WeatherWatch says an area of lower pressure and cold upper air is moving over the South Island today, bringing unstable conditions to many regions.

"Meanwhile, a southwest airflow will bring some showers to western parts of the North," says a WeatherWatch spokesperson.

"A weak cold front is responsible for the changing conditions across the country after what has been a much-welcomed settled weekend thus far."

Starting off with the pick of the regions, areas about the central and eastern North Island are set to remain largely unaffected by the approaching front.

A mainly fine and sunny day can be expected about the Bay of Plenty, eastwards to Hawke's Bay and down to Wairarapa.

Image: WeatherWatch.

"Cloudier weather with southwest breezes and a few showers are likely further west, particularly about coastal areas of Taranaki and Manawatu.

"Partly cloudy with the odd passing shower for Northland, Auckland and the Waikato. Evening rain develops in Wellington and the Kapiti coast as the front makes its arrival."

For the South Island, WeatherWatch says to expect squally showers along the West Coast with thunderstorms and hail possible.

"Showers are due to develop across Southland, Otago during the morning and into Canterbury later in the day as the cold front moves north, accompanied by Nor'westers changing southwest.

"Some showers may be heavy, with isolated thunder and hail.

"Mainly fine about Nelson and Marlborough, although a few showers are also likely to pop up later in the day as westerly winds change southwest."

WeatherWatch says temperature highs on the cool side can be expected for most thanks to the arrival of the colder southwest flow.

"About 10 degrees is likely to be the maximum across Southland and Otago, 12-14 degrees in Canterbury, and nudging slightly higher across the remainder of the country, up to 18 degrees in the far North and East of the North Island."

WeatherWatch