By Phil Duncan, WeatherWatch senior forecaster
After weeks of above-normal temperatures, New Zealand has finally had a cold change with classic autumn weather across the country last week.
Snow fell on the mountains and ranges, hail showers peppered northern New Zealand with thunderstorms and squalls, and temperatures dropped nationwide. In fact, it was the first proper cold southerly of the year – which is remarkable considering we’re just two weeks away from June and the start of winter.
The run of weather that was much warmer than average, noticeable in April and the first half of May, isn’t over yet either. This week northerlies return as high pressure again moves out to NZ’s east. When high-pressure zones are parked east of NZ the airflow (being anticyclonic) tends to bring in milder northerlies and nor’easters (although, granted, nor’easters aren’t warm for everyone – but they do tend to make for milder overnight lows even if the days feel cooler).
When high-pressure zones are parked west of NZ, around Tasmania and New South Wales, that’s when we get the colder southerlies.
In fact, there is a very specific shape to the air pressure zones when NZ gets hit by a major Antarctic blast. For this to occur (that is, when we get snow to sea level in Dunedin and Christchurch and snow on the ranges of the upper North Island), high pressure needs to be basically centred over Tasmania and be a “tall” high (in other words, it’s shaped north to south – so it reaches northwards into Queensland, and southwards over the Southern Ocean).
We then also need a low-pressure zone southeast of Dunedin. When you have the Tasmania high-pressure zone coupled with the low-pressure zone southeast of Dunedin, NZ gets a major cold blast.
When it snowed in Dunedin, Wellington, Christchurch and Auckland in 2011, this was the set-up we had – and we’ve seen variations of that set-up in recent years with other cold blasts (that weren’t quite as major).
While this week saw a decent injection of cold, the high-pressure zone over Tasmania wasn’t “tall” enough – so it didn’t stretch down further over the Southern Ocean to dredge up the cold polar air. The further south that high stretches, the more likely NZ is to get a cold blast.
So as we head towards the end of May, what is happening with autumn’s weather pattern? Next week we are back to warmer-than-average in many places again – all due to high pressure east of the country, where it once again pulls down sub-tropical airflows. We may also see a fast-forming low in the Tasman Sea late next week, bringing a burst of rain and wind to the country.
• High pressure clears NZ to the east next week
• Risk of severe weather late next week
• Heavy rain and strong northerly quarter winds possible next Thursday/Friday
• Low pressure may dominating the North Island next weekend
• Another high may come in towards the end of next week from Australia