The forecaster said low-level moisture and sea breeze convergences were expected to trigger showers over some inland areas of the South Island later today.
There was a moderate risk of thunderstorms over Central Otago, eastern Otago and Dunedin, and further north in Buller, Nelson/Tasman and the Canterbury High Country.
The storms could come with small hail and rainfall rates of up to 25mm/h.
Meanwhile, motorists are being urged to take care on roads across the upper North Island this afternoon and evening, amid potentially severe thunderstorms that could bring localised downpours reaching up to 45mm/h.
MetService has issued a severe thunderstorm watch for some northern regions through to 8pm, with torrential rainfall and hail possible in areas.
In parts of Auckland, Waikato, Coromandel and Bay of Plenty west of Rotorua, there was a high risk of thunderstorm activity – and a moderate risk that they could be severe, with rainfall rates of 25mm/h to 45mm/h.
Across wider affected areas, MetService was giving a moderate risk of thunderstorms from Northland, down to Waitomo and across to the western Bay of Plenty during the afternoon and into the evening.
These storms could come with heavy rain reaching rates of 10mm/h to 25mm/h and hail – and there was also a low risk of small tornadoes or funnel clouds developing, mainly between Northland and Waitomo.
The forecast came as lingering moisture from a slow-moving front combined with daytime heating and cooler temperatures at upper levels over New Zealand.
“Almost the majority of the northern half of the North Island is at risk of seeing some heavier showers today – and it’s all tied up with an old low that’s hanging to the north-east of the country,” MetService meteorologist Lewis Ferris said.
Conditions were already very humid in areas – the dew point broke 20C at Auckland Airport today – and direct sunshine this afternoon could cause this surface warmth to rapidly rise up, creating thunderstorm conditions.
“People will definitely see growing clouds this afternoon, and if they’re unlucky, they might get caught under one of the heavier showers, or potentially even thunderstorms,” Ferris said.
“If that rain is really coming down where you happen to be driving, it’s just about slowing down and taking care.”
Elsewhere, MetService reported a low risk of thunderstorms in other northern areas down to Taumarunui and Taupō, and across to northern Gisborne.
Ferris encouraged people in potentially-affected areas to keep an eye on forecasts.
“The watch covers the potential of severe thunderstorms, if we observe one on our radar network then we will issue a Red severe thunderstorm warning, this means that impacts are likely and people should avoid the area if possible.”
Eastern areas of the North Island were likely to be affected by onshore winds until Wednesday, meaning overcast days with rain, more persistent about higher ground.
While the upper half of the North Island and inland areas of the South Island, was expected to see light winds and clear skies this week, during the afternoons, showers could pop up, with the potential of thunderstorms sprinkled in the mix.
Warm temperatures were also on the menu in places, with Christchurch forecast to reach 26C on Thursday - around 6C warmer than their average daytime maximum for this time of year.
But Cantabrians would need to wrap back up on Friday, when an outbreak of cooler air from the Southern Ocean pushed that high down to a forecast 14C.
