Rainfall of this intensity can cause surface flooding, slips and hazardous driving conditions, while large hail can cause damage to crops, orchards, vines and glasshouses and also make driving hazardous. If any tornadoes occur, they will only affect very localised areas.
A warm front is also forecast to move on to Northland this afternoon and then down to northern Auckland and Great Barrier Island this evening, bringing a low risk of thunderstorms to these areas.
However, this risk of thunderstorms is considered moderate about northern parts of Northland this afternoon and evening. Any thunderstorms that eventuate are expected to bring localised heavy rain of 10 to 25mm per hour or possibly more.
The Ministry of Civil Defence and Emergency Management advises that as storms approach you should: take shelter, preferably indoors away from windows; avoid sheltering under trees, if outside; move cars under cover or away from trees; secure any loose objects around your property; check that drains and gutters are clear; and be ready to slow down or stop, if driving.
During and after the storm, you should also: beware of fallen trees and power lines; and avoid streams and drains as you may be swept away in flash flooding.
While the weather in New Zealand's largest cities, Auckland and Wellington has been stunning today, the weather is on the change for both tomorrow.
Wellington can expect a cloudier day with showers, mainly in the morning, while Auckland will see cloud increasing and periods of rain from tomorrow afternoon.
A Severe Weather Watch for heavy rain has been issued for the upper North Island from tomorrow afternoon through until Tuesday, Christmas Day, in the evening.
This is associated with a complex low pressure system spreading on to the upper North Island. Most parts of the North Island will experience rain during this period, but the heaviest falls are expected across Northland, Auckland, Great Barrier Island and the Coromandel Peninsula.
The low and its associated fronts are expected to continue to move southeast over the North Island, and rain spreads south on to the top of the South Island later on Tuesday.
There is moderate confidence that rainfall accumulations will reach warning criteria from Taranaki to northern Gisborne and northwards to Northland.
The low should move southeast away from the country on Wednesday, followed by southwesterly flow over New Zealand.
On Thursday, a ridge of high pressure should move east over New Zealand from the Tasman Sea.
