Weather: Monster high on its way to South Island from Thursday

Dig out the winter woollies: A cold snap is heading for the South Island this week, with temperatures dipping well below zero as the week draws to a close.

A weak front is headed northwards up the country, followed by a colder southerly flow, which is immediately "locked in" at night by a more calming high pressure, Weatherwatch forecasts.

The weekend high is connected to the "enormous" high easing off Australia, which will this week merge over New Zealand as one powerful high, with the air pressure close to 1040hPa.

It will keep the cold air stuck through inland valleys, where the temperature could plummet as low as -15C in some low-lying areas of Southland.

MetService meteorologist Cameron Coutts said the reasonably fine weather was expected to turn sour from Thursday.

Northern New Zealand will see cloudier, windier and milder weather as it hits, but the South Island will be the worst affected.

One of the strongest areas of high pressure on the planet (near 1040 hPa) could move over New Zealand by next weekend! 🔴 ☀️

This will be associated with well below normal weekly rainfall, not ideal for those continuing to deal with water deficits. pic.twitter.com/fHrubTJsZ5

— NIWA Weather (@NiwaWeather) June 7, 2020

"We're looking at possible northwest gales on Thursday and into Friday in Southland, up into Otago, Canterbury and Wairarapa," Coutts said.

Heavy rain is also on the cards for the west of the South Island this weekend, Coutts said.

"For the most part the South Island will be smack bang under the centre of this weekend high – so the cold southerly change at the end of this week will be locked in, seeing overnight lows plummet to possibly -5C through well inland areas like Tekapo, Weatherwatch says.

"Queenstown, Alexandra and Cromwell have overnight lows currently forecast between -1 and -3C this weekend, but some locations within, or near, these areas may dip to -5C, creating heavy frosts."

Cromwell, Clyde and Alexandra will awaken to a chill -4C on Saturday morning, but the coastal main centres of the South Island won't see temperatures as low.

What's in store for Sunday?
Between Thursday and Saturday front sweeps NZ, with cold air behind it, but what comes next? Image shows 6 main scenarios possible from 50+ weather models. Likely a big high, but also the risk of a localised lingering trough. https://t.co/Yjbq0jxdqz^TA pic.twitter.com/MjotFizWiN

— MetService (@MetService) June 8, 2020

Coutts said exposed areas of central Otago and south Canterbury will be the coldest, with a "pretty frosty weekend" expected in inland areas.

But the brunt of the weather should be over by Sunday, he said.

Niwa meteorologist Ben Noll earlier said the approaching high also came nearly a year after a similarly dramatic system drifted over the country, which pushed air pressure to 1043.2 hectopascals (hPa).

"For the upcoming weekend, it looks to possibly centre over the top of the South Island where the highest pressures will likely be."

Atmospheric pressure is the force of air pulled towards the earth by gravity. The highest pressure recorded in New Zealand was in Wellington in 1889 at 1046 hPa.

Noll said the highest values ever recorded for this month was the 1044.7 hPa measured over the South Island on June 5, 2016.

"The modelling we have for the feature coming this weekend shows it could be in the low 1040s, which puts us in the ballpark of where we could be talking about near record, or even record values, for surface pressure in the atmosphere."

He warned that people would need to watch out for black ice and frosty driving conditions as the worst of the weather hits.

"We are talking about temperatures as cold as -15C in some low-lying areas about the alps on Saturday morning, creating some pretty frosty conditions," he said.

"These are some pretty extreme cold anomalies, so no doubt people will have to watch out for things like black ice."