MetService is forecasting a switch from calm and mostly dry conditions to windy, warm, wet weather starting on Sunday afternoon and continuing into Monday.
A heavy rain watch with possible thunderstoms has been issued for Tasman northwest of Motueka (for Sunday for 15 hours from 11am); the Paparoa Range and the ranges of Buller District (from 2pm on Sunday); and the ranges of the Westland District for 16 hours from 9am on Sunday.
MetService says the watch may be escalated to an orange warning for Westland District.
A ridge of high pressure has prevailed over the country this week, bringing a mostly dry start to June for most.
A high pressure system will keep pleasant today although Northland may be in for some more showers.
On Sunday the West Coast and Tasman will be the first to experience the heavy rain which will head into western areas of the North Island later in the day.
"A weather system arrives from the northwest, bringing warm, moist air to the country, with the risk of severe weather. All the North Island and much of the South Island are expected to experience at least some wet weather as it moves across. Mountainous areas are likely to see the heaviest rain, but other regions could also be affected," MetService meteorologist Mmathapelo Makgabutlane said.
The approaching system will also bring strong northerly winds. Areas such as Wellington, Marlborough, inland Canterbury, and the mountainous regions of the North Island could experience the strongest winds on Sunday and Monday.
Temperatures will also reflect the change. Sunday night into Monday morning will see many places with double-digit minimum temperatures, significantly higher than the seasonal norm.
"Daytime temperatures will also feel unseasonably warm. Napier and Hastings are projected to reach 22 degrees Celsius on Monday, due largely to strong northwesterly winds rather than abundant sunshine. However, there'll be moments of sunshine for much of Aotearoa New Zealand before then," she said.
