around 18C for most of this week, while Christchurch will only drop to 15C on Friday morning – its warmest overnight temperature in over a month,” the forecaster said. 

“The last time much of the country saw overnight temperatures this warm was mid-February, with a similar system from the Tasman Sea. The result will be very different feeling weather compared to the cold fronts from the southwest that have been the norm recently.”