A massive "low pressure generator" covering the entire northern Tasman Sea and stretching as far as New Caledonia was responsible for the drenching delivered to parts of New Zealand this week.
This weather system, detected by weather balloons but which doesn't appear on weather maps, was high in the upper
atmosphere and is also known as an "upper trough", said MetService forecaster Bob McDavitt.
Its "progeny" are the series of low-pressure systems that have moved over New Zealand in the past few days, one a "complex" low-pressure system that brought flooding to parts of the Coromandel Peninsula before it turning to the Bay of Plenty.
It is called a "complex" low not only because of the thunderstorms and rain bands within it, but also because forecasters find it difficult to predict its ferocity.
It had been sitting in the Tasman Sea to the north of the country since last Friday, and was stuck in place because of a high-pressure system over southern New Zealand bumping against it.
The longer it sat, the more moisture it sucked up and the more troughs and thunderstorms it produced.
By early Wednesday morning, rain bands and a line of thunderstorms from the low were moving south down the eastern coast of the North Island past Great Barrier Island in the Hauraki Gulf to the Bay of Plenty, drenching Tauranga with 309mm in the 24 hours to 4pm Wednesday. Mr McDavitt said the deluge was "unprecedented" for Tauranga airport's rain gauge.
"That has never been recorded before at that gauge and it's been going since 1975," he said.
It is not until Sunday that this complex low will move away to the east, but as it crosses the North Island on Sunday, it is expected to produce yet more bad weather, with heavy rain and squally winds forecast for the Coromandel Peninsula, Bay of Plenty and Gisborne on Sunday.
Today and Saturday should be mostly fine in the Bay of Plenty with a few showers possible.
Mr McDavitt said next week was likely to be dominated by westerly winds over much of the country.
