Winter 2020 is proving to be weird, even if you’re waiting for the winter solstice to officially say winter is here – the pattern we have right now is unusual due to high pressure.
The highs (or anticyclones) crossing the country right now are very powerful, with central air pressure up to the 1040hPa range. Put simply, this is as big as most highs get and they are oddly having a positive affect on our climate.
It was spring last year and over our recent summer that highs stretching from Australia put many parts of NZ into drought. Even as droughts slowly end with ‘death by papercuts’ this year (a term WeatherWatch uses to describe months of accumulated showers fixing the problem, rather than one big rain event) we are seeing more high pressure move into NZ.
So why are more powerful highs actually bringing us rain?
It’s all to do with the shape and location of the centre.
Last spring and summer most highs were parked over Aussie and spread east to protect northern NZ. This acted like an invisible brick wall from Aussie to NZ, stopping sub-tropical rain from heading south and limiting how far north any southern rain events went. It lead to drought.
Now in May and June we’ve seen these highs more independent – rather than just stretching out from Aussie, they are breaking away from Aussie and becoming their own system freely tracking over NZ. As a result – we’re getting the colder southerly changes as they move in, then the long lingering sub-tropical northerly flows afterwards – and it’s this key part that is helping produce rain in the dry zones.
It’s rare that high pressure after high pressure starts to make rain – but it’s what we’re seeing as these powerful highs have good ‘reach’ and can pull down moisture-rich air over the country.
In fact, this Thursday NZ will be stuck between two highs. The eastern high feeding in sub-tropical rain to northern NZ while the western high feeds in a snowy, colder, showery southerly. The two air masses meet over New Zealand generating rain – and maybe helping to spin a very weak low too.
Next week it happens again. Another big high with colder nights, then a sub-tropical flow spreads over New Zealand. And if not sub-tropical, at least milder northerly quarter.
This set up matches what RuralWeather.co.nz and IBM forecast in our June outlook issued in May – that rainfall would be returning to normal despite the big highs. Now, we’re starting to see how this unusual set up works – with highs dragging in areas of rain and allowing them to soak dry regions.
Not your normal start to winter – and one that will be a positive for many growers and farmers, not to mention water supply in our largest city, Auckland.
High pressure brings warmer than normal weather for Tuesday and Wednesday with parts of Canterbury climbing into the low 20s (and a few other parts of NZ too). For Canterbury this will be more than 8 degrees above normal for this time of year. However, on Thursday the cold change swings it the other way, more than 8 degrees BELOW normal in parts of Canterbury.
on 15/06/2020 10:31pm
Thanks for the analysis Philip. The present set-up sees a near continuous band of high pressure systems right around the world at our latitude and directly above the high pressure is an almost continuous band of rain. There are only small gaps inbetween the high pressure systems that allow the rain to slip down from the sub-tropics. There is nowhere else on the globe where a weather pattern is almost completely unbroken like this.
on 16/06/2020 8:36pm
Hi Phil C,
Many thanks. Been interesting watching this high pressure belt for the past 12 months. Never seen anything so large or stubborn before. Hopefully showing some signs of breaking up a little more now – but too early to know. Maybe by August we’ll know for sure if it’s breaking up or remaining.
on 15/06/2020 9:16pm
yay patchy rain coming hope i get some nice rain here. 1040hpi thats quite high. what did auckland get up to? hope for thunder but i thinking this yee=ar is a quiet year for t-storms this year. i did get one on the 5th of may tho.
i see jet gone quiet. he may of finally got his thunderstorm