Heavy showers and thunderstorms are expected to develop in the North Island later on Monday, MetService says.
MetService is predicting the rest of the weather to be mostly fine with showers mainly inland until Friday when a large low-pressure system is expected to approach the country, possibly bringing warning amounts of rain.
The weather has been mainly settled on Monday, with occasional showers developing for much of the country later in the day.
MetService said atmospheric conditions will become unstable over the North Island on Monday afternoon and evening, which is expected to produce scattered heavy showers and a few thunderstorms over the central North Island.
"A ridge of high pressure over the country this week brings light winds to many places, which will allow sea breezes to develop during the daytime. Where these sea breezes converge with light winds from another direction it will force the air to be directed upwards, resulting in cloud build-ups, showers and potentially thunderstorms," MetService meteorologist Peter Little said.
"Converging winds alone don't always lead to the development of showers and thunderstorms. There also needs to be enough warmth and moisture near the surface, and cooler than normal temperatures in the upper atmosphere, which makes conditions unstable."
The main risk of thunderstorms is about the Kaimai ranges, Waikato and Waitomo during the afternoon and early evening.
MetService is forecasting that a ridge of high pressure over the country will keep any significant weather systems at bay until Friday.
Temperatures across the country are expected to be above average this week. Inland places are expected to be warmest, peaking in the mid-twenties.
Weatherwatch said Tuesday is likely to be the warmest day this week.
Rain to end the week
On Friday, MetService said a large low-pressure system should approach the country from the Tasman Sea. This large low will be preceded by a strong north-to-northeast wind flow and is forecast to bring rain to much of northern and central New Zealand.
"At this stage, our severe weather outlook has a few risk areas for warning amounts of heavy rain around Mount Taranaki and the northwest of the South Island on Friday," Little said.
There is low confidence that rain will reach warning amounts for Mount Taranaki and the ranges of Tasman, western Marlborough and Buller on Friday and Saturday.
Windy conditions are possible about the north of the North Island on Friday and Saturday and about central New Zealand on Saturday, but at this stage warnings are unlikely, MetService said.
Easterly winds and rain are expected to develop about Canterbury and Otago on Saturday, and there is low confidence that a rainfall warning will be needed for the South Canterbury Plains, North Otago and Dunedin on Saturday.
