Heavy rain, but mainly on the plains

Four-year-old twins Lucy (left) and Amy Law splash out. Photo / Sarah Ivey

Aucklanders be warned - yesterday's rain was just the entree for a meal of poor weather that is due to hit the city this week.

But the serious weather damage will be reserved for parts of Canterbury, where the ground was already nearing saturation point last night.

MetService ambassador Bob McDavitt said a front carrying heavy rain had moved down the North Island yesterday and would stall over the Canterbury Plains for the next two days.

He said Christchurch and the surrounding areas were at risk of floods after Christchurch got 42mm of rain in the 24 hours leading up to 5pm yesterday, with two more days of heavy rain ahead.

Timaru would get more than a month's worth of rain in a single day, said Mr McDavitt.

The heavy rain had cleared from Auckland in time for the Monday morning rush hour yesterday.

It made its way down the country, but not before dumping 46mm of rain in 24 hours on Whangarei, 26mm on Auckland - about a third of the monthly average - 113mm on Whitianga and 78mm on Whakatane.

The strongest winds reached 94km at Cape Reinga at 6pm on Sunday.

Mr McDavitt said the effects of a major low pressure system would arrive in several parts, which he likened to a restaurant meal of several courses with breaks in between.

"We had our entree [Sunday night and yesterday morning]. Our main course is still to come," he said.

"The next step is the westerlies."

Mr McDavitt said westerly winds would arrive tonight bringing big squally showers and heavy swell to the West Coast beaches.

He warned surfers to avoid the West Coast tomorrow and Thursday.

The MetService yesterday issued a thunderstorm watch for Auckland and Taranaki.

Mr McDavitt said the downpours would be brief and would affect only patches of the city.

"You won't get a whole day of heavy rain but you will get a bucket of rain in a short time," he said.

Auckland has already had a fifth more than its monthly average of rain so far this month.

Half of that fell last Thursday and Friday and a further quarter fell on Sunday and Monday.

Meanwhile, South Islanders were last night bracing themselves for more heavy rain expected to hit Canterbury and Otago.

"The front will stop over Canterbury," said Mr McDavitt, adding that it would be pinned in place by the low pressure system which was yesterday over the Tasman Sea.

"That is why we are worried about Canterbury.

"Everybody else is just [receiving] superficial damage."

Big impact but not a 'bomb'

North Islanders may have felt like they were in the middle of a weather explosion when they were hit by heavy rain on Sunday night.

But the drop in low pressure was less than half what it would take to qualify as a "weather bomb".

According to MetService weather ambassador Bob McDavitt, bad weather is officially called a meteorological weather bomb when a low pressure system deepens by 24 hectoPascals or millibars (hPa) during a 24-hour period.

He said the last true bomb in New Zealand was in late July 2008. That cost an estimated $42 million in insurance payouts.

He said the term bomb had its early beginnings when Swedish meteorologist Tor Bergeron coined the measurement of 24 hPa in 24 hours as a unit for gauging quickly deepening lows in the centre of storms in the North Sea.

These days meteorologists say a pressure change of 24 hPa equals 1 Bergeron, or 1 "b" - otherwise known as a weather bomb, said Mr McDavitt.

He noted people had started using the term to refer to all kinds of bad weather events.

The central pressure of a low was only one measure of how much damage a storm would cause, he said.

The low this week would deepen further by tomorrow.

"It is definitely a major event."