With the arrival of December, meteorological summer has officially begun, bringing with it the summer temperatures.

MetService is forecasting maximum temperatures of up to 8°C above average for this time of the year for Gisborne, Hawke’s Bay, and the eastern South Island midweek.

This is due to humid subtropical air, driven by strong northerlies moving over the country.

A mix of warnings, watches, and alerts have been issued for heavy rain, strong winds, and high temperatures.

Rain will begin to fall about the western South Island Monday evening as warm, humid subtropical air approaches from the Tasman Sea.

A Heavy Rain Warning covers the ranges of the Westland District from 6am - 6pm Tuesday, and Heavy Rain Watches are also in place for the first half of Tuesday for northern Fiordland and late Tuesday in the Grey and Buller Districts, as the main rain band moves northwards. 

Northerly winds are also expected to pick up about Wairarapa and southern Hawke’s Bay, resulting in a Strong Wind Watch overnight into Wednesday morning. 

A second rainband with similar impacts moves up the country through Wednesday, and further Warnings and Watches may be issued in the coming days. 

MetService’s Heat Alerts have also begun for the summer season, with the first likely to be issued this week.

"This air is coming to Aotearoa from the subtropics and bringing subtropical temperatures with it," said MetService Meteorologist Clare O’Connor.

"Not only are we expecting daytime maximum temperatures to reach the low 30’s for the eastern North Island, overnight temperatures will also be unusually warm.

"Midweek, many areas of the lower North Island and upper South Island will experience minimum temperatures of 17°C, no doubt disrupting sleep.

"Our first Heat Alerts for higher-than-average overnight temperatures are expected to be issued for locations such as Napier and Blenheim, both of which may even have overnight low temperatures as high as 19°C as a result of this airmass.”

Heat Alerts are issued by the MetService forecasting team when temperatures are expected to be extreme, i.e. hotter than a typical hot summer day.

Heat Alerts are triggered when a single day’s maximum temperature is high, or when sustained periods (two or more days) of very high average temperatures occur, in particularly when overnight temperatures are higher than usual, making keeping cool more difficult, increasing the risk of health-related health issues.

More information can be found on our website at https://about.metservice.com/our-company/national-weather-services/heat-alert-trial/

As the weekend approaches the warmer air gives way to showery southwesterly winds and a return to average temperatures for this time of year.

However, a building high pressure system will bring blue skies to most of the country to end the week.