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• Long-range forecasts suggest unsettled, cooler weather around New Zealand for Christmas Eve and Christmas Day
• But meteorologists say they’re likely to change as the big day nears, with weather models still in disagreement
• Westerly winds are driving widespread , while a shift to “La Nina-like” northeasterly flows is predicted from January

Ten days before Christmas, the latest long-range forecasts suggest that skies over much of the North Island are going to be murky come the big day – but then so is the picture this far out, meteorologists say.

A Niwa forecaster says that, at this point, there are higher odds of cooler, showery weather about New Zealand on Christmas Eve and Christmas Day.

MetService’s extended forecast, just updated to include December 25, similarly shows cloudy, showery weather around the north of the country – including for Whangarei, Auckland, Hamilton and Tauranga.

Further south, it shows a similar picture for centres New Plymouth, Palmerston North, Nelson, Blenheim, Christchurch and Dunedin.

But people shouldn’t be fretting over their barbecue plans just yet.

Metservice forecaster Alwyn Bakker said the weather outlook was highly uncertain, with conflicting model predictions making it difficult to provide reliable long-range forecasts.

“We’ve got some models saying it’s going to be beautiful, and other models saying it’s not going to be beautiful,” Bakker said.

“We just kind of have to hold tight and keep checking as the new [model] runs come in and weed out whichever ones aren’t performing particularly well.”

To help overcome modelling limitations, MetService uses a technique called clustering, where models are run multiple times with slightly different starting conditions, before the average results are analysed.

But this approach has its own challenges when models are providing conflicting predictions.

“Until they kind of agree with each other a bit more, we can’t say which one of them is correct,” Bakker said.

When could people start putting more faith in the forecast?

“A week out is the outside where I’d start saying, this may well be what’s going to happen, but three days out is where I’d be saying I’m definitely making plans based on this.”

Niwa forecaster Chris Brandolino said models currently suggested odds for unsettled weather were greatest on Christmas Eve and Christmas Day, particularly in the upper North Island.

“The theme favours the threat of showers, unsettled weather, maybe a bit of cooler weather on Christmas Eve,” he said.

“There could be a lingering shower or three on the morning of Christmas Day, and then Boxing Day looks to be dry, and that looks to continue on the 27th and 28th for much of the country.”

But, like Bakker, Brandolino cautioned that picking the exact timing and location of weather systems this far out was tricky.

“Oftentimes when you’re seven, eight or nine days away, you have an idea of what’s going to happen, but it’s about the timing and the location – and these things evolve.”

Added to that complexity was the changeable nature of a messy westerly regime that New Zealand has been stuck in for months.

It’s brought a succession of rainy fronts from the Tasman Sea to western parts of the country, while pushing swathes of Hawke’s Bay into drought.

Westerly flows are also behind weather systems that have arrived to soak the upper North Island today.

“For the most part, it’s going to be a pretty bad up there,” MetService meteorologist Oscar Shiviti said.

MetService has issued a heavy rain watch for Northland, Auckland, Aotea Great Barrier Island and the Coromandel Peninsula covering most of Monday.

Heavy rain watches were issued for the Bay of Plenty and Rotorua with a 24-hour watch for the Wairoa District beginning at 1am Tuesday.

The upper North Island will be “muggy, warm, and humid” today, with temperatures in the mid-20s for most places and Hawke’s Bay expected to reach up to 30C.

Shiviti said the rain is likely to linger into tomorrow morning before easing off around midday.

There is also a moderate chance of thunderstorms in the Westland ranges, the Canterbury high country and North Canterbury this afternoon.

As the front spirals off the side of the country, Shiviti said there might be some respite for a few days before another front moves over the country in the middle of the week.

Brandolino expected to see a shift away from westerly patterns from January, when a “La Nina-like” regime began bringing more northeasterly flows – and subtropical warmth, moisture and humidity with them.

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.