Mid-month deluge expected amid warmer slide into winter

New Zealand is in for another big soaking later this month, with Niwa picking plenty more rainfall for northern regions between now and mid-winter. Photo / Dean Purcell

New Zealand is in for another big soaking later this month, with Niwa picking plenty more rainfall for northern regions between now and mid-winter.

The agency's just-issued seasonal outlook also forecast near to above average temperatures everywhere in the country for the next three months.

"The chance for a colder-than-average late autumn and early winter is pretty low, and continues a run of unusual warmth," Niwa forecaster Ben Noll said.

With that warmth came the wet - rainfall levels in the north and west of the North Island, particularly, were expected to be either near or above normal.

That, however, contrasted with a near-to-below normal rainfall outlook for the South Island.

Noll said this trend would be "something to keep an eye on" given hydro lake and soil moisture levels remained low across much of the south, following a particularly dry summer.

"We usually rely on winter to recharge groundwater and those lake levels."

On the other hand, he added, fewer freezing days might lessen the demand on hydro resources for power.

Source / Niwa

Niwa was also predicting lower-than-normal air pressure to New Zealand's northwest - but higher pressure to our east - over the wider period, with more easterly winds likely.

A La Nina climate system that had delivered the country an unusual mix of weather over summer was also expected to soon fade away completely.

But first, the agency was picking more low pressure systems this month - with an "elevated chance" for heavy rain around mid-April.

That was largely down to the presence of a climate driver called the Madden-Julian oscillation (MJO).

The largest element of the intra-seasonal variability in the tropical atmosphere, the MJO was a pulse of rain and thunderstorms that circled the globe every 30 to 40 days.

In certain phases, it could power weather patterns and drive big downpours over New Zealand.

Source / Niwa

Noll said this week's MJO-fuelled weather brought rainfall totals ranging between 40mm to 80mm across the country.

Among some of the most drenched places were Hamilton - where 65mm fell in 12 hours on Tuesday, equivalent to 80 per cent of its monthly normal - and the interior Bay of Plenty, which received about 77mm yesterday.

That rain had been welcome, with virtually all of the North Island, along with eastern and much of inland South Island, running drier than usual at the start of the week.

MetService forecaster Sonja Cooper said Good Friday should start off relatively fine for the upper North Island, with a southerly change creeping up the country, bringing slightly cooler-than-average temperatures.

Similar conditions were in store for Saturday before a northwest flow arrived on Sunday, bringing buckets of rain to the South Island's West Coast but sunshine and warm conditions to the east.