Seasonal outlook: Why Auckland may keep missing out on rain

Niwa's just-issued outlook for the next three months is picking near to above normal rain for the upper North Island – but Auckland could miss out on what it needs to battle back from drought.

There's also a roughly 50-50 chance of a La Nina climate system – which could bring more deluges like this month's Northland storm – developing between now and the end of October.

The seasonal outlook painted a picture of higher than normal air pressure above the country through the period – an atmospheric set-up that typically brings fine, settled weather – and westerly air flows.

However, a building La Nina system in the tropics could bring more northeasterly winds throughout the season – and with it, more moisture.

Temperatures were forecast to be near or above average in the east of the North Island, and north and west of the South Island. Above average temperatures were likely for the east of the South Island, and "very likely" for the north and west of the North Island.

Rainfall was most likely to drop over the west and east of the North Island, be near-normal in north and west of the South Island, and be either near or below normal levels in the east of the South Island.

For the north of the North Island, there was an equal chance of near and above normal rainfall – although not all areas would get the same amount.

"As we saw in late June and mid-July, northeasterly winds can bring a lot of rainfall to places like the East Cape, Northland and Coromandel," Niwa forecaster Ben Noll said.

"But Auckland tends to be a bit sheltered from the Coromandel Peninsula as those east to northeast winds are carried across the ranges – and those air parcels can end up dried out as they finally approach the east and northeast.

"So Auckland, despite being very close to active weather systems, hasn't fared as well as its northern neighbours this winter."

That had resulted in aquifers and groundwater being recharged in Northland while depleted dam levels in the Auckland region received incremental gains.

As at this morning, Auckland's dams were just under 60 per cent of total storage, compared with a normal average of 83 per cent for this time of year.

"So, while parts of the upper North Island may get above normal rainfall, the most likely outcome for Auckland will be near normal – although locally, we'll be hoping for more than that."

Noll expected to see many more "blocks" of high pressure and stormy interludes – a pattern that had characterised the year's weather so far.

Those interludes had been "La Nina-like", with heavy downpours – and even if a system didn't officially develop, Noll said that signature could well be seen in storms over spring.

As it stood, Niwa was predicting equal chances of a La Nina forming up somewhere in the back half of the year, or our climate lingering in its current El Nino-Southern Oscillation (ENSO) neutral state.

But Niwa's criteria for "La Nina watch" have been met because ocean waters have become unusually cool in the equatorial Pacific and trade winds are stronger.

During La Nina, ocean water from off the coast of South America to the central tropical Pacific cools to below average - a result of stronger than normal easterly trade winds, which churn cooler, deeper sea water up to the ocean's surface.

The unusually cool water in the eastern Pacific then suppresses cloud, rain and thunderstorms, as sea temperatures in the far west of the ocean warm to above average temperatures.

In New Zealand, it typically involved more northeasterly winds that bring rainy conditions to the North Island's northeast, and drier conditions to the south and southwest of the South Island.

The last La Nina, in 2017-18, came with a freak marine heatwave that set the stage for New Zealand's hottest summer on record – and there are early indications the Tasman Sea could soon begin heating up again.