"We have a thunderstorm risk for the Bay of Plenty region for today. We do have some thunderstorms offshore at the moment. They may touch down in the evening in the Bay of Plenty area."
The low was expected to stick around tomorrow before moving away on Sunday.
"Tomorrow there is the risk of thunderstorms in the Hawke's Bay in the afternoon and evening as the system moves away to the east and we get that cold air moving up [the country]."
Most places will get temperature highs in the low to mid teens.
Although the rain is expected to ease to "sporadic showers" in Auckland tomorrow evening, a few showers will hang around for the All Blacks first clash against the touring British and Irish Lions.
Even the far south of the South Island, the area that had the best weather today, will be affected when the next front comes in.
"Tomorrow afternoon and evening we're going to get some rain moving into Westland and Fiordland. Then during Sunday morning that's going to move up rest of the country and on to the western side of the North Island."
Skies around the country would dry out later in the day on Sunday but more wet weather was forecast to move in on Monday, Mercer said.
Saturday's forecast
Whangarei:
Rain, with possible heavy falls, easing to the odd shower by afternoon. Strong southwest, easing evening, high of 16C
Auckland:
Rain, with possible heavy falls, easing to the odd shower by evening. Strong southwest easing evening, 15C high
Hamilton:
Rain, with possible heavy falls, easing to the odd shower in the evening. Southwest developing morning, high of 14C
Tauranga:
Rain, with possible heavy falls, easing to the odd shower evening. Winds changing southwest afternoon, high of 17C
Wellington:
Periods of rain, clearing overnight. Southerlies die out evening, 12C high
Christchurch:
Early rain easing to a few showers in the morning, then clearing in the afternoon. Southerlies, 11C high
Dunedin:
Mainly fine, but a few spots of late evening rain. Light winds, 11C high
(Source: Metservice)
