Forecasters eye mid-month rain-maker amid switch in weather patterns

Forecasters are pointing to the potential for a big mid-month rainmaker – offering the chance of some slight relief for north Canterbury farmers battling an ongoing dry spell.

Niwa meteorologist Ben Noll said next weekend and the lead-up to it would be a “period to watch” for an influx of moisture, but added the picture could still change before then.

He said mid-September marked the time when the chilly influence of a rare heatwave in the polar stratosphere started to wane for New Zealand – while La Nina-like weather became more common as the big climate driver began to bed in.

“We’re expecting a bit of a pattern change which may begin to favour northerly quarter winds – and we know that those, whether northwest or northeast, are a moist wind direction.”

Models for September 13 to 15 pointed to a blocking high developing to the south of the country, and a low potentially forming to the north or northwest.

“That’s more like a La Nina-flavoured weather pattern, so mid-month could certainly prove an interesting time in terms of rainfall, and getting some moist flows from the north.”

Added to the set-up was a globe-circling pulse of rain and thunderstorms called the Madden Julian Oscillation (MJO), which was due to pass through the tropical West Pacific, above New Zealand, over the period.

In the past, that’s coincided with large dumps of rain sweeping down.

“I’d say it’s pretty likely that we’ll get something out of that pulse.”

For farmers in North Canterbury – soil conditions in Hurunui have fluctuated between “dry” and “extremely dry” for most of the year – the big question was how much of the rain headed east.

“In Canterbury in particular, there are still some challenges off the back of El Nino,” Noll said.

“I’d say that there is some hope on the horizon for some rain, but there’s the possibility this might not be a particularly long-lasting event.”

While La Nina is known to bring more rain to eastern New Zealand, especially the north-east, Noll said the indications were the system wouldn’t be as strong as previously expected.

Niwa’s recently issued spring outlook gave 50-50 odds of the pattern fully forming before summer.

In the meantime, it picked warmer nationwide temperatures, along with plenty of westerly flows bring wetter conditions to the west, and drier weather for the east.

The outlook follows what was New Zealand’s third-warmest winter on the books – and a particularly dry June and July for many regions.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.