Big Dry: Parched north faces 'cyclone lottery'

The rain-starved north faces a cyclone lottery next week, forecasters say - but even if it does arrive, it will do little to soak concrete-dry soil from Northland to Waikato.

The latest monitoring showed the ground across many parts of Northland and Auckland had a soil moisture deficit of at least 130mm - effectively the difference of two months' rainfall.

After weeks of meagre moisture, no soil in the North Island has normal levels.

The driest spots, compared with normal for this time of year, were found across Northland, northern and western Waikato, and inland Taranaki.

Vast swathes of the upper North Island could now be classified as in meteorological drought - and Northland's Aupouri peninsula and pockets of Great Barrier Island had reached an extreme state.

The one possible rainmaker on the horizon was Tropical Cyclone Uesi, which was expected to head towards New Caledonia next week.

A farm in Tangiteroria, Northland. Soils in Northland are now among the driest in New Zealand, compared to normal for this time of year. Photo / NZME

MetService meteorologist April Clark said it was too early to say if that system would send any rain our way.

"At the moment, it could go many, many different ways - and I think we'll have to wait until mid next week before that will become clear."

Clark added that, if it did bring rain, there was no guarantee the particularly parched areas would see any. From now until Friday, at least, the weather in the north was forecast to stay warm and dry.

Niwa forecaster Ben Noll said that, although models did suggest a good chance of moisture being carried down into the wider Tasman Sea, bets were still off for anything hitting New Zealand.

"We're still don't quite know where it's going, but we have reasonable expectations that it will head into the New Zealand region. That's about the extent of what we know." In any case, the upper north needed much more than one spray.

"A substantial amount of rainfall [is] needed to bring things back to normal," Noll said.

"And that's assuming that everything you get is absorbed into the ground, which is very unlikely, because ex-tropical cylones tend to bring a bout of intense rainfall - and given the ground is more like concrete, instead of its plush, normal state where it can absorb water, you're likely to have run-off, or maybe even flooding. "I don't think there'll be enough but right now farmers will be saying they'll take what they can get."

What's to come?

Forecasters are anticipating as many as three cyclones in the southwest Pacific between now and April.

The odds of those systems forming had risen with the demise of a specific climate driver that's been much to blame for Australia's bushfire crisis - a record-positive Indian Ocean Dipole, or IOD, which had essentially blocked a pulse of rain and thunderstorms - called the Madden-Julian Oscillation - that circled the globe every 30 to 40 days.

"Now that IOD has faded away this pulse can cruise right past the top of Australia and into the western Pacific, which is why we're having that increase in cyclone activity in that part of the world," Noll said.

Models suggested there could be some cyclone activity around the middle of next month.

"Again, it's always a bit like a lottery. There might be a couple of cyclones swirling around in the Pacific, but your chances of getting beneficial rainfall from one of them are never going to be guaranteed," he said.

Walkers make their way up the dried grass of Auckland's One Tree Hill. Photo / NZ Herald

"However, the more that you have up there, the better your chances are - and it's probably just a matter of time before we see some moisture from one of these systems."

But generally, for the north, the background state of New Zealand's climate wouldn't be changing soon to one that favoured rain-making, he said.

For those southern spots that had still seen relatively little of summer, the news was better.

While the tropical Pacific remained in an El Nino-Southern Oscillation (Enso) neutral state - meaning that neither El Nino or La Nina patterns were colouring our weather - a large pool of warm water in the western Pacific was predicted to bring its own flavour here over the next four to six weeks.

At the same time, the influence of a key climate driver called the Southern Annular Mode - whose negative phase over much of summer had added to a cool and dismal season for many in the bottom half of the country - had waned.

Those factors meant fronts travelling up from the Southern Ocean could become less frequent as autumn arrived.

"That's perhaps some good news for people in Southland and Otago, who have had it a bit rough this summer," Noll said.

"Those hardy souls in Wellington might even get a taste of summer as we go into the second half of February."