KEY POINTS:
Forecasters are warning of a large storm due to hit landfall tomorrow before spreading onto all regions.
The storm will bring torrential rain with thunderstorms, creating localised flash or surface flooding, and is in stark contrast to today's weather.
Christchurch has today recorded a temperature of 28C, according to the Radio Network, and Auckland was sitting on 21C at noon.
MetService has heavy rain warnings in place for many regions across both islands.
It said an active front was expected to sweep down the North Island during Tuesday followed by moist northerlies and a period of heavy rain.
Hilly areas exposed to the north should get 80 to 100mm in 12 hour period. In eastern Bay of Plenty and northern Gisborne, the rain is expected to be heavier and more prolonged about the ranges where 200mm is possible in a 24-hour period.
A second front should move east across the South Island on Tuesday afternoon preceded by a moist northerly flow and a burst of heavy rain with 80 to 100mm likely in 9 hours or less over the ranges.
The Radio Network's head weather analyst Philip Duncan said a huge band of thunderstorms in the Tasman Sea stretches well over 1100km.
He said: "While they are often much bigger out at sea, it does show that this storm is packed with energy."
Mr Duncan believes there is a risk of tornadoes in western areas of the North Island as well as across the South Island's west coast.
"This is a weather event that all New Zealanders should pay close attention to," he said. "Take heed of the warnings and keep up to date with the latest thunderstorm warnings."
He said the heavy rain was expected to dump around 50mm of rain across the previously drought-hit Waikato.
"We could be going from droughts to floods. Farmers need to be prepared for possibly a very wet winter," Mr Duncan said.
"The low will be anchored off the south-east coast of the South Island and will likely deepen further on Thursday creating giant swells at sea and a biting southerly that will bring snow to low lying areas and highs only in the single digits".
- NZ HERALD STAFF
