Electrical storms are approaching the top of the North Island as two jet streams - one from the tropics and the other from Antarctica - merge, unleashing days of thunder, gales, rain and even snow.
About 1350 lightning strikes have already been recorded in the last hour over the Tasman Sea.
Niwa describes the incoming storm surge as “nasty”.
No part of New Zealand will be spared with the volatile cocktail set to sweep over all regions and last until Friday.
Most of the country is this morning under weather alerts including 4m-high waves set to pummel the Wellington western coastline and snow to cover the Southern Alps.
The near-dozen warnings and watches began in the early hours of Tuesday morning and last until Thursday morning.
Rain is already falling in Northland and Westland, ushering in up to 18 hours of stormy weather.
Thunderstorms are possible in many parts of the country especially across the north and west of the North Island with a threat of hail and tornadoes.
A severe thunderstorm watch is in force from 2pm to 10pm, spanning Northland to Whanganui.
MetService is warning damaging wind gusts of around 110 km/h and tornadoes may strike along with squally downpours.
The forecaster warns that if tornadoes occurred, they would only affect very localised areas but could cause structural damage, including toppling trees and powerlines and making driving hazardous.
A heavy rain warning has been issued for the eastern Bay of Plenty and the ranges of Gisborne.
Heavy rain watches have also been issued for Northland and the Central High Country across to Taranaki in the North Island.
There are also strong wind watches for Northland, Auckland, Coromandel Peninsula, Bay of Plenty and Gisborne, beginning at 7pm today.
It is likely the strong northerly winds will approach severe gale force in all these areas, possibly gusting 100km/h in exposed places.
MetService meteorologist John Law explained this was due to “a deepening area of low pressure” set to barrel over the North island Tuesday morning, bringing a period of heavy rain, thunderstorms and strong gusty winds with it.
“The extra warmth and humidity that we’ve seen over New Zealand these last few days has helped to spark off some very active thunderstorms, and we are likely to see some more thunderstorms today and Tuesday,” Law said.
Law warns that the thunderstorms are “likely to bring some damaging winds and downpours” and areas such as Gisborne and the Bay of Plenty can expect up to 35mm/h of rain in during downpours.
Winds were also expected to reach severe gale through parts of the Canterbury High Country today.
There is a severe wind watch in place for the area, beginning at 8am this morning and lasting until 2pm this afternoon.
A heavy rain warning is in place for the ranges of Westland south of Otira and Fiordland north of Doubtful Sound, with thunderstorms possible and rainfall amounts approaching 25mm/h or more.
A heavy swell warning is also in effect along the Kapiti coastline for 36 hours from early tomorrow morning and lasting until Thursday evening. MetService is warning 4m-high waves will wash ashore Wednesday afternoon.
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