Weather: Mild winter more like 'sprinter' with westerlies in force

Welcome to "sprinter".

That's the term that WeatherWatch.co.nz analyst Philip Duncan has used to brand this winter, which is blurring the boundaries with spring because of its milder than normal temperatures.

Duncan said many people had been asking when winter weather would be truly arriving, with a few cold patches among an otherwise warmer season.

The neutral weather pattern New Zealand was experiencing had led to "enormous" high pressure systems over Australia, which were fuelling powerful westerlies in the Southern Ocean, he said.

"Basically, we've got a lot of high pressure over Australia and what that does, with its big anti-cyclonic winds, is push a lot of air down into the Southern Ocean."

The air was then being redirected to New Zealand with strong westerlies, he said.

"It just gets caught up in the roaring forties, and comes straight over us. The result of that is kind of like a leaf-blower against a hose, because the southerly is trying to come up to us, but is being cut off at the knees by the westerlies."

He further likened the effect to a motorway, where westerly traffic was going one direction, with no on-ramps for the southerlies to get going.

"So until that changes, we are not getting any true winter weather, but weather that we'd normally get in spring or autumn."

It was a different picture from the near or below average temperatures of last year's winter.

"Because we are two small, mountainous islands in the roaring forties, our weather every year is quite different: but the main difference between this year and last year is that we are not really getting the big southerlies coming in -- the placement of the highs and lows this year are just encouraging a lot more westerlies for us."

How long would it last?

"I think this is what we will be seeing between now and, let's say the second week of August.

"This is the coldest time of the year, traditionally, and at the moment, we've got westerlies blowing in which removes the chance of us having a truly cold winter.

"It doesn't mean that we can't have brutally cold southerlies coming in August and September, but it does limit how cold it can really be across the country, simply because the days are getting longer already."

Between July and September, the National Institute of Water and Atmosphere (Niwa) predicted that temperatures were very likely -- with a chance of between 60 per cent and 70 per cent -- to be above average in all regions, although frosts and cold snaps would occur from time to time.

Seasonal rainfall totals were also about equally likely to be near normal or above normal for all regions.

Warmer sea surface temperatures around the country would remain a "major driving force" for New Zealand's climate over the season, Niwa said, and consequently, there remained an elevated risk for significant rainfall events and severe storms.