The storms are expected to include torrential downpours, with up to 40mm of rain falling in an hour, which can lead to slips and flash flooding.
Wind gusts are expected to reach about 90km/h, and there is a chance of a “damaging tornado”, according to MetService.
Hail the size of gumballs could also fall.
MetService said Aucklanders would see a deterioration in weather conditions from midday and in Northland the conditions would be stormy from the morning until early afternoon.
Auckland Emergency Management warned the wind gusts could cause structural damage, and the rainfall could lead to flooding and slips.
“Driving conditions will also be hazardous, with surface flooding and poor visibility in heavy rain. Do not drive through the floodwater.”
The same watch is in place for Coromandel Peninsula, Waikato, Bay of Plenty, Rotorua, and Taupō from 1pm until 6pm.
“Unstable showery conditions spread southwards on to the central North Island this afternoon, with a moderate risk of thunderstorms.”
The thunderstorms are expected to ease by this evening, but strong winds are forecast to persist.
A heavy rain warning is in place for the Bay of Plenty, including Rotorua and Gisborne, north of Ruatōria from 11am for 12 hours.
A heavy rain watch is in place for Wairarapa, the eastern hills and ranges of Wellington and the Kaikōura Coast from 9am.
Much of the central North Island is also under a watch from midday, with the risk of thunderstorms in the evening as they move off the upper half of the island.
Severe thunderstorms and downpours are likely this evening in eastern areas of the Waikato and Taupō.
MetService has also issued a raft of strong wind watches for much of the North Island.
This includes Auckland and Northland from 9pm, with winds approaching severe gales in some areas.
A watch is in place for Horowhenua to Wellington, Wairarapa, the ranges of Marlborough and Nelson east of Wakefield overnight tonight, beginning from 6pm.
Watches are also in place for eastern areas of the Tararua District and Hawke’s Bay, about and south of Cape Kidnappers and also Mahia Peninsula.
MetService meteorologist Mmathapelo Makgabutlane said it had been a “very active” morning of weather, with 3671 lightning strikes between midnight and 8.30am.
She said 728 reached the ground, causing danger for people, livestock and the electrical grid.
Makgabutlane advised people, especially North Islanders, to keep a close eye on the rain radar as there was a chance thunderstorms could roll in at any point today, bringing damaging hail, heavy rain and more lightning.
“The atmosphere is so primed for thunderstorm activity today,” she said.
Auckland Emergency Management general manager Adam Maggs warned drivers to avoid surface flooding on the roads.
“Turn around and find an alternative route, even if it adds a bit of time to your journey,” he said in a video shared on Facebook.
“Driving through water can cause damage to your vehicle’s engine, something no one wants in the lead-up to the holidays.”
Maggs said floodwater could also conceal debris and other dangerous hazards, and could rise rapidly.
“So the best thing to do is to stay out of it altogether. Take it slow and don’t go.”
NZ Transport Agency Waka Kotahi (NZTA) is urging people to drive with care and caution on Coromandel roads.
“Heavy rain is scheduled between 4am-11pm across the peninsula, including the west from Thames to Coromandel, the north from Coromandel to Whitianga and the east from Whitianga to Waihī,” the agency said.
“There is the potential for localised downpours, thunderstorms and high winds.
“These conditions are likely to bring with them risks such as surface and flash flooding, slippery road surfaces, fallen trees and slips.”
NZTA said drivers should anticipate trees or debris on the road just around each corner.
“Slow down and keep a safe following distance from the vehicle in front. Keep your headlights on to increase visibility.”
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